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Musings of an Energy Nerd

Are Passivhaus Requirements Logical or Arbitrary?

Martin Holladay’s keynote address at the Passive House Northwest conference in Olympia, Washington

Don't be a foam hog! You can save a lot more energy by installing 2 inches of foam under 7 houses than by installing 14 inches of foam under one house. Image Credit: Martin Holladay

What follows is a reconstruction of Martin Holladay’s keynote address at the Passive House Northwest conference in Olympia, Washington, on March 18, 2011. The piece has been fleshed out somewhat, in light of the fact that the original time constraints no longer apply. For the most part, each paragraph corresponds to one slide of the accompanying PowerPoint presentation.

Click here to view the presentation slides

Are Passivhaus requirements logical or arbitrary?

Over the last seven years, it’s been exciting to see the Passivhaus standard take root in the U.S., where several dozen Passivhaus buildings have already been built.

But it’s important to remember that superinsulated houses are not new. Canadian and American researchers and builders began building superinsulated homes in the late 1970s.

A brief look at recent history

My own interest in superinsulation can be traced back to my years as editor of Energy Design Update, a superinsulation newsletter launched by Ned Nisson in 1982. I took over as editor in 2002.

In 1985, Ned Nisson and a co-author, Gautam Dutt, published a landmark book, The Superinsulated Home Book.

The book emphasized the importance of careful air sealing measures, and it provided details for building double-stud walls, Larsen-truss walls, and foam-sheathed walls. It described the advantages of low-e glazing, argon-gas-filled glazing, and triple-glazed windows.

By 1985, superinsulation concepts were well understood. Researchers had studied and quantified air leakage in homes. Books and magazines with superinsulation details were widely available. Builders could buy low-e windows, triple-glazed windows, HRVs, and blower doors. Builders had developed a number of techniques for building homes with very low rates of air leakage. And many successful homes with R-40 walls and R-60 ceilings had already been built.

Eleven years after this somewhat arbitrary milestone, Dr. Wolfgang Feist founded the Passivhaus Institut in Darmstadt, Germany, to promote the…

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  1. Jesse Thompson | | #1

    Foam & Carbon
    Great conversation Martin. Two points:

    1) Individual designers and builders don't have the ability to decide if they get to spread the available resources for a project across multiple buildings. This is the realm of policy, not design & construction.

    If the Energy Apocalypse magically arrives and each community is rationing their last 10 sheets of rigid foam, then we'll talk, but until then it's a distraction to even claim this is a choice of design.

    2) In your discussion of the PH space heating limit, you've totally ignored climate damage reduction as a goal of the space heating limit when PHI developed the standard. 15 kWH/m2/yr in a Central European climate turns out to be a reduction of ~80 - 90% in heating energy from typical building stock, which is right in line with all indications of where we need to get to to stop ripping our climate apart. It lines up with Bill McKibben's initiative, and the european Factor 10 and 2,000 Watt Society's goals.

    It's not just an engineering game, in other words. We need to get our energy intensity down by really staggering numbers as quickly as we can, and the 15 kWH/m2/yr range lines right up with where we need to be, and was one of the fundamental reasons behind the creation of the specific PH limits.

    Jesse Thompson
    Kaplan Thompson Architects

  2. Expert Member
    ARMANDO COBO | | #2

    Great article and good
    Great article and good synopsis of the Passivhaus program. It shows there are many ways to skin a cat… and no one system is the Holly Grail to achieve high-energy efficiency and healthy homes. As in most cases, exaggerations and misinformation is what brings negativism to green building. Good information and education, helps us overcome the bad rap and hype of most programs in the public view and skeptics in our industry.

  3. GBA Editor
    Martin Holladay | | #3

    Response to Jesse Thompson
    1. You're right, of course, that "individual designers and builders don't have the ability to decide if they get to spread the available resources for a project across multiple buildings." But just because "this is the realm of policy, not design & construction," doesn't mean it is irrelevant.

    If a design standard results in buildings that are examples of a bad policy, it surely makes sense to consider altering the standard in such a way that better policy results are achieved.

    2. I didn't "totally ignore climate damage reduction as a goal of the space heating limit when PHI developed the standard"; in fact, I wrote that the Passivhaus standard "sets an energy goal that is in the ballpark of what will be necessary to achieve required carbon reductions."

    Let's look at this issue in greater depth. It is theoretically possible to supply our energy in a carbon-free manner, using a combination of PV arrays, wind turbines, biomass, and tidal power; in fact, the Danish island of Samsø is already close to achieving that goal. The main reason that we haven't already jumped at the chance to supply all our energy in a carbon-free manner (in addition to the fact that the existing energy infrastructure represents a huge investment that our capitalist system is unwilling to abandon or decommission) is that generating carbon-free electricity is very expensive. Dr. Feist agrees with me on this point; he told me, "At the moment, the cost of electricity produced by photovoltaics is in the range of 40 to 50 cents per kWh, which is still ten times the cost of electricity produced by oil or gas. ... I am not against subsidies for renewable energy, but you shouldn’t mix up [the issues] that way. The difference in price is just made by governmental subsidies. You have to be careful with governmental money."

    So, we agree that it's theoretically possible to obtain our energy from carbon-neutral sources. We aren't doing it, however, because it's so expensive. So what possible logic is there to addressing global climate change by choosing a solution that is even more expensive than the renewable energy route?

  4. UphillLarry | | #4

    Response to Martin, re: subsidies for renewables
    I'm not an expert so please don't quote me, but I believe the petroleum industry enjoys many subsidies as well. Only a small percentage of the savings is passed onto the shareholders, and likely less onto the rest of us. The gap between renewable and not, is likely to be smaller than it appears on the surface.

  5. GBA Editor
    Martin Holladay | | #5

    Response to Larry Burks
    The statement about PV subsidies was by Wolfgang Feist; I was just quoting him. It was not my intention to knock PV subsidies.

    You're quite right that the oil industry (not to mention the nuclear industry) enjoys all kinds of subsidies. I would be in favor of gradually eliminating subsidies to oil companies and the nuclear industry.

  6. John Straube | | #6

    Wonderful summary
    Martin, this is an excellent job of summarizing the issues, both good and not so good. This is the kind of discussion we should be having more of.
    BTW. The Austrians and Swedes are not the only ones to challenge and modify PH. The French also have a program which started from somewhere other than PH but ended up near the same spot. It is called -- dont laugh -- effinergie, that does a really nice job too. Tweaked what and how is measured, accomodates French climates and building practise, and gets to the same rough range of numbers, eg around 120 kWh/m2 source energy for all energy uses.

  7. Jesse Thompson | | #7

    I wouldn't agree that we're
    I wouldn't agree that we're not implementing renewables because they're an expensive way to produce energy. We have no problem as a country spending money on areas powerful interests have decided are important (3 concurrent foreign wars?). If we felt like it, I'm sure we could be moving as fast as Germany is in changing our energy mix, but so far we haven't wanted to enough.

    It's not because we've been wasting our money on 16" sub-slab insulation :)

    In our own practice, we're trying to keep our eye on the primary reasons we're working on these problems beyond merely enjoying the technical challenge, and in my opinion the climate destruction issues are first and foremost. You mentioned it as one bullet point in a long list, but whenever I've been around PH advocates, climate change mitigation is the first reason mentioned for the standard's existence.

    Technically, I agree with you, in our colder northern climate we've generally seen that the PV production cost tipping point arrives before a building meets PH, but that's also because we haven't been as skillful (or have been too constrained by our renovation projects or urban sites) at being good passive solar designers to take advantage of all the free heat we have in our climate.

    Our current wave of buildings is doing much better at this now that we're using PHPP for analysis and can really see how every individual window is working as a little heater (or not).

  8. user-723121 | | #8

    Superinsulation revisited
    Passive House has revitalized superinsulation, I have no doubt about it. Superinsulation was the residential building breakthrough and clearly laid the groundwork for Passive House. Since the introduction of PHIUS we now talk matter of factly of R-40 walls and 1 ach50, this is progress.

  9. PFB4bXSZvJ | | #9

    cost issues
    You have to accept basic economics as a fundamental rule of reality or your efforts will have no far reaching impacts. The challenge is to get people to understand the true costs of the choices we make once we factor in the life expectancy of the building and the environmental costs of the energy source we are using. Ignoring economics simply means that you have the luxury to ignore economics because you are one of the elite. Which for most of us in the US, we are, whether we feel it or not. Global climate change will not be stymied by a standard that ignores cost effectiveness, it will merely be a cool niche market and maybe an example to future post apocalyptic societies. If you have money to spare and climate change is your main concern, why not spend the extra money on a massive pv installation and stop at 4" of sub slab foam. Governments don't always make very rational economic decisions (three foreign wars) but private markets generally do, of course when private markets are injected with government backing of insurance for nuclear power than the whole thing gets skewed. I think we we should build the houses that make sense to build based on the cost of wind and solar like Martin says and work to see a change in government policies that allow the true cost of our choices to be more obvious.

  10. GBA Editor
    Martin Holladay | | #10

    Second response to Jesse Thompson
    While I agree that adding unnecessary insulation under the slabs of our buildings is not the reason that our nation lacks a commitment to renewable energy, I fail to understand your apparent disregard of the economic discussion I have raised.

    If a mix of renewable energy sources can provide energy without negative effects on our planet's climate, and if that solution is cheaper than adding 14 inches of foam under our homes, how is this economic discussion irrelevant? It appears to go right to the heart of the matter. Architects who choose the thick-insulation route are choosing an expensive path, and doing a disservice to their clients.

  11. Natur Haus | | #11

    I suggest that you all realize that you will not change each others' minds, that is if you are as sure about your views as I am of mine. We are all on the cutting edge of the learning the best building technologies, implementing what we think is right and what we can afford. While I think Martin's picture pretty much sums up the issue, I don't think Passivhaus is truly, at this time, about moderacy for the masses, it's more about, please read with a sense of humor,"We're German energy Nazis, look what we have done". And I understand it, but 14" of foam is not even responsible, unless you're trying to heat your home with farts in Siberia.......

  12. Envirocon | | #12

    A voice of reason
    As a BPI certified contractor specializing in deep energy retrofits of historic homes, I am caught in the middle of; clients, architects, inspectors, sub-contractors and government and utility rebate programs. Each of these people has his or her own agenda and have gleaned enough disinformation to formulate an opinion that supports that agenda, so when I read an article like this, balanced and thoughtful, it makes me so happy that I have to post. We ( yes us, the ones with the computers, who have done this to the world) are stuck in quicksand. If we do nothing, we will go under, but if we flail around without direction, we will go even quicker. We need to work together to build a rope, not argue about what kind of rope is best. Instead of supporting your point of view to the end, humbly weave yourself into your own community. Be the rope you want to see.

  13. greenophilic | | #13

    Passive House as Rule of Thumb gone awry
    Martin, great article! I dug into comparing net-zero energy design approaches (BEopt for example) against the Passive House design approach a couple years ago, and I came to nearly the same conclusion as you. I think the PH standard was developed as a set of "rules of thumb" that would lead to near-optimal cost-effectiveness, IF the conventional heating system was done away with. Cost-savings/effectiveness though elimination of the conventional system is the crux of the argument, and was the driver behind development of the standard, in the context of Central Europe. But as we know, rules of thumb are often less useful outside of the context in which they were developed, and they are a method that is supposed to simplify analysis. Carefully conceived rules of thumb should get you near optimal solutions without the help of overly complicated technical analysis, and PHPP does not give this. I think the rule of thumb that is the PH standard is out of context in much of the US, and as a result, we get perversely designed buildings with 16" of sub-slab foam. The goal in PH design has become the religion of the PH standard, WITH the best of intentions, of course.

  14. Mike Eliason | | #14

    passivhaus as rule of thumb
    would be correct if PHPP wasn't so accurate and proven across a wide range of HDDs.
    within the context of central europe, it's more difficult to achieve passivhaus than in the US.

    the 16" sub slab foam is the anomaly based on several factors, none of which include passivhaus being used 'out of context' - especially as that context is built upon north american pioneers.

  15. Natur Haus | | #15

    Meaning what? Mike Eliason
    "within the context of central europe, it's more difficult to achieve passivhaus than in the US." Explain your point further if you would sir....I certainly don't see any vague "rules of thumb" I see German detail fixation, which quite honestly I am a victim of......

  16. wjrobinson | | #16

    I am all in with what Bill
    I am all in with what Bill Bradbury posted sort of.... The discussion is great, but the actual building of all kinds of low energy, no energy, carbon neutral, non polluting healthy fun livable homes is... super-active-passive-cali-fragile-istic-extra-all-n-ala-doshiaus!

    I gots lots of rope and am even pretty darn good at a weave splice of sorts....

    Here's my nickel's worth...

    Let's rebuild a 100,000 homes to net zero this year and then do 10% more each year following. Rules? Any damn way you feel you can get there. With or without LEED, PH, EnergyStar, and all the rest. Foam, no foam, some foam. Do it your way and in groups and individually. Be a do-er. Be a proud rebuilder of a home to Net Zero!

    The 100,

    Off to ready me rope for some weave splicin...

  17. Mike Eliason | | #17

    ease of achieving PH

    When I take a project in PHPP and migrate to every climate in the program, tracking resulting specific heat demand, the EU climates are trending higher (meaning more difficult) than North American ones. I've done this to several projects, and the results are all similar to the graph posted yesterday (

    So to say that the envelopes here are perverse, when most EU envelopes aren't as perverse and it's harder to achieve PH there - tells me that there is a disconnect. The rules of thumb are there, but it's possible they've been ignored. They relate to orientation, massing, etc.

    We need better rules of thumb in designing a passivhaus. The technical analysis will get you the rest of the way. Frankly, I've never found the analysis part to be difficult or complicated, and we use it as an integral part of the design process, constantly refining so we can avoid those perverse assemblies.

    How are you a victim of details?

  18. GBA Editor
    Martin Holladay | | #18

    Response to Mike Eliason
    You write, "the 16-inch sub slab foam is the anomaly." But observers in northern climates are seeing R-50+ sub-slab foam and R-90+ attic insulation in PH project after PH project, from very smart designers who can't easily be dismissed as idiots or misguided fools.

    So it's not enough to tell those of us in New England and Minnesota, "Just sharpen your pencil. There must be a better way to do it." Because these smart PH consultants tell me, "I keep going back to the spreadsheet and I can't hit the number without R-50 under the slab and R-100 in the ceiling."

    So, if these very smart designers can't do it, and Mike says, "Well, they should be able to do it with a lot less foam," then there's something about the software that makes it particularly hard to use. Because these designers in New England and Minnesota aren't idiots.

    And if the software is that hard to use -- and if the software even confused Katrin Klingenberg, who after all built a simple cube without any bump-outs -- then I think it's the Passivhaus standard that needs to sharpen its pencil, not cold-climate designers.

  19. homedesign | | #19

    Mike Eliason,
    I admit that I have not designed any extremely high performance homes to date.
    I have only designed a few "Half-Assive' homes (Homes with A HERS Index around 50)

    Can you provide links to some of the Passivhaus Homes that you have designed and Built?

  20. GBA Editor
    Martin Holladay | | #20

    Response to John Brooks
    Do your homes fully comply with all requirements of the HalfassivHaus standard?

  21. Mike Eliason | | #21

    built Passivhaeuser

    re: PH homes - none built yet, but we're working on it.

    I can point to two certified Passivhaus projects in 8000 HDD+ North America climates that don't have R-100 assemblies or 16" sub-slab insulation.

    1. Osterreichhaus, Whistler BC:
    ground: R-47
    wall: R-47
    roof: R-54

    1. Bagley Nature Center, Duluth MN:
    ground: R-53
    wall: R-64
    roof: R-85

    The Bagley Center, based on our research, could have lowered their insulation levels with better glass. In our findings, the Cardinal 179 is decent glass, and is still energy positive in this project, but routinely comes up short compared to EU glass.


    The R-50slab /R-80 roof is the upper limit we're seeing for projects we've migrated around North America. But that's not 16" in the ground, and it's only in the more extreme portions of the Midwest and New England. Most of the East Coast needs less.

    For maximizing solar gain (or daylight, or natural ventilation), a cube isn't the most efficient shape. I know these consultants aren't idiots - in fact most are a hell of a lot smarter than we are. But maybe the biggest problem is going back to the spreadsheet - but not going back to the drawing board. Dr. Feist's Kranichstein project is a great formal model - compact shape oriented east-west with no bumpouts, simple roof line, maximizing glazing to the south...

    Honestly, when you start there - the numbers start to look less dramatic than when you start by shoehorning a design into a Cape Cod or Saltbox. Perhaps this means Passivhaus and modern architecture are better paired?

  22. Natur Haus | | #22

    Mike, the victim comment refers to my fixation on details. I don't view the Passivhaus standards as ambiguous "rules of thumb", but as fairly acute prescriptions. Rules of thumb would be superinsulate, passive gain, grid tied solar, solar hot water, energy efficiency, massing, etc; this is how I build homes. Although my homes are efficient and well detailed, I am sure I don't meet the Passivhaus standard, and believe doing so is actually arbitrary in itself. As with LEED, I believe it is divisive, saying THIS is good, and THAT is not, meaning the way the rest of us are building. Not to say I don't believe in the core elements of Passivhaus and LEED, as I try to implement building styles that appease both. As far as the Cardinal 179 glass, triple pane American wood window with the low-e on the #3 surface, I believe it gets close to the performance of the uber fiberglass and uber German wood windows' performance, at roughly HALF the cost(of the serious windows anyway).

  23. albertrooks | | #23

    Don't get too wound up... It's still early.
    Obviously it's a little early to say that there is a last word on how much underslab foam is needed for an American PH Project in a northern climate. It would be naive to think there is enough data to support a statement that the Passivhaus requires 16" of foam in high HDD climates.

    A little more patience and time is needed before a "rush to judgment".

    As Martin admitted (and thereby aging himself -smile) He was involved in super insulation in the US at it's inception. I'm sure that he would not say that they "had it down" to an optimized approach in a few years. There were only a few working in the field and you can only learn so fast...

    The PHPP is the crux of the underslab foam issue. Getting a project through it is demanding. The PHPP is a tool created in Germany. When it's used (today) in Germany, the values used are based on products and values that are available in that market... And at market competitive prices due to concentrations of vendors in competition with each other.

    Currently, the American designer does not have that luxury. And in this case -it is a luxury. The American Designer is having to: 1, Learn an extremely complicated new tool designed in a non english speaking country. Work with materials that are not native to the tool, trying to "shoe horn" North American products that don't have the high values common to the practice. And then deal with building shape orientation options that for the most part, have not even made it on the radar... as even a variable that one would adjust.

    Considering the handicaps in being an early PH designer/builder in the US, is it any wonder that underslab foam is one of the first "go to" points for getting it through PHPP? It's far less expensive than Import windows and doors with the values needed.

    And what does that mean? That PH is too foam reliant -or- that our supply industry is behind the supply industry of Europe? We don't have the product choices here... And we have to work with what we can economically get.

    Too say that the only PH option is for far too much foam is, at this point, is a rush to judgement.

    I mean come on... We're just getting warmed up!

    (No pun intended.)

  24. GBA Editor
    Martin Holladay | | #24

    Mike Eliason's R-values
    Your examples of R-50 floors prove my point. An R-50 floor, according to John Straube's calculations, requires between twice as much and 2.5 times as much sub-slab foam as can be justified by comparing the energy savings of the foam with the energy output of a PV array.

  25. albertrooks | | #25

    R Values continued

    Now that the examples are out of the range of your sketch (thank goodness it was not to scale! What was that: R-150 compared to R-2?) and into the R 50 range, I've lost the thread of the issue:

    MH: "I am proposing that the cost of PV is a useful benchmark representing the high limit of likely future energy costs; for this reason, it makes sense to avoid envelope measures that yield a smaller energy return than a PV array. If you add more insulation than this benchmark justifies, you are planning for a future that will never come."

    It seems to me suggesting that: in Mike's above cases, R-25 foam is better than R-50 foam is semantical. If either measure is equalizing the same demand, does the issue then turn to a suggestion that PV is a better strategy than sub slab foam?

    I can see the case for R-70 and up. I make no secret of the fact that I'm not a big fan of foam, but if there is foam going under the slab, than going from R-25 to R-50 seems to me to be in a reasonable range, and not what I would consider a "foam hog".

    If I have it right that your advocating the same load reduction is better delivered by PV than foam (in this R-25 load range), then I feel it is important to bring up the points that you were not interested in discussing:

    MH: "It's important to note that I'm not advocating that builders actually install a PV array; nor am I particularly interested in arguing over whether insulation usually lasts longer than PV modules. (For the record, it usually does.) "

    Those above points are important to the "lasting effect" of load reduction. It eliminates the issues of owner care, lack of owner care, and the failure of the PV array to remain in place due to remodel, re-roofing, storm damage and other cases that can cause the PV to not deliver the load reduction. The point after all is- that a load reduction actually happens.

    It's not just a semantical cost argument: PV is better than foam -or- foam is better than PV. Durable and Passiv envelope improvements should always trump roof top gadgets.

    If buildings can be built in this range, with durable results, then I don't see how 15kwh/m2 is arbitrary. There is nothing wrong with an R-50 slab.

    Why settle for mediocre results as a benchmark? I'm hoping that getting the envelope load down to PH levels is the starting point. Project budget or not, there are those (certainly myself) who see this load reduction as a way to leap frog over zero and head to plus homes. It'll be a disappointment if we get there and you've already filled the roof with PV to get to zero just to make up for poor windows and slab demand.

  26. user-788447 | | #26

    i'm between Martin and Mike
    I haven't read this whole discussion carefully but here are a couple points to add -

    I don't know who Martin has been speaking with in Minnesota but I think the R values Martin points to (R-50 slab, R-100 roof) are elevated based on my experience with the PHPP in the same climate. Even though smart people are working on this stuff there is too much going on to nail it down in the first two designs even. Martin give us some time to get to our third project and beyond and get some feedback from how the projects act in the field. Passivhaus is still maturing in my area and part of the elevated cost is the fact that it is new.

    Mike, I would caution anyone trying to achieve Passivhaus, in a cold climate at least, to not try to widdle down insulation levels by maxing out solar heat gains. The day/ night temperature fluctuations need to be considered as well as periods of no clear weather. Balance between high levels of heat retention and allowing only as much solar heat gains in the winter as needed is key (you can have an excess of heat gains even during the coldest Minnesota weather). Light construction lacks the thermal mass of construction more common in Europe so watch out for increased day/ night temp fluctuations.

    My opinion still stands that learning Passivhaus is the best method for any cold climate designer serious about making an energy efficient buildings. The fact a designer needs time and experience to fully understand the software isn't a bad thing.

    As for cost optimizing software, what designer wouldn't love to have cost optimizing software!

  27. Mike Eliason | | #27

    Straub's calcs

    I don't know what kind of project John ran through PHPP to determine those calcs, so it's hard to get around his thinking on them.

    Here is what I do know. I put together a compact 1440 gsf (TFA=1236), 3 BR project. I migrate that to Minneapolis (8033 HDD), and have to add a little insulation...
    Walls: R-44
    Slab: R-34 (4" slab on grad over 7" type IX EPS @ R-4.55/inch)
    Ceiling: R-57
    Windows: R-6.5 (installed)

    The resulting PHPP specific space heat demand comes in at 4.67kBTU/ft2a

    If I peel the ground slab back to the BSC R-10 approach (walls and roof are already at BSC levels), the specific space heat demand jumps to 7.47kBTU/ft2a.

    this is a difference of 2.8kBTU/ft2a. I then go to the PE Value tab, and can find out how much PV is needed to make up that difference: PHPP says just over 1,000kWh/yr.

    1. Is 292 (700 sf * 5") cubic feet of EPS, plus scraping a little more dirt for the slab really more cost than enough PV to produce 1000kWh/yr, which in Minneapolis appears to be about a 0.6kW system?

    2. With the additional 5" of EPS in my Passivhaus, it's a one-time 3,205 CO2 output. PHPP shows the CO2 emissions of the PV at 0.41lbsCO2/ft2-yr, which equates to 507 lbs CO2/yr total. Over 30 years, that PV is responsible for 15,210lbs of CO2. Is the active approach, which emits another 6 tons of CO2 before taking into account the embodied energy of the PV panel the right approach?

    To me, it's a no-brainer, especially if we're talking about reducing energy bills AND CO2 emissions - that tuning the envelope correctly (which may mean thicker slab assemblies than we're used to) is the way to go.

    [edited to reflect correct TFA]

  28. GBA Editor
    Martin Holladay | | #28

    Response to Albert Rooks
    It seems to me that much of the desire to hit the Passivhaus number is emotional rather than logical. The argument seems to be, "R-50 instead of R-25 isn't that much more -- why are you making such a big deal about it? After all, that's what I need to get the certification, so it's worth it."

    I feel that basic math and economics aren't enough to convince Passivhaus die-hards, so perhaps it's not worth trying. After all, it's a voluntary program, and if you want R-50 foam under your slab, go ahead.

    I'll make one last stab at introducing logic, however:

    1. To set up a basis for understanding PV costs, I'll quote Dr. Wolfgang Feist, who is respected by the Passivhaus community: "At the moment, the cost of electricity produced by photovoltaics is in the range of 40 to 50 cents per kWh, which is still ten times the cost of electricity produced by oil or gas."

    2. If someone is building a net-zero-energy house, they have to assume that their energy costs are equal to the current cost of PV. That's expensive electricity, but some people want a net-zero-energy house anyway.

    3. According to Dr. Straube's calculation, if you are building a cold-climate net-zero-energy house, you should put R-20 or R-25 foam under your slab. After that, the foam costs more than PV. Almost no one puts R-20 or R-25 foam under their slab, though, because it's really expensive to build a net-zero-energy house.

    5. Passivhaus proponents are proposing R-50 foam under a cold-climate slab -- twice as much as is needed for a net-zero-energy house. They aren't just assuming that the cost of electricity will rise to 45 cents a kWh like people who build a net-zero-energy house. They are assuming, bizarrely, that electricity will cost much, much more than the current cost of PV. When? I don't know.

    6. So, net-zero requires R-25. It's wicked expensive, so it's rarely done. Passivhaus says, "Install twice as much as is needed for a net-zero-energy home." Well, you can do it if you want the PH certification -- so go ahead. But the math doesn't make any sense.

  29. GBA Editor
    Martin Holladay | | #29

    Response to J Chesnut
    I think you made an important point about south glazing area when you wrote, "I would caution anyone trying to achieve Passivhaus, in a cold climate at least, to not try to whittle down insulation levels by maxing out solar heat gains. The day/ night temperature fluctuations need to be considered as well as periods of no clear weather. Balance between high levels of heat retention and allowing only as much solar heat gains in the winter as needed is key (you can have an excess of heat gains even during the coldest Minnesota weather)."

    I've seen two or three Passivhaus designs that make me nervous -- big walls of south-facing glass, obviously driven by the numbers on the PHPP spreadsheet. More solar gain appears to make the spreadsheet work, but at the risk of overheating during sunny weather in March and April, and at the risk of poor performance during long cloudy stretches of weather in November and December.

  30. GBA Editor
    Martin Holladay | | #30

    Response to Mike Eliason
    You wrote: "I don't know what kind of project John ran through PHPP to determine those calcs, so it's hard to get around his thinking on them." If you want more information on John Straube's calcs, see the extensive explanations in my earlier blog, Can Foam Insulation Be Too Thick? and in the comments to that blog.

    Here are some of Straube's assumptions:

    — “A slab on grade insulated to R-32 in Finland had an average heating season soil temperature of 12.5°C (55°F). Hence, during the heating seasons the average temperature difference between soil and indoor air is about 15°F.”

    — “If you account for a coefficient of performance of 2.5 for the heat pump over the season (3.3 in 40°F weather but 2 in -10°F weather), the cost of PV-powered heat is no more than 60/2.5 = 24 cents per kWh. Note that many central forms of renewable electricity production work at 25 cents per kWh, such as wind, microhydro, tidal, biomass, concentrating solar thermal, etc. So this seems like the high end of electric production costs.”

    — “The cost of insulation becomes more than the cost of generating energy for the walls in a typical house in a 7,200-HDD climate at about R-60 (using the Building Science Corporation approach), and slabs [on grade] at about R-20 to R-25, depending the cost of placing EPS (which costs around 10 cents per R per square foot).”

    — “If I base my design on real measured results, rather than someone's model, I repeatedly find that the temperatures of the sub slab range from the 10°C to 15°C range (50° to 60°F).”

    — "By measuring the soil temperature under a slab, the impact of insulation on reducing heat flow and the insulation and thermal storage capacity of the soil are directly considered. No estimates or fudge factors needed. All but the Cardiff slab were measured in a cold climates with lots of insulation (4 to 8 inches), so the impact of heat loss should be mostly accounted for. Heat flow through a layer of insulation, whether under slab, roof, or wall, is driven by the temperature difference across it. For roofs, the challenge is to account for the solar impact on the surface temperature. For slabs, the challenge is to estimate the soil temperature. So, I repeat, regardless of the fudge factors, standards, estimates, and computer models, heat flow across an insulated slab is due to the temperature difference across it, and the limited measured data provides consistent information about the size of this temperature difference. The real-world measurements simply do not match the standard approaches and assumptions.”

    — “I use an average of thousands of measurements (hourly) over the year. As you must know, the thermal mass of the soil under the slab means that the temperature varies incredibly slowly. I did not use a steady state model. I am using measured boundary conditions that are far from steady state. I have also done these types of calculations with Heat 2D, a dynamic 2D program, and was brought up on Mitalas's brilliant and still relevant basement heat loss models, which include 3D effects, dynamics, etc but unlike all other models, was carefully benchmarked against multi-year heat loss studies of DOZENS of REAL basements (the reason I trust his results more than most other paper studies).”

    — “Of course the perimeter is different than the edges. Of course geometry has an impact. But both of these do not affect the answer to the basic question of how much insulation should be under a slab. I hope it is also obvious that the perimeter should have more insulation if it is easy to do so, and the center less. My calculations are normally based on a 7.5x12 m floor plan, which … reflects the lower 50% of the housing market in North America, and probably 70% in Europe. A 10x15 plan is OK for a ranch house, and will mean the perimeter zone is about 25% of the total slab area, depending on your definition of a zone. Physics of heat flow across a slab much wider than it is thick exposed to slowly varying temperatures: Q = U A Delta T. The only issue is the Delta T.”

  31. bxPc5pH8mc | | #31

    Question for Mike Eliason
    In 2nd through 4th sentences after the number 2 in your comment, do you say that PV panels EMIT CO2?

    If I am reading your comment correctly, could you explain how a PV panel emits CO2 during use (ie separate from the emissions created to manufacture the panel)?


  32. SDQQQiYWp6 | | #32

    Passive solar
    I really appreciated Martin's recognition of the confusion caused by using passive house vs. Passivhaus. I have been designing passive solar homes in WA state for 24 years and feel like my specialty label has been co-opted. I'm sure this causes some folks to make assumptions about my design strategies because they have heard of Passivhaus, and those strategies seem extreme for our mild climate. I have worked very hard teaching classes, giving talks, writing articles and developing a criteria for cost effective passive solar homes in WA state. I feel like I have been hit with a double blow when Passivhaus proponents' considerable marketing confuses my potential clients into associating me with them and then further hurts me by using all my good work to get audience for their techniques. I am familiar with double wall construction and super insulation from my education in the early 80's. They were called envelope houses and didn't make much sense in western WA's mild climate. Even with higher energy prices 30 years later, I'm still not convinced that taking up more floor space and using more resources to achieve super duper energy efficiency is worth the trouble and expense, especially with a limited budget. I have attended a few introductions to Passivhaus standards and have always specified above code insulation and windows. Since air leaks are typically the biggest heat loss area I am very diligent with air sealing details. As a Lung Assoc. certified healthy house designer, I am also concerned about making houses super tight when so many finishes and furnishings available in the U.S. are made with petrochemicals. Hi tech ventilation systems are only as good as their installers and operators. I appreciate designers pushing the envelope of energy efficient design. I wish the Passivhaus folks would get their own term for it, like high performance or super efficient. Because as you said, these phpp houses are not passive.

  33. Mike Eliason | | #33

    CO2 output of PV

    PHPP doesn't consider grid-tied PV as carbon neutral, but carbon reducing. the house still draws energy from the grid at night and during winter, and while you can net meter a house - you aren't zeroing out the CO2 emissions from grid-utilized source and transmission losses. As such, there is an associated emissions factor, and PHPP kicks out the resulting CO2/ft2a.

  34. kevin_in_denver | | #34

    Great Article, BUT
    The real congratulations goes to the brave Passivhaus folks who invited Martin to speak. They must have known they would get a healthy dose of real science aimed squarely at their sacred cows.

  35. albertrooks | | #35

    The cows and their tenders in the field are doing just fine

    I do get your point about a cost valuing insulation levels in the PHPP as being an improvement. It's a good point and would be a good feature. No doubt. I am not "die hard" to the point that I would not respect your opinion on this or any other issues. I come to GBA to get it so don't hold back.

    Frankly it's great that we have the opportunity to push these issues around. I certainly hope I don't come off as "it's Passivehaus or nothing". The reason most of us are in "low energy building" is to get more good buildings built. Regardless of how much sub slab foam, or what "program" they are built in.

    So thanks for the opportunity and the "gentle nudge" to examine what we're doing and why.

    I hope I can be forgiven, and still be viewed as "sane" if I have a "favorite path". Obviously I can't answer the issue of how Dr Feist chose 15kwhrs/m2. I think that I, along with many more, understand why that mark was chosen. I've gone to both US and european conferences and heard the presentations and they match: It's a reachable level of load reduction through passive envelope improvements and orientation. That's not to say it's an easy one. Nor is it to say it's for every project.

    I also want to acknowledge all of the positive things that both you, and Dr Staube, have said in this post about what the introduction of Passivhaus has brought to the US. MH:"The Passivhaus standard is now attracting wide attention, and designers are thinking and talking about design details in a new way." Hopefully that continues and doesn't stifle other developments, values or viewpoints.

    It would be presumptuous of me to speak for PHUIS or PHI, but I can say that the hopes of Passive House Northwest are that PH in our region adds to the overall atmosphere of low energy building to both it's practitioners and the general public. Regardless of what our personal favorite is, regionally we support every form of energy efficient building.

    With that, I'm going to let the subslab foam issue drop. I really don't think it's important today to have a final word on what is possible with the PHPP as it stands now. Both you, me and J cheastnut have admitted here that there are really not enough projects on the ground to see what American designers can do with it. Even Dr Staube pointed out that the PH standard was "tweaked" in France, but wound up near the same values. It means to me that it's attainable and bears further investigation before applying the label "arbitrary".

    Ps... I love the "HalfAssiveHaus". We'll need to develop the HalfAssive Planning Package. (HAPP).



  36. user-970944 | | #36

    thoughts on conservation vs production
    I guess I'm going to join in here and though I may be on the 'illogical'' and non-scientific standpoint, my experience comes from being in the building trades for 25 years, living through the ‘70s & ‘80’s in a well-monitored & well-engineered (for the time) Solar Home (passive and active), and also being a numbers geek.

    The main point of contention I see flowing through the conversation is the idea of the most cost effective method of meeting the energy needs of a building's occupants in an environmentally conscious method. In our reach for this I really don't think we can set aside the soft sciences, namely human behavior. This is why a passive approach holds the most promise in my view.

    Utilizing active systems demands higher levels of human involvement. Call me cynical but my hope for humanity-in-general's motivation to maintain an optimized PV system is rather low. Maintaining a substantial thermal envelope is generally no different than non-super-insulated structures.

    As for the high–cost of the thick blanket in a Passive House approach, cost effectiveness in any project always rests with the designers and builders. As folks have said above, sharp pencils, experience and optimal use of available materials are the main means there. As for conservation vs production (foam vs PV), one of my main concerns is how an alternate method will affect the building production. We can’t just look at material costs isolated from installation. The Passive House I’m finally completing does have thick layer under the slab field. Placing 4” under the perimeter grade beam took no less time than the center 11” (ok maybe 1 hour less). Is this overkill in a 4900 HDD climate? Possibly. However, having the higher inner portion also allowed me to reduce the need for fill. Spending a good amount of time crunching a numbers and using PHPP as a tool enabled me to create a very cost effective sub-slab assembly sequence. And the extra few inches? They helped allow the use of a less expensive and local window package.

    Our pool of successful Passive House projects is still rather small. Examples are being added regularly, currently I’m bidding a PH Single Family House that uses just 5” of sub-slab foam. One of the beauties of PHPP is it does allow the designer to readily adjust placement, material and thickness of the blanket.

    Don’t get me wrong, I am a fan of a PV, co-generation, wind +++. However I think they are best utilized in concentrated situations. The cost effectiveness of a significant installation on a nearby institutional/industrial or such structure would be soooo much more cost effective (from both installation and maintenance standpoints) than on a single family house.

    Yes I’ll agree the 15 kWh/m²∙year is set somewhat arbitrarily. Some think it should be more stringent, especially for our climate here in Seattle. Is that illogical? Less logical than concentrating populations in climates with 7,000 or more heating degree days and deciding that supplying the energy to keep comfortable (at the flick of a switch) is more important than political and environmental instability?

    As for how the Solar Home from 30 years ago doing? It’s still there and reportedly warm/cool, but all the active Solar elements have now been stripped (and not replaced) while the passive ones continue.

  37. GBA Editor
    Martin Holladay | | #37

    Response to Dan Whitmore
    You've raised many good points. I couldn't agree more with your point about passive systems outlasting active systems; the researchers who built the Saskatchewan Conservation House in 1977 came to the same conclusion, and there are lots of examples of passive solar homes from the 1980s that still work well but have abandoned one or more active elements.

    I think you misunderstood my point about the cost of electricity generated by PV arrays. This is what I wrote: "I'm not advocating that builders actually install a PV array; ... I am proposing that the cost of PV is a useful benchmark representing the high limit of likely future energy costs."

    Most homeowners want to leave the hassles of electricity generation in the hands of their local utility, and that's as it should be. In the future, as now, almost all Americans are likely to get their electricity from their local utility -- they won't be generating electricity on site.

    At the risk of repeating myself, I'll try to explain why I think the cost of generating electricity from a PV array is a useful benchmark: (1) It's a technology that already exists, and (2) It's expensive -- more expensive than several other types of renewable energy, notably utility-scale wind.

    So that if our utilities make a total switch to renewable energy -- a mix of wind, tidal power, PV, and biomass, with a little bit of deep-hot-rock thermal generation thrown in -- it will never cost more than the current cost of PV-generated electricity.

  38. GBA Editor
    Martin Holladay | | #38

    Another rebuttal - this one from Graham Wright
    You can read Graham Wright's rebuttal of my keynote speech here:
    Response to Martin Holladay's critique of the Passive House movement.

  39. ePA582Dt3d | | #39

    swiss standard
    interesting observations about the standard. you might also want to look into the swiss standard of passivhaus, which has a much better name: minergie [minimal + energie (energy)]

    btw, if you take a look at old (and yes, 100+ years) houses in the alps, you would find them to be quite possibly the first energy efficient homes, with stone walls 3+ feet thick, "powered" by wood burning stoves.

  40. Mike Eliason | | #40

    minergie isn't equal to passivhaus, though they do have more stringent levels of certification, including one based on passivhaus (minergie-p) and a 'green' label, minergie-eco. both can be combined for a super green high performance building (minergie-p eco)

    minergie goes off ERA (energy reference area) which is gross floor area, instead of the more stringent treated floor area (TFA) of passivhaus (basically livable area from gyp-gyp minus a fraction of storage/mech, stairs, etc)

    there are a number of other subtle differences as well, different source factors, performance criteria, etc.

  41. user-731553 | | #41

    Losing the forest for the trees
    I have really appreciated GBA, John Straube etc.'s critiques of Passive House... this is what evolution is all about, continually questioning & rethinking & improving. These discussions are invaluable, not to mention entertaining (I was sad to miss the fun up at PHNW!) I've been perplexed & a little frustrated by some Passive House evangelists that are so defensive, seem unwilling to accept the notion there could be potential improvements to be made w/ PH... it's the Holy Grail, can do no wrong, is perfect and will save the world all by itself. Don't get me wrong, this is a wonderful standard and I'm a huge proponent--but of course it's not perfect. And while this work we're doing is very important, it's still just a small piece of what this world needs. How will our world look with automobile-dependent suburbia filled with Passive Houses? Or an Afghanistan where every building is a Passive House but a woman better not flee from her abusive husband lest her nose and ears get cut off?

    Back to the point, these are my main observations:
    1) People can and do get caught up in the numbers and sometimes end up missing the point and doing REALLY illogical (and certainly not cost-effective, no matter how you slice it) things with their projects. But I don't see this as such a terrible thing--rather part of the evolution of high-performance building in this country. It's too bad when a "bad" example is overly harped on, potentially misrepresenting the standard on the whole, giving the public the wrong idea of what it takes to get to PH. But I think this comes with the territory of learning and applying what is, in this country, a new standard. Bring on the critiques--everyone needs to hear and learn from them.

    A challenge I've seen is that we in the U.S. lack experience with the standard, and tools, those in Central Europe have had for 10+ years. As Martin said, many smart people are building PHPP models. But--these smart people haven't necessarily been taught all the aspects of the PHPP, or might not understand the importance/sensitivity of certain data that can hugely (and positively) impact the numbers. We don't yet have the luxury of a comprehensive book of details from which to pull thermal bridging coefficients, so certain details that might be beneficial to a project are often ignored in modeling. Likely, some of these potentially overdesigned PH projects are based on models that are too conservative.

    2) Using one set of numbers to define a standard--for any climate, any building size, any building type, will by default give you arbitrary (or you might say unscientifically determined) numbers, right? I actually find this aspect of PH quite compelling--while the numbers might be somewhat arbitrary, there are just 3 of them (compare that to a green building checklist!) and they provide something simple, solid and absolute to help guide decisions and provide a sense of how a project is doing. It's up to the project team (or wise consultant) to have some perspective about the numbers and decide whether it makes sense to achieve them "at any cost" for a given project. While I would like to see some adjustments made for climate & occupancy types, there's power in the simplicity of the PH criteria.

    3) The fact that a building standard can inspire such passionate debate is amazing, and very telling about the compelling nature of PH! What nationwide "movement" have we seen that causes builders to work double-time to fanatically airseal. Overkill on such a project, perhaps--but who cares. I've seen PH transcend what's logical/pragmatic/cost-effective and give many people in this industry renewed passion for something they long ago forgot they loved. I'm all for us running with it, missteps and wrong turns and all. Let's keep constructive debates going, but not get caught up in fighting ghosts.

  42. GBA Editor
    Martin Holladay | | #42

    Response to Katy Hollbacher
    Great post! And thanks.

    As you might have guessed, I'm a big proponent of the Passivhaus standard, which is why I want to see the standard changed and improved -- and why exaggerations pain me. It's exciting to see these projects get built.

  43. GBA Editor
    Martin Holladay | | #43

    An e-mail from Gary Proskiw
    Today I received an e-mail from Gary Proskiw, one of the authors of the study mentioned in Solplan Review and cited in my speech in Olympia. Proskiw wrote, "Someone just sent me your article in Green Building Advisor and I must say you are right on the money. Designing a Net Zero Energy House is easy, the trick is doing it without spending a truckload full of money. I have been working on the whole business of NZEH optimization for a while and have concluded that many of the things we are doing may, or do not, make sense."

    Gary kindly agreed to allow GBA to publish the presentation that he and Anil Parekh gave at the BEST 2 conference in Portland, Oregon, in April 2010. Here it is: Optimization of Net-Zero-Energy Houses

  44. Jesse Thompson | | #44

    Economics of low-energy
    To add even more variables to the conversation, so much of what we're talking about is based on ever-shifting costs as well. The Passivhaus standard was developed at a time when PV was much more expensive than it is currently (early 1990's).

    I was reminded of this when we had to revise our net-zero design spreadsheet this week and dropped the price per installed kW of PV from $8,000 to $5,000 before tax credits. Insulation prices sure aren't dropping that quickly, in fact they seem to rise every time oil goes up in price...

    The current PV cost curve may not continue, but it's currently behaving in a way that is disruptive to conservation economics.

  45. GBA Editor
    Martin Holladay | | #45

    Dropping PV prices
    You're right. But designers of net-zero-energy houses can take comfort from one level of certainty: they know exactly what their energy costs will be. Other designers have to scratch their heads and throw darts at the wall as they try to guess what energy prices will be in 15 years. Not designers of net-zero-energy homes: they know that they are designing a building that is optimized for a known energy price. That price is the cost of electricity generated by a PV array purchased at today's price for PV modules.

  46. david_white | | #46

    i regret that i haven't taken enough time to read the full article or all the comments. however, i'd like to address some of the apparent demerits.

    "Calling these superinsulated houses 'passive' is problematic." i used to work for a company called Transsolar and people frequently harrassed me about the fact that my company didn't design solar systems. one even suggested that i design solar systems for trannies. let's get over it. passive solar houses have heating systems too, and we don't complain about that, do we? the Passive House name is a matter of history - it stemmed from a research project to improve comfort in unheated houses in china.

    "Delivering heat through ventilation ducts makes no sense." i believe the original intention of PH was to deliver heating via the fresh air supply, and it did work, and continues to work in many cases. if it leads to the elimination of a furnace and forced hot water distribution system without loss of comfort, that seems to make sense to me. not to say that i'm opposed to recirc.

    "The annual space heating limit of 15 kWh/m²∙year is arbitrary." Juergen Schneiders at PHI told me that the original definition of PH, "a house that can be heated only by heating up its fresh air supply," led to overventilated houses in some cases, so first a peak load limit, and later an energy standard were set to close the loophole. 15 kWh/m2a is about what a house needs if it can be heated by its fresh air. this varies a lot with climate and especially glazing ratios. but it's not arbitrary.

    i was disappointed to read these "demerits" because the information is freely available.

  47. GBA Editor
    Martin Holladay | | #47

    Response to David
    I'm sorry to hear that my article disappointed you.

    On point one, your advice is "get over it." That doesn't sound like disagreement with my point -- just an acknowledgment that the problematic name has been in use so long that it's too late to change. Unfortunately, however, this problematic name is still leading to misstatements, including a whopper -- the Passivhaus Institut's definition of a passive house, prominently featured on its Web site: a passive house is “a building in which a comfortable interior climate can be maintained without active heating and cooling systems.” As long as these misstatements continue to be promulgated by the Passivhaus Institut, I'm not going to "get over it."

    On point two, you say that delivering space heat through ventilation ducts "continues to work in many cases." I agree; I don't dispute that. I object to the continuing presence on Passipedia of a passive house definition that says that space heat MUST be delivered that way.

    On point three, you note that the origin of the heat load limit is the requirement that space heat be delivered through ventilation ducts. This is circular reasoning, especially if it is indeed true that the Passivhaus Institut has rescinded the requirement that space heat be delivered through ventilation ducts. In light of the irrelevance of the requirement that space heat be delivered through ventilation ducts, your explanation strikes me as arbitrary.

  48. Kopper37 | | #48

    Gary Proskiw's Presentation

    In Gary's presentation he mentioned that they assembled 50 different Energy Conservation Measures (ECMs), then evaluated those measures against their PV Value Index---in four different climates, using three different house designs.

    Do you know if the detailed information from this study is available to the public?

    I looked on the NRCan website, only found mention of NZE cost optimization here:


  49. MPFadR5c6L | | #49

    why only the PV vs foam comparison?
    My comment regards homes in the mostly heating climate zones. PV vs underslab foam costs have been compared, but why isn't the discussion including solar thermal? (unless I missed that comment) I haven't done any math, but off the top of my head, I can't see why not install R-20 below slab insulation, then heat pex and run glycol through the collectors. The insulation wouldn't need to be as critically sized, nor would the relatively high PV costs come into play. 1 more point: I'm sure I'm the only nut who actually cares about environmental impact, but less foam = more environmentally responsible.

  50. GBA Editor
    Martin Holladay | | #50

    Response to Deniz Bilge
    Although solar thermal equipment appears at first glance to be more cost-effective than PV equipment -- because of all the BTUs you can collect in July -- it turns out that when used for space heating, solar thermal equipment is usually LESS cost-effective than PV. There are two reasons for this:

    1. Most of the energy collected by a solar thermal system is collected during the summer, when it isn't needed. Storing heat energy for more than three or four days is very difficult and expensive.

    2. Net-metered PV systems have a wonderfully efficient storage system -- the grid. As long as a homeowner has access to the grid and is able to sign a net-metering contract with the local utility, 100% of all PV energy is credited to the homeowner. (This contrasts sharply with the owner of solar thermal equipment, who collects a lot of energy that can never be used.)

    I discussed this issue at length with the designers of the Riverdale net-zero energy house in Edmonton, Alberta. I reported my findings in the September 2008 issue of Energy Design Update. Here's what I wrote:

    “The Riverdale design team assumed that an active solar thermal system was an essential component for a zero-energy house in Edmonton. As the project progressed, however, the solar thermal system became increasingly large and complex. ‘Even though we made the house envelope as efficient as we could, so that the house could be heated at minus 32 degrees Celsius with the equivalent of six hair dryers, we were still amazed at the size of the solar thermal system we needed,’ [Gordon] Howell told EDU. ‘We needed to extend the solar thermal collectors and the PV array beyond the roof line. We were also surprised at the huge complexity of the solar space heating system. We really did not expect solar space heating to be this complex.’

    “[Peter] Amerongen echoed Howell’s observation. ‘We spent more on solar thermal than we ever expected,’ said Amerongen. ‘The solar thermal system is not hugely practical. We have a fair amount of technology chasing small amounts of energy, going after the last little bits of thermal load. When it comes to chasing net-zero energy, in some ways it actually makes more sense to go 95% of the way on a number of houses than to go 100% of the way on a single house.’

    “As the building neared completion, Howell began to doubt the wisdom of including a solar thermal system. Howell recounted, ‘I said, “Peter, this is ridiculous. Let me go back and see what would happen if we had eliminated the solar thermal equipment.” After running the numbers, I was dumbstruck. The cheapest house would have been a house without a solar thermal system, using just PV and geothermal heat. That would have cost $8,000 less than what we did. If we went with a PV-only system — with no solar thermal and no geothermal, only passive solar heat, electric resistance heat, and a PV system — it would have cost only $1,000 more than what we are doing now, and it would be hugely less complex. I was stunned with this. The increased area of the PV array would have been identical to the area of the existing solar thermal collectors, and the PV array could have been integrated into a single sloped roof.’

    “Heating a house with PV instead of a solar thermal system is counterintuitive. ‘PV is way more expensive and less efficient than solar thermal,’ Howell noted. Although a large solar thermal system can collect a lot of heat during the summer, the heat is difficult to store. ‘The big difference between PV and solar thermal is that we haven’t figured out how to convert solar thermal heat to natural gas and feed the gas back to the grid,’ said Howell.”

  51. Ken Levenson | | #51

    perhaps i'm missing something....
    I enjoy your articles but the advice isn't adding up for me:
    1. PV has embedded energy and toxicity and forward maintenance costs that should put a premium value on insulation. Sure PV is better than coal, but I'd say a cost premium on cellulose is preferred to PV. (And i don't think we should be distracted by this seeming fetish with high amounts of subslab insulation on a few projects....let's see a few more PH project get realized before we declare an Achilles heel.)
    2. PH is not alone, as you note, in using area metrics - this is not a unique weakness. And I don't see how a per person metric is much better - when the kids go to college does the house get decertified? (And the PHPP does actually give a "large house" penalty of a sort, in that for certification is increases the population to 1 per every 377sf - hugely contributing to primary energy demand in oversized houses and materially effecting whether the house meets verification requirements.)
    3. Your claim that PH claims that they are "homes without heating systems" is a critique of translation - not intent. There are all sorts of weird statements reading through text of PH German translations to English - yet one can figure out what is actually meant with a little effort. To make better translation a major item (or that some are repeating such mistranslation) seems a bit weak.
    4. To follow your logic on the confusion of "Passive House" I was expecting your next piece of advise to be that the Germans should stop calling it Passivhaus and instead call it Passive House. Seems inconsistent that the Germans can use their language to call it Passive House, but we can't.
    5. Yes, climate specific goals - for the extremes - this is the issue that vexes many. But if my uncertain understanding of oddly translated German is correct, the idea of keeping the standard flat across the globe is this: By sticking to the benefits of such a stringent goal - the problems associated with construction and cost in more extreme climates will will be rapidly addressed through accelerated market transformation (new products and methods invented, mass produced etc...) that will drive down costs rapidly - making PH cost effective if not everywhere today....then tomorrow. PH is an innovation driver.

    Having replied all that, I am NOT declaring the infallibility of PH - there are many improvements to be made. And as I work on several PH projects right now we are struggling with a myriad of issues such as: PHPP's accounting of AC, retrofit standards, historic building accommodations, product availability and so on. There is much to critique - but I think the critique would be more useful if you dug in a few more layers. I hope you do.

  52. GBA Editor
    Martin Holladay | | #52

    Response to Ken Levenson
    I'll respond to your numbered points in order.

    1. At the risk of repeating myself: I am not advocating that homeowners install rooftop PV arrays. That's why I wrote in the blog above, "I'm not advocating that builders actually install a PV array; ... I am proposing that the cost of PV is a useful benchmark representing the high limit of likely future energy costs." In my response to a comment posted earlier (4/5/11, 11:02) by Dan Whitmore, I wrote, "Most homeowners want to leave the hassles of electricity generation in the hands of their local utility, and that's as it should be. In the future, as now, almost all Americans are likely to get their electricity from their local utility -- they won't be generating electricity on site. ... I'll try to explain why I think the cost of generating electricity from a PV array is a useful benchmark: (1) It's a technology that already exists, and (2) It's expensive -- more expensive than several other types of renewable energy, notably utility-scale wind. So that if our utilities make a total switch to renewable energy -- a mix of wind, tidal power, PV, and biomass, with a little bit of deep-hot-rock thermal generation thrown in -- it will never cost more than the current cost of PV-generated electricity."

    2. I agree that finding a solution to the small-house penalty is complicated, but many programs have come up with workable solutions. The simplest solution -- one adopted by several green building programs -- is to establish lower energy limits per unit of area for larger homes than for smaller homes.

    3. I think that Passivhaus proponents are being disingenuous when they write (as you do) that my criticism of “PH claims that they are ‘homes without heating systems’ is a critique of translation - not intent.” I have raised this question directly with Dr. Feist and Katrin Klingenberg, more than once, face to face and on the phone. Nevertheless the Passivhaus Institut Web defines a passive house as “a building in which a comfortable interior climate can be maintained without active heating and cooling systems.” This is false, and Dr. Feist is not telling the truth if he tells people that he doesn't understand the implications of this English sentence. Dr. Feist is a physicist, and is familiar with the precise use of scientific terms and technical language. There is no misunderstanding his intent or meaning when he writes, "without active heating and cooling systems." Moreover, a wide number of English language sites, in Ireland, Norway, Sweden, and the U.S., repeat this false claim. Translation problems have nothing to do with this fiasco, and Passivhaus adherents who shrug off the problem as a misunderstanding due to poor translation are being deliberately blind.

    4. Of course I have frequently noted that both "Passivhaus" and "passive house" are misleading terms. As a journalist, I have no choice when reporting on these issues but to choose one of the two spellings; I prefer to refer to these buildings as "Passivhaus buildings" so that English-speaking readers can distinguish them from passive solar homes. But it would have been better if Dr. Feist had called this the "Wunderbar standard" instead of the "Passivhaus standard," and if his organization were called the Wunderbar Institut. That would have led to much less confusion.

    5. The question of climate-specific standards is, of course, debatable. But when a standard forces builders to choose an alternative (thick sub-slab insulation) that is more expensive than available alternatives merely to meet an arbitrary standard, then I think it's reasonable to discuss climate-specific standards.

    Finally, I hesitate to take you up on your challenge to continue digging deeper into the Passivhaus standard in order to discover the fact that there is still "much to critique." I think I would end up ostracized if I did that. I look forward to reading your exposé of these issues, however, if you care to post it here.

  53. Jesse Thompson | | #53

    More PV Cost

    What surprised me about running the numbers on current PV costs is that the cost you quoted of $.28 - $.75 is actually far above where costs sit right now, and the economics of deep conservation (Passivhaus) look worse from an economic perspective. Deep conservation still makes a great deal of sense for other reasons (passive survivability, actual carbon output in winter when the panels aren't producing but the building is heating, etc).

    If you can currently buy a 1kW system for $20 / month (borrow $3k @ 6% for 25 years, expected life of the panels) it will produce ~110 kWH / mo in our area. That's a cost of ~$0.18 / kWH locked in for 25 years.

    At those prices we should all be borrowing as much money as we can as quickly as we can and covering every roof in sight with PV. PV is no longer expensive...

  54. GBA Editor
    Martin Holladay | | #54

    Response to Jesse Thompson
    Of course, as PV prices drop, my central argument -- that cold-climate Passivhaus buildings are installing too much insulation -- gets stronger, not weaker.

    The upper end of the PV cost numbers I quoted refer to off-grid systems with their associated battery costs.

    Thanks for pointing out that, as PV gets cheaper, we all need to reconsider our assumptions about optimized designs.

  55. user-634791 | | #55

    PV vs. insulation is like comparing apples to oranges
    To Martin

    Sorry if this has already been addressed, I have not had the time to read all of the comments.

    I am stuck on the Insulation vs. PV argument you make.

    I am assuming you are applying that argument only to Passivhaus or houses with electric heating. Obviously if we are talking about a house which uses any kind of fossil fuel appliance for heating, less insulation is only going to create more greenhouse gas etc.. and more PV will do nothing. Even a Passivhaus or an all electric net zero energy house (with net metering) is still going to create greenhouse gas, pollutants etc when it draws energy from the grid. Lets not fool ourselves. When you say
    " makes sense to avoid envelope measures that yield a smaller energy return than a PV array. If you add more insulation than this benchmark justifies, you are planning for a future that will never come...... "
    We live in a country run by the oil,coal and gas industry, the future which will never come is a grid powered by renewables, at least in our lifetime. We are going to have a global crisis if we do not mitigate global warming now I do believe there should be a place for quantitative analysis to make educated decisions but you cannot just look at the straight energy cost when comparing. We are paying enormous environmental costs every day. We need to reduce our consumption now, and if we are talking about heating, the best way to do that is with insulation.

    I choose to look at the long view, so Investing in the fixed shell of a home with additional insulation which will last at least 100 years and save 1000's of tons of carbon emissions 24/7 is a no-brainer. Investing thousands of dollars in temporary high tech systems only lasting 20 - 30 years and probably obsolete in 5 or relying on a renewable energy grid to make up the difference in thermal deficiencies doesn't make a lot of sense to me. Those should just be the icing on the cake.

    PV vs. insulation is like comparing apples to oranges.

    David Pill

  56. Jesse Thompson | | #56


    The consistent use of the word "arbitrary" in describing the numbers behind Passivhaus is unnecessarily contentious. Repeatedly describing the Passivhaus standard as "based on random choice or personal whim, rather than any reason or system" feels like you're trolling for web hits.

    The economic arguments for the Passivhaus standard might be being eroded by the crashing costs of renewable energy production or they might have an inherent German conservatism embedded in the approach, but it's a system based on metrics and numbers that requires selection of a defined number. Without a defined numeric range, it's not a conservation program, it's a hope and dream.

    Instead of Arbitrary, please try out Specific or Fixed, or another word that doesn't needlessly raise people's hackles. It's a little too reminiscent of a certain poster formerly on this site...

  57. GBA Editor
    Martin Holladay | | #57

    Response to David Pill
    Since I know that you are the designer of the net-zero-energy home that won the NESEA challenge, it's surprising that you wrote, "Investing thousands of dollars in temporary high tech systems only lasting 20 - 30 years and probably obsolete in 5 or relying on a renewable energy grid to make up the difference in thermal deficiencies doesn't make a lot of sense to me." Maybe you're right -- but look in the mirror.

    When anyone designs a net-zero-energy home, like you and Andy Shapiro did, you need to perform a simple calculation to determine when to stop installing additional energy-efficiency measures and when to install a renewable energy system. You ended up buying a $40,500 Bergey wind turbine -- your final cost was less due to incentives -- but others have chosen to install a PV array. You just can't ignore the fact that when designing a net-zero-energy house, you ALWAYS have to compare the cost of energy-efficiency measures with renewable energy generating costs. It goes with the territory.

  58. GBA Editor
    Martin Holladay | | #58

    In defense of "arbitrary"
    When designers of net-zero-energy homes are optimizing the design, they always compare the cost of energy-saving measures with the cost of renewable energy equipment. That balance is affected by the climate where the house is located, as well as the insolation (if the home will have a PV system) or the average wind speed at the site (if the home will have a wind turbine). That's a system that ensures that the home's energy targets will not be arbitrary.

    However, anyone building a Passivhaus uses a different method -- hitting a number chosen by an institute in Darmstadt, Germany. That results in designs that are far from optimized -- the designs are occasionally absurd. I see nothing wrong with calling the heating load target established in Darmstadt as "arbitrary" -- unless someone can come up with a better explanation for why it makes sense in colder areas of the U.S.

  59. Ken Levenson | | #59

    dick cheney and german accents
    Martin, Your arguments for PV scarily parallel Dick Cheney's for oil production. Perhaps Katrin and Wolfgang's German accents got in the way of your understanding that "no heating system" means no TRADITIONAL heating system? (Seems like way too big a divide.....and isn't liar a bit strong?)

    Jesse, Martin IS contentious...

  60. GBA Editor
    Martin Holladay | | #60

    I'm beginning to sound like Dick Cheney...
    I don't know how to respond to the suggestion that my discussion of PV costs sounds like "Dick Cheney's arguments for oil production."

    Where do I start? Most Americans use electricity. (I lived without it for a few years, and I ended up missing it.) We need to come up with a plan to produce some electricity (or a viable plan for living without it). I painted a picture of an energy future based on a mix of renewable energy sources: wind turbines, PV arrays, tidal power, biomass, and perhaps (if the drilling can be done without causing earthquakes) some thermal electric production using heat from deep hot rocks.

    I dunno ... it might not be the best plan ... but I think it's better than Cheney's. What's wrong with talking about the cost of renewable energy generation? Do you have a better plan?

  61. P_Murphy | | #61

    Passive House issues
    As I understand it, Martin is suggesting that the PV option to slab insulation is more competitive when using the grid rather than batteries, since the battery costs will be very high. Thus there is a very wide range in the cost of PV. ($.28 - $.75)

    There is an assumption that the energy that goes into the grid is actually used which is questionable. In two talks I have had with power companies they have acknowledged that at least today in the US the electricity bought from homeowners is not effectively used – much of it grounded out. To date here, it is still more of a marketing scheme. Until we know what part of renewable energy is actually used via a grid system, we should be cautious about its effectiveness.

    Renewables have other considerations as well. Ted Trainer’s book Renewable Energy Cannot Sustain a Consumer Society goes into detail as to why.

    It seems all the rating systems are arbitrary e.g. Energy Star, LEED, etc. One might asks the reasoning behind a number for LEED Gold vs. Silver. It’s not clear to me why any of these systems, included Passive House, have to explain the process in detail, which may not even be possible.

    The debate about the language of Passive House seems to be limited to a sentence or two and maybe Martin could suggest a new sentence. e.g. “Passive houses heating load is designed to use 80-90 % less energy and the small amount can be obtained from small backup systems like tiny woodstoves, mini splits, electric base boards, etc.” I am not sure this is a good sentence but it would be nice to devise an alternative rather than continue the argument.

    As far as the source of the term here is a quote from William Shurcliff to whom Feist gives lots of credit for his work on superinsulated houses.

    “Consider the Saskatchewan Energy Conserving Demonstration House. Or consider the Leger House in Pepperell, Mass. They fit none of the … listed categories [of solar houses]. The essence of the new category is:“
    1. Truly superb insulation. Not just thick, but clever and thorough. Excellent insulation is provided even at the most difficult places: sills, headers, foundation walls, windows, electric outlet boxes, etc.
    “2. Envelope of house is practically airtight. Even on the windiest days the rate of air change is very low.
    “3. No provision of extra-large thermal mass. (Down with Trombe walls! Down with water-filled drums and thick concrete floors!)
    “4. No provision of extra-large south windows. Use normal number and size of south windows — say 100 square feet.
    “5. No conventional furnace. Merely steal a little heat, when and if needed, from the domestic hot water system. Or use a minuscule amount of electrical heating.1
    “6. No conventional distribution system for such auxiliary heat. Inject the heat at one spot and let it diffuse throughout the house.
    “7. No weird shape of house, no weird architecture.
    “8. No big added expense. The costs of the extra insulation and extra care in construction are largely offset by the savings realized from not having huge areas of expensive Thermopane [windows], not having huge well-sealed insulating shutters for huge south windows, and not having a furnace or a big heat distribution system.
    “9. The passive solar heating is very modest — almost incidental.
    “10. Room humidity remains near 50 percent all winter. No need for humidifiers.
    “11. In summer the house stays cool automatically. There is no tendency for the south side to become too hot — because the south window area is small and the windows are shaded by eaves.
    “What name should be given to this new system? Superinsulated passive? Super-save passive? Mini-need passive? Micro-load passive? I lean toward ‘micro-load passive.’ Whatever it is called, it has (I predict) a big future.”

  62. GBA Editor
    Martin Holladay | | #62

    Response to Pat Murphy
    The William Shurcliff quote is a favorite of mine that I have highlighted in several blogs and presentations. The term "passive solar" was originally used to distinguish between passive solar solutions and active solar equipment -- the idea being that the passive solar components of a house (basically windows and thermal mass) did not require pumps or blowers like active solar systems.

    I certainly disagree with your implication that the electricity produced by grid-tied PV systems is mysteriously "grounded out" rather than used. That's simply untrue, and you'll have to come up with a more convincing and authoritative source for the information before I'll believe it.

    Concerning your contention that the distinction between LEED Gold and LEED Silver is arbitrary: I couldn't agree more. You're right. That's why metrics based on energy consumption make more sense than most green standards. The Passivhaus standard is absolutely right to focus on energy consumption; what we are debating here is whether the Passivhaus targets make sense for cold climates in North America.

  63. GBA Editor
    Martin Holladay | | #63

    Response to Daniel Ernst
    Yesterday, you posted a comment asking where you could find the full text of the paper written by Gary Proskiw and Anil Parekh, "Optimization of Net-Zero-Energy Houses." Gary has kindly shared the paper with me by e-mail and given permission to publish it here at GBA. I've provided a link at the same GBA page where I posted the PowerPoint presentation, so check it out:
    Optimization of Net-Zero-Energy Houses

  64. Bruce Wilson | | #64

    Are Passivhaus Requirements Logical or Arbitrary?
    Unfortunately logic has very little to do with how most houses are built.
    I have worked for the last seven years educating about green building through the Lehigh Valley Green Builders ( ), an organization my wife and I co founded.
    I worked for a builder of super insulated houses in the 80's that used rigid foam outside the framing to prevent thermal bridging. He limited air infiltration, had R 30 walls and R 50-60 ceilings and triple glazed windows. At least R 10 under the floor slab helped temper the basement temperature. He sometimes used heat pump water heaters. Some of the houses were so efficient they were heated by a hot water heater connected to a thermostat, pump, controller and heat coil in the same ductwork that circulated fresh air from the ERV. While this did not eliminate the need for heating equipment it saved a lot of money compared to a boiler or furnace. And let’s face it Martin, if the heat loss is very low, there is no problem heating through the ventilation air because very little heat is needed.
    He got the concept of integrated design before it was coined.
    Basically, if you triple your insulation budget and tighten your envelope you will ad about 10% to the cost of a house. If you also incorporate ample but not excessive south facing glass that is shaded in summer you will reduce the heating and cooling load by 90% or more. If you right size the heating and cooling equipment you can afford high efficiency.
    If you use a standard 30 year mortgage your savings in energy use will be about double the extra payment on your mortgage.
    Of course you will have to have an ERV to introduce fresh air, but that only costs about 50-60$ a year to run. The replacement filter will probably cost more than the energy to run the ventilator.
    Unfortunately most designers use "value engineering" to reduce construction costs with the emphasis on initial cost reduction rather than using long term cost benefit analysis.
    If bankers understood the amazing return on investment of super insulation, they would be requiring energy modeling on new homes and insist on loaning more to achieve better performance thus making their clients a better credit risk by removing most of the burden of energy costs.
    I find that most designers and builders are not aware of what is possible, and would rather keep building what they are used to building.
    We must keep pushing the industry to produce better houses.
    I prefer the term high performance building over super insulated or passive house.

  65. user-634791 | | #65

    Response to Martin
    We can take this offline if this gets boring for others....You didn't really address my point about PV vs insulation.......but you are right I should know about high tech, I do use it in my house, but the house is based on the simplicity of super insulation and passive solar gain but I wish I had actually used even more insulation than I did, even though at the time it seemed like a HUGE amount. Things change, and we learn. The fact of the matter is my house only uses between 6000kWh and 7000 kWh before any renewable credit, and as I said, the icing on the cake should be the ability to offset that, and I was fortunate enough to be able to afford the wind turbine. The only other high tech component (if you want to call it that ) is the GSHP, and had they been more available I probably would have used an air to air mini split, because you can if you have very low loads afforded by having lots of insulation.

  66. GBA Editor
    Martin Holladay | | #66

    Response to Bruce Wilson
    We're on the same page. I've been advocating for all of these elements -- high-performance windows, good air sealing measures, beyond-code insulation -- for years, and I agree completely that these measures drastically reduce the need for space heat.

    It's a simple calculation to determine whether you can introduce enough space heat into the low air flows (40 to 50 cfm) required for ventilation to satisfy all of the heating needs of a house at the design temperature. It's possible in mild areas of the U.S., but it can't be done in colder regions.

  67. GBA Editor
    Martin Holladay | | #67

    I agree with you, David
    As you probably know, I agree with your approach. You designed a net-zero-energy house, so you know what I'm talking about. There's a trade-off between energy-efficiency measures and renewable energy, and everyone who is designing a net-zero-energy houses has to know where to draw the line.

    You have a house that uses very little energy, and you STILL needed a $40,500 wind turbine. These houses are expensive, so we need to consider economics. I'm suggesting that it makes no sense to choose the more expensive option when less expensive options are available, and I don't doubt you agree. If anyone wanted to choose an energy-efficiency measure that would cost MORE than your $40,500 wind turbine, it would obviously add to the total cost of the project.

    And, as you and I have discussed many times, PV is much less of a hassle than a wind turbine. My oldest PV module is 31 years old and counting...

  68. Mike Eliason | | #68

    issues comparing net zero PV to PH
    The biggest issue I see comparing the cost of Net Zero PV to Passivhaus, is that Net Zero builders aren’t dealing in terms of source energy but in terms of zero’d out site energy over the course of the year. Of course this is going to appear more cost-effective than Passivhaus – you’re dealing in two significantly different numbers between Site and Source energy. To really compare ‘apples to apples’ between the cost of PV and Passivhaus, you would have to price out enough array to be Source Net Zero (which is a lot more than Site Net Zero) AND carbon neutral (which is again significantly more than just Net Zero).

    Grid-tied PV (or wind turbine) isn’t carbon neutral, and still leads to much more CO2 emissions than the conservation approach of Passivhaus. Yes, the PV (or wind turbine) leads to less CO2 emissions than a power line, but is that enough? My answer is no.
    The beauty of Passivhaus isn’t just energy reduction, but significant carbon reductions – and Martin even acknowledged this in his presentation, “It sets an energy goal that is in the ballpark of what will be necessary to achieve required carbon reductions.”
    It is these carbon reductions in Passivhaus that cause electricity to be dinged so much in the specific primary energy demand over biomass, propane, wood pellet, etc.

    Passivhaus appears to be a better (and dare I say, more cost effective?) approach for energy reduction AND significantly reduced CO2 emissions – even with a huge chunk of foam insulation (which may or may not be ‘huge’, depending on approach/designer). Alternatively, some genius entrepreneur could develop a schaumglas granulat factory here in Washington, and we wouldn’t need to source foam at all – although the sub-slab foam we do source can contain a high recycled content, meaning that much less embodied energy in the Passivhaus approach.

  69. GBA Editor
    Martin Holladay | | #69

    Response to Mike Eliason
    I'm saying that even when the heavens part, and the angels sing, and the last coal-fired plant is decommissioned, and all of our electricity is produced by renewable energy (including wind turbines), that electricity will not cost more than the current cost of PV-generated electricity.

    Now, Passivhaus homeowners may be an exceptionally selfless group, willing to invest more in foam than can be justified in energy savings -- even when calculating the cost of energy at the high cost of PV -- because of the nagging thought that there are coal-powered plants out there and they should do their bit to whittle down their energy use, even if it means investing in insulation with a return that calculates the cost of the energy saved at $1 a kWh.

    There are a few homeowners out there like that, and God bless them. But they are building very expensive houses. And I think that the moral contribution of homeowners who build a net-zero-energy house (using less insulation than a Passivhaus) is adequate to allow them to set their head gently on their pillows at night without guilt.

  70. GBA Editor
    Martin Holladay | | #70

    One more point
    Here's one more point: I don't think that the clients of Passivhaus consultants are even being given the choice of which path to take. Do Passivhaus consultants really tell their clients, "It will take $8,000 more in insulation to bring this envelope up to the Passivhaus standard. Of course, we could install $8,000 of PV modules on your south-facing roof, and for the same investment, your energy bills would be even lower than if you built a Passivhaus. Your choice."

    Has any Passivhaus consultant presented the choices to their clients that way?

  71. user-634791 | | #71

    The angels will not be singing soon enough
    It may just be me Martin, partially because I don't like math, but I still don't understand your argument for PV vs insulation. For all the reasons I stated and Mike stated much more clearly. These are two different strategies doing similar things but NOT interchangeable. I wish the angels were singing but we are going to live in a fossil fuel world for a while and if the point of building efficient housing is to protect the planet and reduce emissions, we need to eliminate the load. You can certainly offset it with PV but you still end up with CO2 emissions. Am I missing something because this seems pretty obvious.

  72. user-634791 | | #72

    RE one more point
    We are doing a Passivhaus here in VT, and yes is the answer to your question.......and the client chose to invest in the insulation, because for the same price they make a better choice for the environment.. They realized that they can always buy PV, but will not ever jackhammer the slab and add more insulation.

  73. GBA Editor
    Martin Holladay | | #73

    Designing a net-zero-energy house
    You designed a net-zero-energy house that required a $40,500 wind turbine to balance out. Let's be arbitrary, and say the house cost $260,000 -- I'm choosing a number for the purposes of illustration. So the package cost about $300,000.

    Now, if you choose insulation that costs more than renewable energy, your house can be built for a higher price -- $310,000, say -- but the energy bills will be the same. It's an interesting idea, but not many Americans can afford to bite.

  74. Kopper37 | | #74

    Thanks Martin
    Martin - Thanks for following up and posting the optimization paper! Maybe I'm wrong, but it appears only part of the paper is attached? At least---I didn't see any of their PV Value Index figures.

    One point I'd like to raise out of this VERY lively and informative discussion: I don't see a lot of disagreement between ZEH and Passivhaus when it comes to wall and roof / attic insulation values. The disconnect appears at the foundation level, where the building couples to the earth.

    Is there a significant difference between PHPP model assumptions (ground temperatures / delta T), the assumptions in HOT2000, and the temperature values others are pulling from measured data (i.e. John Straube)?

    If not, where is the disconnect?

    A quote from the paper authored by Gary Proskiw and Anil Parekh:

    "Basement Floor Slab - Surprisingly, the benefits of insulating the basement floor slab were not as significant as had been anticipated. In milder climates, including Maritime and Eastern locations, the recommended treatment was to leave the slab uninsulated whereas in colder climates, such as on the Prairies or in the North, a perimeter skirt of RSI 1.76 (R-10) insulation was recommended."

  75. GBA Editor
    Martin Holladay | | #75

    Response to Daniel Ernst
    The question you just raised was much debated in the comments posted on my earlier blog, Can Foam Insulation Be Too Thick? John Straube is a serious building scientist who combed the research literature for actual measurements of sub-slab soil temperatures. He based his calculations on that data, and I trust his numbers. No one has presented a rebuttal to his calculations, as far as I know, although many Passivhaus consultants were surprised to read them.

  76. Mike Eliason | | #76

    I think clients are


    I think clients are pushing consultants to meet Passivhaus, the choice exists and is not the consultant’s to make (unless they are the client). Not all roofs have that much south facing area! Some designers like to ensure neighboring buildings have decent solar access!

    Are you talking about $8,000 worth of insulation beyond the Net Zero approach? I don’t know any homeowners putting $8,000 worth of insulation into their slab. Even with the high cost of oil right now, that would be approximately 1000sf x 20” for type IX EPS. In the example where we migrated a house to Minneapolis, that would be over 5.5x the amount of insulation we needed to achieve Passivhaus. I would rather take $1,500, put it in the ground, and the balance in better glass to ensure reduced energy bills, comfort and significantly reduced CO2 that comes from achieving Passivhaus.

    In that example, we also calc’d that the additional cost of insulation (about $1,500) would be less than the 0.6kW PV, as well as saving over 6 tons of CO2.

    I think the biggest problem is still perceived cost based on some less than stellar examples. And in any case, this argument is only valid for detached single family houses (talk about a penalty!!). Commercial, institutional/ multifamily all require less insulation/investment to achieve the standard. In some cases, it is actually less to achieve PH (in Europe) and even here in North America it can be achieved for almost same cost as code minimum. John Semmelhack’s 4700sf Waldorf school in Charlottesville (R-18 slab, R-26 wall, R-57 roof) required only $4,000 additional investment total to hit the standard. The increased amount of insulation for the entire building was under $8,000.


    In our modeling, the envelopes for ZEH and PH can be very similar with the exception of slab. PHPP shows significantly more heat loss through ground.

    The PHPP climates I’ve been comparing show that Europe has a significant disadvantage to North America - mainly soil temperatures don’t appear to be warming up to the degree here in North America. My guess is this is due to significantly decreased insolation in Europe. As a result, the ground doesn’t warm up as it does in North America, meaning it cools off faster and stays cooler longer. Over the course of a year, European buildings have more heat loss through the slab than North America. I think this is the reason slab assemblies in Europe tend to be thicker. And one of the reasons I think slabs can be fairly normalized here for the most part, if proper care is taken.

    Also, I take umbrage w/ Straube's assertion in the Thick Foam posting that Passivhaus designers only have a few ‘dials’ to manipulate – there are several, you just have to know how to manipulate them. And, obviously, I’m still far from sold on Straube’s argument.

  77. GBA Editor
    Martin Holladay | | #77

    Response to Mike
    Good points. I agree that my example of $8,000 was a number I pulled out of thin air to make a point, so your response is appropriate. If the additional cost of thick insulation is $1,000 or less, many homeowners might choose it, even if it costs more than PV. (But it still might be fun to have $1,000 of PV on the roof ... just to watch the ammeter on a sunny day...)

  78. Kopper37 | | #78

    Soil Temperatures
    Mike Eliason - Thanks for the response on soil temperatures.

    Actually, Marc Rosenbaum made the statement on 'dials' in PHPP. Later in the blog comments:

    "As Martin quotes Straube quoting me, there are only so many "dials" one can adjust to meet the PH heating criterion of 15 kWh/m2/year."

    Regarding the North American climate, where does the PHPP derive its soil temperature values? I'm guessing these are calculated values based on location HDD and insolation?

    Surely, a lot of this debate would dry up and blow away if we had more data. Do you know of any PH certified houses in North American that have sub-slab temperature data loggers?

    In the long run, computer models need to be refined with real world data. I would like to think PHI will make that a primary goal as their reach expands in the coming years.

    Note: edited for spelling

  79. GBA Editor
    Martin Holladay | | #79

    The origin of the "only so many dials" quote
    The place I first heard Marc Rosenbaum say that there are only so many dials you can turn when refining a Passivhaus was in Joe Lstiburek's backyard, during an August 2009 conversation between Marc Rosenbaum, John Straube, and myself.

  80. as9bRQru9C | | #80

    Let's not ignore the manufacturing impact

    Thanks for looking at this with an analytical eye. I'm a CHPC and have designed Passive Houses and will continue to do so. I'm a proponent of the standard and think it (and PHPP as a design tool) is a fabulous path for meeting the demands of the changing climate. However, I think it's unwise to dogmatically assert that Passive House is always the answer and more insulation is always better, which is the attitude I think is coming across in many of the dissenting comments.

    I was at David White and Alex Wilson's presentation at the NESEA conference where David debuted his calculator for determining total GWP of various insulation materials (inclusive of operational carbon savings and manufacturing embodied energy/blowing agent GWP). Using his calculator, I find that, in the Boston climate (6,000 HDD, 0.6 ground reduction factor, 50 year lifespan, heating with an ASHP [COP=2.8]) somewhere around R-40-45 is the inflection point for Type IX EPS foam under a slab. In other words, beyond this level of insulation the GWP increase due to manufacturing exceeds the GWP reduction due to reduced heat loss. If you use XPS insulation, the inflection point is around R-20-25. To be clear, this is under a particular set of assumptions - different climates will be different, extending the life of the house increases this level. (However, using any set of assumptions, there has to be an inflection point somewhere.)

    You can argue that this is a temporary problem and blowing agents will get better in the future or non-foam products for sub-slab insulation will be the norm. However, that doesn't eliminate their contribution to climate change today or change the fact that the vast majority of Passive Houses in this country are built with foam under the slab today.

    I think it’s to everyone’s benefit if we continuously challenge our assumptions to make sure we’re doing the best thing for the right reasons. To that end, keep up the good work.

    To give David full credit: you can find his calculator on his website:

  81. GBA Editor
    Martin Holladay | | #81

    Response to Jordan Goldman
    I was also in the room at the NESEA conference when David White and Alex Wilson unveiled their new calculator. Thanks for sharing your calculations -- they point to another important reason why more sub-slab foam is not always better.

  82. user-971945 | | #82

    Whole Systems Design Please
    I think that you fail to mention is that the studies that you refers to - where PV is "cheaper" than insulation - rely upon the idea that you can use heat pumps as a means of fiddling the accounts. Once your annual PV divide the cost of heat from PV by 3-4 (theoretical rather than measured HP) then you starts to get "cheaper" energy from PV... As has been pointed out befire this naturally leads to the observation that PV does not provide the energy required when the heat is needed. In essence you have fallen into a similar trap to the one that you acuse PHI of having fallen into.

    Anyway, all this talk of expensive insulation is, in my view, a red herring about optimising the energy efficiency around the "most profitable" U-values. The most profitable U-value in the UK is, over the lifecycle, about 0.2 W/m2K - so putting climate to one side why bother with PH? ....If you look at the elemental cost per kWh saved then you find that we are generally dealing in 3rd and 4th decimal places. U-values of < 0.1 W/m2K are actually affordable (at only about 0.5 p/kWh more expensive.) Okay foam insulation is two times more expensive but that only means that it costs 2 p/kWh saved at 0.09 W/m2K.

    ..... Of course these cost are for insulation alone and exclude any additional labour and materials needed to achieve this kind of U-value (materials such as fixings and structure). Even these additional costs can be offset by:

    i) assuming a compact form - as shown at last years International PassivHaus conference,
    ii) by some smart design that minimises labour,
    iii) by reducing material use and waste etc. etc.

    In fact I have begun to think that PHI started with ETHICS (external ins) because labour costs do not increase with EPS - you just get a slab of insulation as thick as you need. All other things remain pretty equal (longer ties do add some material cost.) Blown fibre insulation is a similar story (though forming the cavity does have some costs though as more fixings and material and labour is required compared to EPS.) So whilst I recognise that we may - for now - run into some technical limitations for wall/roof/floor construction I find myself arguing that they are "just challenges" that need to be overcome. In a nutshell cost limitations may have more more to do with the current construction technology that is being employed rather than the insulation itself. ....Even on a PassvHaus project that I'm working on, with its 450mm I beams factored into the cost /kWh saved it ends up with 5-6p/kWh saved. (Yes it is more than gas 4p/kWh but it is still a damn bit cheaper than the real cost of PV, plus it is is paying back the entire price of the structure.)

    Food for thought?


  83. gusfhb | | #83

    The concept that keeps
    The concept that keeps rattling through my head when pondering underslab foam is that no matter how much foam you put there, it never generates any heat.

    Let me try to explain.

    If you have a delta T of 2 degrees, a spreadsheet will tell you there is a heat loss there. If you put an inch of foam, you will still see a heatloss, 2 inches, 3, 5, 10, still the spreadsheet tells you there is a heatloss.

    There is no real world feedback that would tell you that there is 'too much' insulation. The insulation never overheats the building, showing the designer that 'whoa better dial back the sub slab' the way, say, too much south facing glass would.

    It probably isn't possible to 'overinsulate' a house. It would just shorten the amount of days that people keep the windows closed.

    I think many here feel [and have numbers to support] PH overstates sub slab losses.

    If you were to pretend that with an extremely well insulated perimeter that there was effectively no heatloss through the slab, then it would be virtually impossible to tell the difference between 2 inches and 12 inches of insulation, because there is no difference

    If to meet a standard, you are putting more insulation under the slab than in the wall, it would seem the standard is 'sub optimal'

  84. GBA Editor
    Martin Holladay | | #84

    Response to Mark Siddall
    Concerning heat pumps and whether or not a PV array produces electricity at the same time that it is needed: I'm happy to stipulate that most space heating is needed during the winter, when the output of a PV array is at its lowest. Clearly, a PV array cannot provide space heat for an off-grid home. But almost all people in the U.S. live in homes that are grid-connected.

    Moreover, the majority of U.S. homes are located in areas where local utilities offer net-metering contracts. That means that they can size a PV array to meet their annual electrical needs, and be credited for excess electricity production during the summer. Whether or not you agree with the political decisions that led to net-metering contracts, net metering is now a fact, and many Americans are taking advantage of the opportunity to sign net-metering contracts.

    Moreover, ductless minisplit heat pumps are a very common method used to heat Passivhaus buildings in the U.S. These Passivhaus buildings have heat pumps anyway, so the equipment does not represent an extra expense.

    For those of us who remember the superinsulated construction details of the 1980s, the current discussion about the best ways to build a high-R wall are nothing new. People have been debating Larsen trusses versus double-stud walls versus thick exterior foam for at least 25 years. These discussions are all fine and good, but it's only the newcomers who assume that some magic breakthrough is about to occur to make thick walls cheaper to build. We've been doing this a long time, and the construction costs are fairly well known.

  85. GBA Editor
    Martin Holladay | | #85

    Response to Keith Gustafson
    Good energy modeling is essential for choosing the thickness of sub-slab foam. In hot climates -- for example, in Florida -- the optimized thickness of sub-slab foam is zero inches, because a ground-coupled slab helps reduce cooling costs during the summer.

    As I'm sure you know, as long as there is a Delta-T across the slab, there will never be a situation in which there is "no heat loss through the slab." Insulation doesn't stop heat flow; all it does is slow it down.

  86. gusfhb | | #86

    My exaggeration of


    My exaggeration of zero was simply attempting to illustrate my point.

    Is not one of your points that the energy modeling involved in sub slab losses is in some ways flawed?
    I am simply trying to say that when PH advocates claim that the results prove that the system is sound that they have no way of proving that in the field[other than to build an infinite number of potential PHs with varying levels of sub slab insulation and measuring the energy loss]

  87. GBA Editor
    Martin Holladay | | #87

    Response to Keith Gustafson
    For the time being, unless someone presents clear evidence that I'm mistaken, I'm operating on the assumption that PHPP is accurate.

    I think that most PH consultants know that when they go from 10 inches of foam to 12 inches of foam, they aren't saving a significant amount of energy. But because they have to hit the 15 kWh/m2∙yr, no matter what the cost, they keep adding foam until they hit the number -- even if the last few inches of foam save only a small amount of energy.

  88. Ken Levenson | | #88

    Cheney, energy mix, future elec cost and the PH target
    The Cheney comparison goes only so far as his predisposition toward energy production - just substitute PV for oil/coal.
    The cost of electricity is likely only to rise, rise, rise.....current PV cost is no ceiling. We are going to see major disruptions and transformations - so actually the only stable electricity cost will be that from the PV panel on your roof....the grid supplied cost is going up and away. ....making the cost of insulation a very prudent upfront investment.
    Regarding future energy mix - I look to Joe Romm for guidance .....but I do think he is underestimating the potential of building energy efficiency.
    Regarding PH targets, my experience has been that what the ultimate target is varies a great deal from client to client: all using the PHPP and PH components, some really want to hit certification (for sales purposes), some just want a high performance building and aren't interested in certification, and of those if it looks like we need to do something "extreme" to hit 4.75 they are perfectly happy with 5 or 5.5 etc.... this goes hand-in-hand with air sealing too - we are trying to make the projects as tight as possible but if we are under 1 and how to get it tighter seems a mystery we'll stop at 1. 4.75 and better and .6 and better are our guideposts.....and for safety factor we'd like to hit them....we should have a few projects online soon - will be interesting to see just how they perform!

  89. user-971945 | | #89

    Compact buildings, surface area to volume ratios and heat pumps
    I can not assume that I know about the construction industry in the USA and I was not suggesting that there is a miracle cure for construction technology, nor that the end goal of zero marginal cost will actually be achieved using a given construction technology alone (whilst it is a worthy persuit.) That said holistic design has been shown in to result in PassivHaus buildings that do not cost more than the average building (at least this is the case in Europe). A compact building form, whilst not the sole preserve of PassivHaus, has, in the context of PassivHaus, been shown to result in no additional marginal cost compared to the average building. This, I would argue, is an important factor that is deserving of attention. (Interstingly Amory Lovins has also drawn attention to such considerations.) A complimentary consideration is that the design, and the eventual cost, of a building is subject to certain boundaries of standard deviation - the same moneys can be spent in various ways. It is down to the designer to resolve how to best satisfy the range of requirements. In esence, if compared to the range of standard market rates and no marginal cost is incurred, then PassivHaus certainly make a good investment. If additional costs are borne by the end user/owner then this is of course their decision (if they are not happy with the cost then some form of value engineering will be required). Fianlly, there is at least one study (a paper in Energy in Buildings as I recall) that shows that Kartin Klingenbergs PassivHaus, if PV were added, had no increased lifecyle cost compared to one net zero energy home. The project was in this case also less costly over the life cyle than a range of other (built) net zero homes. Whilst this is just one study placing one PassivHaus in context it does begin to suggest that such PassivHaus projects (certainly in that climate) can be a feasible and realistic proposition. ...This is also one of the projects that tends to fall within the sights of your floor slab analysis ;-)

    Stepping slightly aside of the PH debate: My primary intetest is not so much PassivHaus but the delivery of buildings that perform "as designed." The quality assurance requirements are therefore quite onerous. (For me the beauty of PassivHaus is that the design tools have been varified time and again.) In the context of building performance, in the USA, is there research into the delivered performance of heat pumps?.... In the UK they have proven to be rather disappointing (the Energy Savings Trust . and Leeds Metropolitan University have had similar findings - EST had more industry "support" if you know what I mean In te UK our experience is that in reality annual fuel bills rise, carbon emissions increase and that little value is experienced from using heat pumps (theory and lab condcitions do not translate to reality). If this is not the case in the USA I would be intersted to know how the use of the technolgy has been perfected (a new article perhaps...).


  90. Paul Eldrenkamp | | #90

    adapting the PH standard to US needs?
    Fascinating, important discussion. What I don't see here, though, is much talk about possible alternatives to the Passive House standard. The fact is, the PH standard has generated so much attention because it's aggressive, it's quantifiable, it's focused, and there's few or no opportunities for backsliding or gaming the system. It's a "green" standard that stands for something and that represents fundamental change in our industry. That's why I and so many others find it so compelling.

    But it's true the US includes 8 climate zones, and all of Germany (home of the PH standard) would fit into 1 or 2 of those 8. One-size-fits-all might work OK in Germany, but it does pose extra, probably unnecessary, challenges in the US.

    I've often thought that it might be useful to have a version of the Passive House standard that takes into account local climate -- and also takes into account occupant loads. It turns out that such a standard has been developed, after a fashion, and it's part of the Thousand Home Challenge; go to and download the "Threshold Calculator."

    Though developed for retrofits, I think the Threshold Calculator is a really interesting tool for setting an energy budget for any residential structure. Like PHPP and the PH standard, this tool has its issues. It addresses some of the criticisms directed at the PH standard, but probably opens itself to other criticisms. (For several homes I've run through it, the energy budget ends up being less than what would have been allowed by the PH standard.)

    I think the US needs to learn all it can from the European Passive House experience, but ultimately it probably needs to develop its own version of the standard, and to do so in a way that inspires respect rather than derision from European practitioners. Maybe some blend of PHPP and Thousand Home Challenge Threshold Calculator could get us there?

  91. GBA Editor
    Martin Holladay | | #91

    Air-source heat pumps
    Mark Siddall,
    Concerning ductless minisplit air-source heat pumps from Japan and Korea: If you search the GBA Web site for "ductless minisplit," you will find many articles that discuss this method of heating.

    The latest cold-climate minisplits from Asia are nothing like the air-source heat pumps of yore. Here in the U.S., our experience with conventional air-source heat pumps has been similar to your experience in Britain. They are not suitable for cold-climate use, and their performance has been disappointing.

    The new units from Fujitsu and Mitsubishi, however, are entirely different. Several researchers have measured their performance and been startled to learn that their output exceeds the factory specs while their energy use is lower than advertised. Marc Rosenbaum has been surprised at how little electricity they use in Massachusetts, and these units are performing well in Vermont and New Hampshire, where winters are much colder than they are in Britain. So it may be time to discard our prejudices against air-source heat pumps.

  92. gusfhb | | #92

    Re Martin
    Sorry Martin, I thought in your responses to your previous blog regarding thick sub slab insulation you had called into question the assumptions of the PH software regarding sub slab temperatures

  93. user-971945 | | #93

    Thanks. I will look into the minisplits though I'm somewhat risk averse when it comes to heating systems. (The mantra of "In God we trust, all others bring data.") Measured data assessing the performance of the seasonal COP is what is required - for the minisplit and the whole minisplit system (i.e. overall household performance). I look forward to an in dpeth nerdy review of some monitored results :-)

    Also, if I understand it correctly these ductless systems mean that you have a heat pump in each room. How noisy are they? Every heat pump that I've ever heard is very off putting.


  94. user-659915 | | #94

    Heat pumps in the UK
    “Also, if I understand it correctly these ductless systems mean that you have a heat pump in each room. How noisy are they?”

    A minisplit, as its name suggests, is in two parts. The compressor unit is much the noisier part and that’s outside the home. Inside is the heat exchanger/air handler. Do you need an air handler in each room? Depends on the house. If the home is new construction, super-insulated and relatively open plan a single inside unit per floor would be sufficient. But while this component of the system is far quieter than the window air conditioners that many Americans have tolerated for years and is comparable to an average ducted air heating system as found in many American homes it is not as silent as the hydronic radiator systems that British users are most familiar with.

    But here may be a bigger problem: air, with its low specific heat, is not the ideal medium for heating high thermal mass structures (for US readers: many British homes have both external and internal walls of solid masonry in addition to uninsulated slab floors). Hydronic radiator systems are standard there for good reason: heat the occupants, not the walls. Despite the favorable climate conditions, minisplit heat pumps may remain marginal in British markets until high-efficiency air-to-water systems are available.

  95. GBA Editor
    Martin Holladay | | #95

    Response to James Morgan
    Right after World War 2, American visitors to Britain were surprised to discover that most British homes did not have central heating. Rooms were cold, and the British kept warm with sweaters and tea.

    I really don't think this is still true. I imagine that if I took a thermometer into the average British home in January, the indoor temperature would be similar to that of the average American home.

    So I don't think your statement makes any sense: "Hydronic radiator systems are standard there for good reason: heat the occupants, not the walls." British heating systems bring the interior conditions to the setpoint on the thermostat, and there's no reason that you can't do that with a ductless minisplit. It's not as if your interior air temperatures are cold, and the British occupants are warming their hands at the nearest radiator.

  96. user-626934 | | #96

    Response to Keith Gustafson
    Keith, you wrote, "If to meet a standard, you are putting more insulation under the slab than in the wall, it would seem the standard is 'sub optimal'"

    In my opinion, if someone is putting more insulation under the slab than in the wall, their design is "sub-optimal", not necessarily the standard they are trying to meet.

  97. user-659915 | | #97

    Response to Martin
    I think you miss the point, Martin: air temperature is secondary if you have a full complement of relatively chilly masonry walls and concrete floor sucking the warmth out of you, and using air as a thermal transfer medium is simply not the best way to bring these heavy construction elements back to room temperature if you let them get cold. The old saying was, build a stone house and give it to your worst enemy rent-free for a year. If you live in a really high thermal mass building with inadequate insulation, as I have done and as many Brits do, you'll take all the high-temperature radiant you can get. And bear in mind that the North American expedient of just cranking up the wood stove to supplement a less than stellar background heating system is simply not an option in most UK homes: no cheap firewood, high population density, major air pollution issues.

    I just mention this traditional UK preference for high-temp radiant systems as one of the factors which is likely to be limiting air-to-air heat-pump take-up there: this is not just a cultural thing, it's a realistic response to the damp climate and the nature of the common construction materials there. There are plenty of other potential factors at play including the absence of a need for A/C and a huge installed base of natural gas-fired hydronic radiator sytems, most of which are relatively new. On the plus side is the mild climate which seldom pushes the range limits of heat pump efficacy, which is why I suggested that a high-efficiency air-to-water system might be part of the answer - something which could plug right in to the hydronic distribution systems already in place. Of course that just trades diminishing reserves of natural gas for equally problematic electrical inputs, but that's another discussion.

  98. user-659915 | | #98

    And by the way,
    on my last visit to the UK just a few years ago, Brits were definitely still wearing sweaters, drinking tea, and backing up to the radiators - not just to warm their hands but their butts as well. And that was in June.

  99. KyleA | | #99

    Great Article
    A well though out article with some good perspectives, thanks for writing!

  100. GBA Editor
    Martin Holladay | | #100

    More on sweaters, tea, and stone houses
    Your arguments about the need for hydronic systems instead of ductless minisplit systems in Britain contains a mix of irrelevant issues and red herrings.

    You're all in favor of hydonic systems in Britain. So are the British using these systems or not? Assuming they bother to turn them on, I'm going to assume that these heating systems bring the indoor air temperature to the thermostat setpoint. Otherwise the systems are undersized or poorly designed.

    A ductless minisplit system can do the same thing -- bring the indoor air to the thermostat setpoint.

    Your counter-argument: "Air temperature is secondary if you have a full complement of relatively chilly masonry walls and concrete floor sucking the warmth out of you." This problem is real, but it applies to either heating system. When cold surfaces suck heat from occupants -- the problem also occurs with windows -- the response of the occupants is usually to raise the thermostat setting by a degree or two. Either a hydronic system or a ductless minisplit system will respond in the same way to a change in thermostat setting.

    You wrote, "using air as a thermal transfer medium is simply not the best way to bring these heavy construction elements back to room temperature if you let them get cold." That is an argument against nighttime setbacks, not an argument against ductless minisplits. Nighttime setbacks are overrated, and I agree that they often don't make sense.

    You wrote that hydronic systems are "a realistic response to the damp climate." But the dampness of the climate has nothing to do with it. We have damp climates in North America, too. During the winter, when outdoor temperatures are cold, any heating system will bring the relative humidity of the indoor air to comfortable levels.

  101. user-659915 | | #101

    Dear Martin, I'm not "all in
    Dear Martin, I'm not "all in favor" of hydronic systems in Britain, the Brits are. I'm not trying to sell them anything, just suggesting why that might be so. Your points e.g. about night-time setbacks etc. are perfectly correct, although whole-house thermostats and hence night-time setbacks are actually uncommon in Britain. Most central heating systems are controlled on a room by room basis by simple mechanical thermostats on the individual radiators themselves and occupants save energy by turning down the settings in temporarily unused rooms.

    But that's a digression. If you read the study cited in Mark Siddal's initial comment you'll note that the great majority of the installations monitored were connected to hydronic distribution systems. The study notes that air distribution systems would be advantageous in respect of needing a smaller temperature lift than a hydronic system but does not comment on why they were not generally deployed. I would respectfully suggest that this might be because:

    1) UK houses are predominantly cellular, not open-plan and would therefore require multiple air handlers or a ducted system

    2) Ducted air systems are problematic in the average UK home for simple space reasons: the standard is a small (less than 1200 s.f.) 2-story 3 bedroom townhome with 8' ceilings and no crawl space. Even traditional radiator systems were too bulky for these tiny homes: central heating did not take off in the UK until the development in the 1970's and 80's of microbore systems with compact pressed steel radiators, piping no larger than a Romex cable, and a compact on-demand gas heat source that could hang on the kitchen wall among the cabinets. The discovery of vast reserves of natural gas in the North Sea (now depleting) was of course the other key component of Britain's central heating revolution.

    3) The UK has a huge proportion of its housing stock predating even the modest insulation standards of the 1970's. The masonry construction makes anything but the attic difficult and expensive to upgrade. Therefore setbacks, whether room-by-room or whole-house, will continue to have function in saving energy and a quick rebound will continue to have value for the occupants.

    4) The low specific heat of air makes that quick rebound problematic in a high thermal mass condition. This last seems to be your major point of disagreement, Martin, and perhaps I am overemphasizing it.

    5) The Brits are not used to the sound of rushing air coming from their home heating system. Though new low velocity high volume ducted air systems in the US are pretty silent, ductless systems are still pretty noticeable. This may seem a small point to Americans who have known it their whole lives but I can testify that it takes some getting used to. Brits are conservative about their homes (who knew?) and a shift to air distribution will take some time.

    This is a far lengthier discussion than I ever anticipated of why I suggested that more-efficient air-to-water units, which not incidentally might reduce the payback period of a conversion from gas of upward of 30 years, might be the key to heat pump adoption in the UK. I didn't dream it would be such a contentious idea. Sheesh.

  102. GBA Editor
    Martin Holladay | | #102

    Another response to James Morgan
    I never argued in favor of ducted forced air heating systems. I was merely responding to your proposition that hydronic heat made more sense in stone houses than ductless minisplit systems. The reasons you cite in your latest post make more sense than the "cold walls, damp climate" theory.

  103. Tom_Bassett_Dilley | | #103

    cost effectiveness
    Martin, I deeply appreciate your work in elevating our understanding of PH in the US. As an architect, recently certified passive house consultant, and co-author (with CTI/OneWatt Construciton) of a recent "near-PH" in Oak Park (near Chicago), I grapple with these issues regularly when I talk to potential clients and students. I'm going to leave the economics to the rest of you (and learn from it, thank you!), but want to add a design perspective to the cost-effectiveness issue. Having tried about 8 designs for small prototype houses, PHPP led me to simple form (compact), with enough windows for solar gain, but not so much that the house would overheat. Paying attention to the energy information coming out of PHPP during design made me realize that simplicity pays energy dividends, and it's cheaper to build. Lots of high-performance windows and a complex envelope are more expensive than a saltbox with just enough windows. Contrast that approach to a recent net zero house in Chicago wrapped around a courtyard with tons of glass and a monster PV's a different world. Plus I like the timelessness of the traditional (pre-HVAC) forms being re-invented with PH. I'm not discounting your foam/PV argument, but want to point out that you can use PHPP to help you design a more efficient envelope, which, if insulated with cellulose as much as possible, helps hugely in cost control. Those who choose to design inefficient shapes will need more insulation to meet PH, possibly including more foam under the slab. I don't think there is any energy design software that has cost-effectiveness built in. But good design with an eye to economy is key.

  104. David_Gregory_CZ3_CA | | #104

    'Passive' in Hot Humid Climates?
    Apologies for not having done _all_ my homework on this topic before chiming in (103 comments on this post alone!) but thought that the discussion about set standards in the face of highly variable climates warrants fleshing out...certainly I could stand to learn a lot more...

    I recently saw a competition announced, "Challenge: Design a Passive House for New Orleans"...(note the spelling)...
    [ ]

    At first, I thought 'Surely we have reached the nadir of the passive house / passivhaus trajectory...good luck achieving _affordable_ comfort in NOLA without AC...) Then I see the Saft residence in Lafayette at $110/sf? I haven't yet found annual energy costs, but found references that it includes an 'efficient 1-ton air conditioner' plus 3.2kw PV. (Interesting side note: the GBA photos show windows flush with exterior, no shading, causing my heart to skip a beat...other photos show very deep angled shades. The peril of the rush to publish?).

    I'm curious about a couple things:

    1. How is comfort defined / quantified in PHPP / the standards? I don't see a comfort metric discussed very prominently, but isn't that the ultimate datum? I'm sure we can assume sweaters in Yukon winters and bare skin (mostly) in the tropics (especially near Bourbon Street...>..< least the lights are usually T5/CFL].

    Again, a lot has to do with your personal thermostat...

    Look forward to the continuing discussion...

  105. GBA Editor
    Martin Holladay | | #105

    Response to Tom Bassett-Dilley
    You wrote, "I don't think there is any energy design software that has cost-effectiveness built in."

    Actually, such software exists. The software, BEopt, was developed at the National Renewable Energy Laboratory in Colorado. I have written about BEopt many times in previous blogs.

    To learn more about BEopt, which for the time being is only available to researchers and has not yet been released to the public, check out the following links:
    Building Energy Optimization Software

    BEopt Software for Building Energy Optimization

    Program Design Analysis using BEopt Building Energy Optimization Software

    BEopt: Software for Identifying Optimal Building Designs on the Path to Zero Net Energy

  106. GBA Editor
    Martin Holladay | | #106

    Response to David Gregory
    Thanks for your post; you raise many interesting questions. I invite GBA readers who are involved with the development of Passivhaus buildings in hot, humid climates to comment, since this topic is evolving rapidly.

    You are correct that the Passivhaus standard was a residential standard developed in central Europe, where residential air conditioning is extremely rare. For all intents and purposes, the standard was developed with the assumption that summertime cooling is unnecessary.

    As the use of the standard is now beginning to expand into climates (like that of New Orleans) where summertime cooling is the norm, it will be interesting to see how the developers of the standard respond. People have lived in hot, humid climates for thousands of years without air conditioning, so the question, "What level of comfort is assumed?" is central.

  107. Tom_Bassett_Dilley | | #107

    comfort: hot and cold
    David Gregory, comfort is a primary concern in PH. In a recent interview, Dr. Feist, one of the founders of the Passivhaus movement, puts it in a very human way, essentially saying the the objective is to provide a high quality of life, not just to count BTUs and Kwh. PH brings comfort and efficiency together. With a superinsulated shell, you have even temperatures throughout the house--no drafts, hot spots, cold spots. Window U-value is determined such that the interior temperature of the glass is not more than 4 degrees F above/below the interior room temperature; that was determined to be the comfort level (though Martin, in slide 60/66 calls this an "all-or-nothing posture", it isn't), because beyond 4F you feel it. There is also a calculated Frequency of Overheating (above 77F); if that frequency exceeds 10%, comfort is not achieved, and cooling must be supplied. Once cooling is supplied, you look at your Primary Energy consumption. If PE is too high, you'll need to look at shading, lowering the SHGC of the glass, changing glass size/location, etc. Ultimately, PHPP is the most humane approach I've seen, because even while it's an exacting and aggressive energy standard, it is fundamentally concerned with occupant comfort and long-term building life. Which brings me to another bone to pick with Martin: that slide 60/66 also criticizes the .6ACH50 airtightness as an "all-or-nothing" posture. But that standard was determined to be a safe level at which one would avoid condensation in the walls through air leakage--it's based on building science, and it's proven to be achievable. Not sure why you're picking on that, Martin. Go to the Passipedia and check out the slide on airtightness: with indoor air 68F/50%R.H., outdoor air 32F/80%R.H., a 1mm wide gap allows 360g--that's 12 oz.!--of water to condense in a day. The standard exists to provide reasonable "all-or-nothing" criteria for performance.

  108. GBA Editor
    Martin Holladay | | #108

    Second response to Tom Bassett-Dilley
    You wrote, "But that standard [0.6 ACH50 airtightness] was determined to be a safe level at which one would avoid condensation in the walls through air leakage--it's based on building science, and it's proven to be achievable."

    First of all, I have no disagreement that investing in building practices that result in very low levels of airtightness is cost-effective and desirable. Reaching 0.60 ach50 is achievable, and well worth the effort and investment in gasketing materials, tape, caulk, and labor necessary to achieve it.

    We now have lots of experience building homes with low levels of air leakage, and plenty of research results in the lab and in the field. We all know that for a wall to be durable in the long run, the drying rate must exceed the wetting rate. If, an an annual basis, drying potential exceeds wetting, walls are generally safe. We have many modeling programs to study these drying and wetting rates, including WUFI. These modeling programs are regularly validated by comparing modeling predictions to field observations and data from monitored houses.

    It's a gross oversimplification to say that 0.6 ach50 is required to avoid structural failure in walls. Plenty of wall assemblies have very high drying potentials, and can easily accommodate much higher air leakage rates. Certainly thousands of homes have been built with air leakage rates of 1 to 2 ach50 without structural failure.

    Moreover, some wall assemblies (such as ICF walls) are virtually immune from condensation damage. Such examples further undermine the broad-stroke oversimiplification inherent in the statement, "You must achieve 0.6 ach50 to avoid structural failure."

    We need to use our brains when we design our wall assemblies. Materials differ, air leakage rates differ, climates differ, rain exposures differ, and interior RH levels differ. It takes more thinking to design a wall than an oversimplified rule like "you need to achieve 0.60 ach50 or your wall is at risk of structural failure."

  109. David_Gregory_CZ3_CA | | #109

    Comfort: Hot and Humid (@Tom Bassett-Dilley)
    Thanks Tom B. for the response (M.Holladay too)...

    So PHPP says no more than 10% over 77deg? Again, at what relative humidity? Wind speed?

    I haven't signed up / purchased the software, so maybe that would give me answers, but I'm trying to figure out first more generally how it has been / might be applied in hot/humid climates first. In particular, I'm interested in the difficulty that there's a tipping point on the way from temperate to tropical where the ideal building form/design shifts from mostly open / high (shaded) surface area / 'natural' (or fan-assisted) ventilation, to sealed & compact / very well insulated walls / mechanical cooling/dehumidification. If my reasoning is right, it's very different from the more steady 'incremental' movement from openness (temperate weather) to tight enclosure as you move the other way toward 'polar'.

    My sense is that more generally we need economical ways to 'de-couple' functions within integrated (physically, or via network) components ('magic box[es]'), to have [am]bi-valent options. Separating components for ventilation from view/light is one approach, but most examples I've seen don't give you enough opening for hot/humid. The 'economizer' approach doesn't seem to be universally applied to HRV's/ERV's yet; and the debate about which is better - where and when - suggests ideally we (or a logic board) could decide. In Guangzhou, having the option to occasionally dehumidify without cooling is valuable, but when a dehumidifier costs almost as much as a mini-split...

    Example: I'm fortunate to have a mini-split [unfortunately not reversable] where the compressor / condenser coil is located on an enclosed porch, so by controlling which doors/windows are open, it can do cooling, dehumid without net cooling...and clothes drying. It's also of course a room fan. Why couldn't it be a whole-house exhaust fan (compressor bypass), evaporative cooler, and a water heater too? I know; because when it failed you'd be SOL...

    Will post a graphic of the various bi-valences I'm struggling with...if I ever figure it out...hopefully someone will provide a link to someone who has already done it first...or better yet, _built_ it...

    David/Guangzhou [lat23.13, Long113.26]

  110. user-975699 | | #110

    Numbers games

    Good to see you are still up for the challenge. To jump back to earlier threads, yes, PHPP is a numbers game like any modeling based standard. Ultra high levels of insulation, whether under-slab foam or anywhere else, is an attempt to hit a target. Humans are into competition and scoring points. Go figure.

    It just so happens that it is a lot simpler to replace fairly expensive washed rock with more foam, or add more cellulose to an attic, than it is to make walls increasingly thicker. In the very harsh climates, I agree, we need to fix the standard not go nuts trying to fit it to a condition it was not designed for. This paradigm is obviously quite stuck so beating this particular dead horse may not be productive.

    On the other hand, comparing this practice to grid tied PV is every bit as much a numbers game. The current reality of grid tied PV is that it adds little or no resilience to the grid and no energy to the building it sits on when the grid fails. In Japan this summer, any grid tied pv will shut down when they are forced to institute rolling blackouts. We live on a very thin veneer between wishful thinking and infrastructure failure. While, I agree, the last few inches of foam under a slab also offer little additional robustness, the differences between solid state efficiency and something like grid tied PV that is reliant on the grid being always up and stable, is huge.

    I offer that we are beating the wrong horse here. What matters is what happens when the shit hits the fan. If another large earthquake hit japan right now ( or California, or Indian Point) no one nearby would give a damn about incremental CO2 savings from foam or PV. In Tokyo this summer they may be wondering why the hell the windows do not open and looking for the idiot that designed all the buildings that way.

    Regardless, I appreciate all the efforts here.


  111. user-975699 | | #111

    Horses and nukes
    I really have nothing against horses so please insert better metaphors into my last post at will.

    My references to Japan and earthquakes also has to do with my feeling that we may well find ourselves backing away from our current 20% reliance on nuclear fission to boil water and generate electricity. If I'm right, and we want to cut CO2 emissions (or just keep the lights on), we had better worry more about dropping primary energy from 120 to 60 kwh/(m2a) (or lower) and worry less about 15 for heating.

    PH's fascination with electrons as primary drivers is now more worrisome than ever.

  112. Tom_Bassett_Dilley | | #112

    Aww, I have to use my BRAIN? .6 ACH, hot-humid
    Martin, yes, it is an oversimplification to say leakage>.6ACH=structural failure, but that's not what PHI says--they explain it as a safety factor in addition to the conservation inherent in airtightness. We use WUFI and THERM, and design assemblies to dry. One thing we've learned is that in superinsulated walls and roofs, the interior face of exterior sheathing is much colder than in conventional assemblies, so the potential for condensation is high if there happened to be a leak allowing moist air into the cavity. Your ICF house will likely have a wood-framed roof (brain is working!), and one good leak in the wrong place could be disastrous. So, if you were establishing a standard for airtightness, knowing that many PHs would have timber framing, you might consider that risk...again, knowing that .6ACH50 is attainable, and that it helps in conservation as well. Though imperfect, generalization is one of the purposes of a standard.

    David Gregory, Corey Saft designed and built a PH in Lafayette, LA, USA--I'm sure you could learn much about the hot-humid PH strategies from that house.

  113. GBA Editor
    Martin Holladay | | #113

    Response to Michael LeBeau
    Thanks for your comments, Michael. If you are designing a house for passive survivablity -- that is, if you're preparing for earthquakes -- you are obviously introducing new criteria not found in the typical energy modeling program.

    Your analysis might make sense, but one's response depends on one's personal take on preparing for low-likelihood, high-consequence events. Plenty of people choose to live in California, despite your analysis, and they aren't necessarily wrong. They're just making different choices from you.

    I live in an off-grid house, and my root cellar is full of potatoes I grew myself. I ended up living in the woods because of an analysis like yours. However, I'm not sure my response was entirely logical or defensible. Certainly during the last 35 years -- the time frame of my life in the woods -- the grid has been fairly dependable.

  114. user-971945 | | #114

    The Brits, comfort and heat pumps
    Being a Brit and still living on that side of the pond I have a couple of points for James and Martin:

    1) Most homes in the UK now have central heating using hydronic systems - even the old ones. (More control over comfort and yes the seductively cheap gas from the North Sea)
    2) Thermal mass does not "require"the use of hydronic systems. Hydronic is useful for addressing a high peak load - say if you switched the heating off for an extended duration (in an uninsulataed home this would be a few hours).
    3) Once you have a building that is super insulated (say a PassivHaus) then the radiant temperature of the surfaces are such that you will not experience radiant asymetry. In fact on the coldest day of the year the walls will be no more than about 1C colder then the air temp. In respect to thermal comfort PassivHaus addresses ASHRAE 55. Mass does not cause radiant discomfort it is heat loss that causes discomfort.
    4) In terms of energy savings there is little real fiscal benefit in switching off the heating system in a PassivHaus even for an extended duration, however, if you went on a 2 week holiday and turned off the heating then you would require a larger heating system to manage the subsequent peak load (which has a cost). For this reason PassivHaus type buildings opperate best at a constant temperature (with limited use of set backs during holidays.)
    5) Heat pumps work better - have a higher COP - when they do not have to supply at a high temperature. Whether or not heat supplied by either hydronic and air systems is irrelevant you have to i) insulate the building or ii)increase the area/volume of the heat emitor or iii) a combination of both. The problems being witnessed in the UK, and elsewhere I would tender, include (a) the design of the heat pump system (b) quality of the install (c) the lack of comissioning (d) the poor integration between the heat pump system and the whole building system.

    ...and yes some of us also drink tea and wear woolly jumpers to keep warm.


  115. GBA Editor
    Martin Holladay | | #115

    Still chilly in the U.K.
    According to the news, today's wedding day was chilly in London. So I guess it's time for another cup of tea.

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