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Part 3 of GBA’s Passivhaus Video Series

“Superinsulated Slab” is the third episode in a series of videos on the construction of a Passivhaus in ­Falmouth, Massachusetts

Posted on Jun 19 2014 by GBA Team

At the Passivhaus job site in Falmouth, Massachusetts, architect Steve Baczek specified the installation of 10 inches of EPSExpanded polystyrene. Type of rigid foam insulation that, unlike extruded polystyrene (XPS), does not contain ozone-depleting HCFCs. EPS frequently has a high recycled content. Its vapor permeability is higher and its R-value lower than XPS insulation. EPS insulation is classified by type: Type I is lowest in density and strength and Type X is highest. under the slab-on-grade foundation. After the concrete had been placed, more rigid foam was installed above the slab, to bring the finished floor assembly to R-50.

Using foam this thick meant developing some new techniques. The crew used a reciprocating saw rather than a tablesaw or circular saw to cut the foam; they drilled a pilot hole for each penetration, then drilled from both sides of the foam with a spade bit; and they tacked the inside pieces of foam to the first layer of foam to keep them from shifting.

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Here is the link to the “Superinsulated Slab” video.

To see Episode One of the series, click here: “Passive House Design.” (The first episode is available to all GBA readers, including non-members.)

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  1. Fine Homebuilding

Jun 19, 2014 6:44 PM ET

Not enough slab insulation
by kyeser

I would have gone with 20" of under slab insulation.... And shredded up $100 bills for dense pack insulation in the walls.

Jun 19, 2014 7:40 PM ET

Are you serious?
by JackofAllTrades777

Is this April 1st? I never knew Massachusetts was part of the Arctic Zone 8 climate.

R-50 on a slab? This is getting ridiculous. It will take 1,000 years to recoup the cost of doing that slab labor and all that foam.

Articles like this will turn people AWAY from Passive House designs when they see things like this. Millions of people in the world live in poverty and don't have homes and here we are throwing tens of thousands of dollars into an R-50 slab.

Jun 20, 2014 2:47 PM ET

Edited Jun 20, 2014 2:48 PM ET.

maybe it was being built over
by ntisdell

maybe it was being built over an area with near surface magma activity?

Or possibly a weird subterranean liquid nitrogen flow?

agreed! not to mention the ecological impact is there a burn barrel in back where they dispose of their plastics too?

Jun 20, 2014 5:20 PM ET

A possible solution
by user-1132906

Why don't governments cover the whole surface of the earth that isn't needed for food production with foam so people could build anywhere they wanted?

Jun 21, 2014 8:40 PM ET

Is there a question in there somewhere?
by user-417066

Gentlemen, is there a question in there somewhere, or does your absence of understanding (heightened by bad jokes) suffice as your course of action. While I also believe that 10" of rigid foam is an extreme measure, I understand the need for such measures. If you have the capacity to, or desire to understand, and can successfully compose a cerebral question in which we can all benefit from, please do not hesitate to ask.

Jun 21, 2014 10:29 PM ET

by user-1132906

This is the blog section of GBA which accepts comments, unlike the Q & A section where posters solicit advice.
The underlying tenor of the comments that seem to have offended you is that we consider the construction techniques shown to be absurd. Perhaps you could flesh out how you came to your understanding of them as necessary.

Jun 21, 2014 11:09 PM ET

by user-417066

I get it, I'm not proposing it become a Q&A, and offended I'm not, my justification solely lies with each of my clients. I deal with absurdity on a daily basis. In this particular case, my client desired a Passive House, and I promised to deliver one. For those that own or have lived in one understand their value first hand. The reduction in energy use is a direction we should all be choosing to go. In Passive House you proportionally turn a series of "dials" (insulation levels, window performance, air leakage, etc) In this house we turned the underslab insulation dial as one of the last. Proportionally, the R-50 was the tipping point to a "Pass" mark. As for cost, per board foot, it is in the ball park of closed cell spray foam. The total cost of the 10" of foam was not nearly as outrageous as one would think, and was simply the requirement of meeting the PH standard. I'll be the first one to admit, when I was told we needed 10", I chuckled heartily and said you gotta be kidding. But as you can see, 10" was the call.
For many years I worked at looking at building failures and inadequacies. For someone who loves working in the building industry, it is somewhat depressing to know we have the ability to build great things but yet we pursue avenues of default and continue to build severely inadequate products. While much of Passive House is extreme, this series is set out to show all that extreme can be pretty easily met with the methods and tools used everyday, just with a little more consciousness.
So let me blog about the effects of extreme building (including an R-50 slab package) This house's heating energy is welcomes 52% of its requirement as sunlight thru the windows. After an additional 24% taken care of by internal gains (yes that's 75% of the heat load), the remaining 24% of the load is provided for by ONE mini split. The house is over 2500 sq ft, and heated by a device many install to heat/cool their family room. The kicker is the mini split we spec'd (the lowest available) is twice the capacity required for remaining heat load. The house is around 7000 sq ft of surface area, yet its cumulative air leakage is about the size of a business card. This house's annual heat/cool/hot water budget is less than many friends month of February budget
When I come to understand the effects of extreme measures such as an R-50 slab, I have switched from asking why do we need it? To "why don't we do it? So we can all joke, and I have too, but the reality is that extreme measures result in extremely effective solutions

Jun 21, 2014 11:28 PM ET

What I find absurd
by user-417066

What I find to be absurd is that some design and build homes with the choice of a less than mediocre insulation and air leakage package, and then solve for the comfort issue with an HVAC system costing tens of thousands of dollars to install and hundreds of dollars to operate monthly, and upon each instance of operation causes an internal environment subject to potential health concerns and not nearly the comfort one could achieve - That's absurd!!. I think a step back, and a hard look will reveal some real absurdities. Maybe I should shred some $100 bills and throw them into the furnace to keep that monster fed, or simply open the window and toss them out.

Jun 22, 2014 12:08 AM ET

by jinmtvt

I am not a PH diehard fan, but i like PH for what it has brought up,
and how it is a good basic standard for super high efficiency building.
We need such a standard because with it, you can give a definite performance rating to a client,

It is very hard for some to think in terms of 20-40-100 years range.
Then understanding that some of the initial investment such as under slab foam, is a long term investment but is recoup from the money saved on mechanicals as you pointed out.

The problem with underslab insulation is not the insulation cost,
but the system + labor cost .

EPS is cheap, very cheap . If you consider the price of the insulation itself, the paybacks are much quicker than you might think.
Problem is that we usually have no way to installing it efficiently and for cheap labor-wise.

ON this particular project, why is the slab no the floor ???
why is this nice thermal mass semi-insulated from the room ??

Jun 22, 2014 4:04 AM ET

Response to Steve Baczek
by user-756436

This home's performance is impressive. Still, I would be interested to know the cost of the sub-slab EPS and the polyiso installed above the slab.

The criticism of very thick sub-slab foam has nothing to do with whether or not the home above the slab performs well. Your answer only addresses part of the controversy; it misses the core of the question.

If your floor assembly is rated at R-50, the most important question is: how would the house perform if the floor were rated at R-25? In other words -- we all know that the first R-25 of rigid foam saves a lot of energy. But what about the last R-25 of rigid foam?

Would the homeowners, even in bare feet, be able to tell the difference between a floor with R-25 foam and a floor with R-50 foam? I think not.

How many BTUs (or dollars) are saved per year when you go from R-25 to R-50 foam? The savings, I maintain, are so low that the payback period (I know, it's dreadful of me to bring up the question of payback) for the last R-25 is probably 100 or 150 years. It's certainly longer than that for the last R-5 of rigid foam. (In other words, for the foam added to change an R-45 slab to and R-50 slab).

Jun 22, 2014 8:06 AM ET

Let me try again
by user-417066

The 10" foam and installation of it was just under 10k. At about 1,000 sq ft of space - it comes to just under $1 per board foot of insulation (installed). That's further broken down to 25 cents per R / board foot. Closed cell spray foam is roughly half of that. And I agree, and will always agree, the last dollar spent to make the Passive House standard is the longest payback, because we are operating on the near flat part of the curve (see episode one video). So yes, as you pile the R-5's on the payback stretches longer and longer, and I am guessing I couldn't tell the difference of R-25 vs R-50. I said I get it. I do many homes that are "pretty good houses" and our underslab is usually around R-15-20. So , if you make the argument on cost /payback, I can't and won't argue. What I think Passive House does best is that it forces us to think about what we are doing, the decisions we need to make, and the fact that our answers are on a 150 year + train, and when that train leaves the station, the cost of a second chance is twice the initial cost. I commend my clients for their leadership and their acceptance of cost to provide us answers and direction. Yeah, ok we spent a little more on EPS and we strung out the payback a little bit, but its a step in the right direction. a step that gets us thinking about underslab insulation, and whole house performance in general.
Imagine our position today if builders in the 20's, 50's or even 80's thought just a little more about what they were doing, think about our current existing housing stock, how horrible it is, and how much it costs to just bring an existing home to 1/4 of PH standard. You want to talk about cost, have those conversations.
As for the PH standard, the one other thing it does is that it allows everyone, builders, clients, architects, anyone, to understand there is a pass / fail line in the sand. This at the very least provides us a common language and direction. The R-50 slab, isolated looks pretty bad, but understood in the context of the whole house it becomes a part of overall system to accomplish a certain metric, and right now it is the answer, whether I like it or not.
And dreadful is sugar coating the idea of payback. We as an industry have dug ourselves one hell of a hole. Our abilities and methods far exceed our level of decisions and leadership. Our typical housing is embarrassing, straight out horrible, but what makes it exponentially worse is our unfiltered desire to justify it based on payback. And as I think about it, this is the concept that initialized my slight fury. We joke about R-50, the cost of Passive House, etc. But we are in an industry with serious problems we choose to defend on concepts like payback and absurd cost (in isolation and not understanding) and joke about them. There is a luxury tax in doing the right thing, its a tax we can incur, or at least begin to incur, or we can simply pass it on as business as usual, or even worse we can increase the tax level. We need to make up for years of absurdity in my opinion, and start paying down our industries tax bill.

Jun 22, 2014 8:11 AM ET

little add on
by user-417066

Just as I hit post, I immediately thought of all the projects I have done, and currently working on where the 10k for the R-50 is just part of the cost of taking an old basement and trying to get the basement floor to R-10ish. That tax is a very costly one!!

Jun 22, 2014 8:15 AM ET

Answer to Jim
by user-417066

And Jim, the reason for not exposing the floor was simply the client's desire for a hardwood floor. In the northeast it takes a very special client to accept a concrete floor.
And for the record, and those of you that remember the article on my first passive house where we insulated the floor with cellulose. I initiated the design of this house with a similar concept but was told it was no longer an acceptable detail, my list of absurdities continues to grow............

Jun 22, 2014 1:13 PM ET

Edited Jun 22, 2014 1:14 PM ET.

by user-1132906

Thanks for the thoughtful replies. I don't think there are many people reading here who don't share your view that the way we commonly build right now is inadequate to meet the problems we are facing. That's why we are looking for a viable alternative. In times of uncertainty it is comforting to look for ready-made answers. You see it in how people flock to simplistic political, religious or psychological doctrines. These sorts of "don't think, just follow me" solutions can be as wrong-headed as the things they replace, and surely it is incumbent on us as professionals in the field to constantly assess and question what the new orthodoxies suggest - as much of what has gone wrong as due to us not questioning the old ones.
Passive House standards contain obvious absurdities. The under slab insulation is one of them. I don't see saying they aren't important because at least it is a move in the right direction as an adequate response. We are either open to examining the efficacy of various standards or we aren't. If we aren't then we have to give up pretending we are engaged in anything more than an ideologically driven solution

Jun 22, 2014 2:44 PM ET

Malcolm Taylor
by Onions

"Passive House standards contain obvious absurdities"
Not so obvious to me, which Passive House standards contain obvious absurdities?

Jun 22, 2014 7:33 PM ET

by user-1132906

The entire quote was:
"Passive House standards contain obvious absurdities. The under slab insulation is one of them."
So we could start there...

Jun 22, 2014 8:18 PM ET

We need a scoreboard
by kyeser

Martin's point hits home. How much diminishing value is there in huge amounts of Underslab insulation?

How about a GBA section that allows members to post home specs along with energy usage? It could be be separated by zone, and builders could enter in insulation values, blower door scores, windows, etc.
From there we could gleen what is actually producing results, and maybe get rid of some of the hype(r 50 Underslab) or maybe not.

And yes I know, occupant behavior has a huge impact on energy usage but a dashboard of results would sure help to give some clarity to some of these often fussed upon topics.

Jun 22, 2014 9:42 PM ET

by user-1132906

I agree that payback is a pretty fraught subject. Perhaps a better way to look at it is that spending money on one thing precludes it being spent on another. That's where I find real problems with the prescriptive nature of Passive House standards. The extra money which came from the budget to buy and install the R50 under slab foam might well have been used in other more productive ways - some off which may not have anything to do with the building envelope. Examples might be to have installed PV, the money might have allowed the owners to use a more efficient form of transportation, or indirectly benefit the project by putting it into landscaping which restored the site's ecological balance. Rigid standards preclude these types of decisions.

Jun 22, 2014 10:29 PM ET

Proportion is the key
by user-417066

Hello All,, here's another view. Absurdity lies on a subjective spectrum. More importantly, the breadth of the spectrum makes an easier or harder argument. To say R-50 is absurd is an argument I would agree with in a code built home, or even slightly above. But when I am designing homes with R-100 roofs, R-75 walls, and using R-7+ windows, an R-50 slab system loses much of its absurdity. The spectrum is pretty tight at that point and the R-50 slab becomes less of an option and more of a requirement. In a code built or energy star house the rest of the house is still pretty performance poor to warrant anything above R-20. But in this specific house the heat load gets so low that typical absurdity becomes the solution.
The problem with any standard is that it draws a line, a line means choices, and choices mean others might disagree, hence where we are. Regardless of the final level of performance, I will always believe "proportion" is the key. Basement, AGW, and roof insulation all need to be proportional, as one increases, the others look proportionally worse.

Jun 22, 2014 11:40 PM ET

Right but...
by user-1132906

The difficult discussions around proportion are exactly the type we need to have. R-50 may well be proportionate to R-100 roofs, but on what basis was that figure arrived at? Was it dictated by a standard or did was there some analysis of the project done which arrived at those numbers?
I recently designed a house here in the PNW which, with no insulation beyond what is typically used by builders here, has a total electrical bill of $70 a month, including heat. How much more should I have improved the building envelope? R-100 walls and R-50 under the slab? What is the difference for the owners and the larger community whether I improved it until it consumed, say, 30 bucks a month instead of pushing for net zero? Those are the types of questions we need to ask, and not sidestep them

Jun 23, 2014 12:16 AM ET

Malcolm Taylor
by Onions

Under slab insulation is not a Passive House Standard. Malcolm I think you are confusing Passive House Standards with what was designed into this house to meet the Passive House energy use requirements. Passive house has no insulation requirements. They have suggestions for insulation based on climate but ultimately it is up to you to design the house to meet Passive House Energy requirements. I am sure you understand this. If you think Passive House requirements are not reasonable maybe that is where you need to focus your discussions.

Jun 23, 2014 3:45 AM ET

Edited Jun 23, 2014 8:11 AM ET.

Response to Steve Baczek (Comment #11)
by user-756436

I was encouraged (at first) by the logic and frankness of your response. You wrote, "I agree [that] ... the last dollar spent to make the Passive House standard is the longest payback ... So yes, as you pile the R-5's on, the payback stretches longer and longer, and I am guessing I couldn't tell the difference of R-25 vs R-50. I said I get it. I do many homes that are 'pretty good houses' and our underslab is usually around R-15-20. So, if you make the argument on cost /payback, I can't and won't argue."

So far, so good. We are all on the same page.

And I also understand that, as an architect, it is your job to deliver the home that your client wants. When your client requests a Passivhaus, it is your job to deliver a Passivhaus.

After this promising start, however, the logic of your response weakened significantly. You wrote, "We spent a little more on EPS and we strung out the payback a little bit, but it's a step in the right direction." If you spent $5,000 of your client's money (the cost to go from R-25 foam to R-50 foam) -- oops, I forgot to include installation labor, so let's make that $6,000 -- to save $40 a year, why is this a step in the right direction?

Jun 23, 2014 3:52 AM ET

Response to Debra Glauz
by user-756436

I didn't use the word "absurdities"; Malcolm did. However, I understood what Malcolm meant.

You wrote, "Under-slab insulation is not a Passive House Standard. Malcolm, I think you are confusing Passive House Standards with what was designed into this house to meet the Passive House energy use requirements."

If I may rephrase Malcolm's point on Malcolm's behalf (a dangerous precedent, I know): The adsurdity (or, in my phrase, the arbitrary taget) in the Passivhaus standard is the requirement that a house meet the goal of 15 kWh per square meter per year in every climate on the planet.

This arbitrary target results in R-100 attic insulation and R-50 sub-slab foam when the Passivhaus standard is used in New England. These are unjustifiable levels of insulation. These levels of insulation are unjustifiable because the last R-25 of insulation saves such a tiny amount of energy.

Jun 23, 2014 7:26 AM ET

moving in the right direction
by user-417066

"The step in the right direction" is a reference to my client's leadership in requesting and doing a Passive House, not doing an R-50 slab. I said I get it and I agree. Working with the PH consultant we had to deliver a PH (in this case the client/owner is the PH consultant). We were pretty much max-ed out in the wall and ceiling, but as I spoke in the "proportional" concept, because the attic and wall looked good, the slab looked bad and needed to be upped. I'm not defending, the financial benefit of an R-50 slab. What I do defend is that it was required for me to deliver a PH, and my clients decision to exercise some responsibility and pay down our energy tax debt, they, my client made a move in the right direction.
They are a retired couple, financially sound, they could have easily built a better than code house and faded of into life quite comfortably, but they didn't .............

Jun 23, 2014 8:09 AM ET

Respose to Steve Baczek (Comment #24)
by user-756436

Thanks for your clarification. It looks like we are in agreement.

Congratulations on the performance results you obtained.

Jun 23, 2014 10:39 AM ET

Why does it matter?
by user-1132906

I agree with Steve that an affluent client building a single family house choosing to go with the Passive House standard is a step in the right direction over code minimum or blowing the cash on other features. It is the knock on effects that worry me.
Although right now those projects are largely limited to these small custom houses, proponents of Passive House have been clear that the real gains are in larger buildings, and there is a lot of talk about lobbying for the standard to be adopted into building codes. There is a huge difference in playing this game with your own money and imposing it on others.
Let's have some clear-headed debate about the implications of the standards without resorting to them being justified as a move in the right direction or using realistic lifespans for the buildings not supported by anything unique about their construction.

Jun 23, 2014 10:43 AM ET

by user-1132906

I understand the difference between the standard and requirements but you are just hair-splitting. As an analogy: If I say the amount of reinforcing in concrete foundations is too high due to a local code standard for seismic resistance, what's the point of replying that there isn't any prescriptive requirement to use rebar, when that is the only way to meet the standard?

Jun 23, 2014 11:44 PM ET

Added clarification
by user-417066

Hello All, so I thought it worthy to call my client and discuss the issue relative to the PHPP. First of all, he plugged in R-25 and we failed to meet the standard. He reminded me that we could have turned up the wall insulation dial a bit but we had some zoning restrictions on the project and that he didn't want to give up any floor square footage to more insulation as we had a fixed limit on the size of the house footprint. Our option was then to increase the sub slab insulation to the R-50 to keep our square footage where it was.
What is interesting is that we tested out at .37ach 50. (better than the .60 requirement). If we go back and input the .37 into the PHPP then the house easily makes the passing grade with an R-25 subslab. So in hindsight the R-50 slab is too much as the house stands today. The problem in the situation is that we don't know what our air leakage number is until we build the house and long after the foundation needs to be constructed. Secondly, PHIUS wouldn't let us lower the .6 in precertification because we were told they wanted the buffer space (I think that decision should be at our risk). Regardless, we kinda got shoe horned into the R-50 subslab. If the house was in the middle of a pasture somewhere where our square footage wasn't under restriction then it could have very well been down in the mid 20's for sub slab insulation. God, glad we got that cleared up!!!

Jun 23, 2014 11:47 PM ET

by user-417066

And Martin Buddy, anything I can do to put a little sunshine in your day.

Jun 24, 2014 11:09 AM ET

by Onions

Malcolm I understand your analogy but I am just not seeing it. From my thick headed perspective I am not seeing myself as hair splitting. There are too many dials to turn when doing the energy balance for a Passive House. How do I know that a Passive house design is an efficient Passive House design? I am referring to any Passive House not the above example. Maybe bettor windows could have been used. Or maybe there is a view of a rock and the client is taking an energy penalty to get that view. Or maybe the design is inefficient and the designer discovers a bettor design but the client doesn’t want to pay for a redesign so they turn the slab insulation dial. For me it’s too complex to boil it down to rebar.
Martin said“This arbitrary target results in R-100 attic insulation and R-50 sub-slab foam when the Passivhaus standard is used in New England. These are unjustifiable levels of insulation. These levels of insulation are unjustifiable because the last R-25 of insulation saves such a tiny amount of energy.” So Martin is what you are saying is, in zone 5,6,7 of New England it’s just too difficult of a metric to meet? You also don’t like the fact that it is arbitrary (absurd?). Is that right? How would you change a performance target so it is not arbitrary? Maybe you don’t like performance targets?? I understand you don’t like to waste materials, labor and money. I agree, by definition, waste is not good. PH probably doesn’t want to be known as a system that is wasteful. The PH spread sheet could certainly create a cut-off, beyond a certain amount once a near flat line was reached no additional credit will be given. Or the 4.75kBTU could be raised for given areas. I think that is what Phius is doing for certain climates.
I am not arguing for or against PH or PH requirements. Nor am I for wasting insulation. I am from California zone 3 and 4- fire, heat, termites and lots of regulation and no humidity to speak of, so I am not intimate with building across three zones in New England. Passive House here is easy pezy to meet but difficult to accomplish, hard to breach Architects love affair with glass.

Jun 24, 2014 11:20 AM ET

Edited Jun 24, 2014 11:25 AM ET.

Response to Debra Glauz
by user-756436

Q. "So, Martin: Is what you are saying, in zones 5, 6, 7 of New England, it’s just too difficult of a metric to meet?"

A. No, it's not difficult. It's just not cost-effective.

Q. "You don’t like the fact that it is arbitrary (absurd). Is that right?"

A. Yes, that's right.

Q. "How would you change a performance target so it is not arbitrary?"

A. For years, I have consistently proposed that designers stop specifying additional energy measures (thicker insulation, better performing windows, etc.) when the incremental cost of the proposed measure yields a smaller return in energy savings than the same investment in a PV array.

Jun 25, 2014 11:51 AM ET

Steve Baczek
by jinmtvt

Your report about the additional R-25 that was at the end, not necessary , is appreciated.

I do not believe that insulating the ground concrete slab was a smart move.
At the end, why pouring a concrete slab at all if you are going to hide and break it ??

When you say 10 000$ for the slab insulation, what is comprised in this price ??

I hope that PHIUS can work out a climate based mofidicator soon ,
so the PH movement in NA gets going at last.

There is probably more carbon in the last inches of insulation than it will ever save in 50-100 years.
Even more if we move toward clean electricity. ( we = NA )

Jun 26, 2014 6:36 AM ET

Morning Jin
by user-417066

Jin, let me start with, as an architect, I can make suggestions, I can argue those suggestions, I can provide proof of those suggestions, but in residential architecture, the client has the last word. All of my projects are "quilts" with many, many parameters to making all the decisions. Any one of those decisions when isolated and taken out of the context of the "project quilt" could seem crazy, or even stupid. As a young architect I would run around saying that was dumb, this is a bad decision, etc. Then I had to do a couple large projects, and the clients heavily relied on me. It was an education in understanding just how much all of these decisions are inextricably linked.
The additional R-25 as a trade off to loosing more interior square footage to an increase of exterior wall was a desirable one by the client. The additional $$ of insulation simply gave him more square footage of living area. As part of the resale equation, more space = more $$, so some of that money is simply an investment.
We needed a concrete slab, and I believe it is the best, most cost effective solution. As for hiding it, the client desired the wood floor, should I tell them they can't have what they want?
As stated earlier the 10k is labor AND materials for 1,000 sq ft of 10" insulation, That equals $1 per board foot of insulation -INSTALLED. That' not really a bad price.
I hope can do a lot of things.......
Because I live for being educated by those wiser than me, please let me ask;

Beyond an initial R-25, and not losing any interior square footage to increasing the exterior wall thickness, and the attic insulation is maxed out - How would you have solved for meeting the Passive House Standard?

Given the client's desire to want a hardwood floor, how would you have detailed the elimination of the floor slab?

Please also make the argument for exposing the slab? Its thermal storage component while it does store some heat and then provide a release when needed, in a Passive House the loads are so so small it simply doesn't do that much for you. But, again please, I love to learn, so please help me understand the worthy benefits of exposing the slab?

What would your Passive House slab foundation detail detail comprise?

What is it that you believe PHIUS should be modifying?

I appreciate your comments... Thanks

Jun 26, 2014 8:31 PM ET

by user-1132906

Let me start by both sympathizing with and acknowledging the difficulties we all face working with clients and then being judged although we had complete control over all aspects of the project. I'm a lot happier hearing that you listen to their needs and desires, rather than simply dictating to them. You sound like a good and thoughtful architect.
The tradeoffs you describe all seem logical and necessary - but only if you accept the constraints put on the project by the Passive House standards in the first place. I understand that the client wanted a Passive House, so my criticisms aren't directed at your response, but at the underlying problems that the standard has brought to your work. The increased insulation was necessary to increase the floor space. But the necessity seems to me to stem from having to adherence to an inflexible standard, You wouldn't have deemed it was needed if Passive House didn't, and I'd bet that you don't spec that much when you have the flexibility of designing to levels your professional experience tells you make sense. I hope you take what appears to be criticism of the project in the way I think it is intended - as doubts about the appropriateness of the standard, not your work within it.

Jun 26, 2014 10:34 PM ET

by user-417066

I totally agree!!!! I think we all, architects, builders, PH consultants, and PHIUS, have a lot to learn. The good thing about Passive House is that they have a simple energy goal and it is up to us to achieve it. I continually self-evaluate my projects, and I actually greatly appreciate forums like this to discuss, we all get an education from it. I have no problem being scrutinized. I'm in the business of getting things built, and my projects have schedules and budgets that I have to deal with. I am confident I am making the right decision at the time I make them.
I often equate Passive House to someone looking to get in shape, hire a personal trainer, and then being trained to be a world class bodybuilder. It isn't the path for everyone, and getting anywhere near it is still a dam good house. At the very least, the Passive House craze has drummed up a ton of conversation, and that is good, BUT, conversation doesn't get things built, and we need to build them. I think PH is in its infancy here in the US, and the only way to solve for it is straight thru it. Build some, scrutinize them, and scrutinize them hard, learn from them, and most importantly, close the loop on learning. My personal evolution increases ten fold with each one I do. But I need to build these to work out the kinks. My hope is that PHIUS sees there process as an evolution of solutions, not simply a rigid product.

Jun 28, 2014 1:09 AM ET

by jinmtvt

We had a discussion last year ( was it ? ) about the code required concrete slab on full basements in most places .
If you are to install several inches of dense EPS under the floor as in your case ( and most PH ) ,
and then cover the concrete slab with a wood floor or even additional insulation ,
why have a slab at all then ?
What is the purpose of the slab ?

You coul've just specified a wood framing onto which the wood finish would've gone,
all that layed on top of the EPS blocks.
The saved money from the slab would've help with the underslab insulation budget a bit.

All thermal mass is a benefit, even if some around here disagree ,
i think that it does not help much with the energy but rather is a comfort helper as it really stabilize the temperature of the rooms.
( i live in an all concrete house and the stability of the temperature even with the unfinished state, is very impressive )

In a solar design, the mass helps reducing the overheating of the winter solar gains, while it is absorbing heat is has to give away some of its lower temp to the room ,
and when we get toward the night it releases its daily acquired energy down to equilibrium with the heating system settings.

The effect might be of a lesser magnitude in a PH, but it is always present nonetheless,
and if the design has an important day SHG, it smoothes it.

If i get it right, once solar energy enters the house and hits flooring/walls etc.. ,
it is pretty much "stuck" in the building and goes away with the thermal loss of walls/windows surfaces , so the total energy will be the same for a building with and one without much thermal mass, but the one with the more important mass will not overheat as much .

For a basement slab though ..might not participate in comfort as much as for the first floor.

About the PH tradeoff to meet criteria ...well you are probably right that it was the less intrusive way to meet your goal .
How would the additional money spent on the last R25 account for? 3-4000$ ?
Adding 3-4000$ to the windows would've change energy as much as the slab ? maybe .

Insulated window shutters could be another option,
automated during the night. Probably custom and more expensive than the foam.

PHIUS , or another body, will have to work on the energy goal to adjust it to reality.
As discussed with Dana on the carbon topic, it is simply unjustifiable in most of NA to go at PH with the current energy goal, in terms of Co2 reduction ,
and will become even less with the advent of renewable energy.

So maybe the energy goals could be adjusted according to the climate,
according to the energy source, but it needs adjustment.
Do not tell me you really believe that this same goal is right for all of NA climates,
because we all know that it creates problems with extremes.

Doesn't mean you can't do it, just that is is not a good investment for the homeowners who are paying, environmentally and energy wise.

We should always strive for perfection , so if better compromises are found , move forward.

Jun 28, 2014 9:10 AM ET

by user-417066

My guess is that a framed floor over 10" of EPS would have cost more than the slab. To install a wood floor and maintain enough rigidity so it doesn't bow or move would mean some type of joist system rather than a slab system. I think a 2x8 wood framed floor over the EPS would have been the same, or actually a little more than the slab. The second issue is that I get a rigid substrate in 4" to build upon rather than 8". That 8" then becomes questionable as to what do I do with it, leave it vacant?, insulate it? Lastly, in a wood framed floor,because of structural surface area contact of the wood frame (probably a beam line) I concentrate the floor load to a smaller area and risk differential treatment. In the case of the slab, I have full thermally broken contact that should it move slightly, the slab will become a little bridge, or it will move as a whole (which I don't really see happening)
As for the cost of eliminating the second R-25, it is not as linear as one would expect. You save directly on the material expense, but everything associated with prepping and initial insulation installation is the same. Once we had the first 6" piece down and finished, the second 4" pieces were a quick puzzle.
As for the slab, we have ground, then R-35 EPS, then a 4" slab, then R-6, then the conditioned space. I would think the slab even though it is not in direct sunlight, takes on heat given its R-35 insulated backside, why wouldn't it? As far as I know heat moves from hot to cold, if the slab is colder than the room during the day it will heat up, as the room temp falls below the slab temp at night the heat would reverse. I do realize that the slab in direct sunlight would heat up warmer than the room, but why isn't the wood floor taking on that task, granted it doesn't have the capacity of the slab, but this house's heat load is so dam low, the slightened capacity probably isn't even aknowleged by the house.
And I agree, the US is far too wide a spectrum for a one size fits all solution, and some areas get penalized more severly than others.

Jun 29, 2014 12:09 AM ET

by jinmtvt

agreed that i might have cost just as much to use a wood framed floor instead of a slab.
But you could've install it on less EPS because installing some cheaper insulation would've been possible withing the wooden frame . So total thickness is not an issue,
and you could've used another greener type of insulation .. but it may not make a large difference neway.

About the additional cost of the last R25 ..
What you are suggesting is that the cost of this additional R25 might be almost solely based on the insulation price itself, as it did not add much additional labor .

If you consider only the base price of the insulation, the impact of the additional R25
on the ROI should be much less. ( still does not change the Co2 relation impact )

As i said in another blog, one of the current problem we have with super-insulation system is the associated labor cost. Usually there are materials available that still make sense , but either the changes to the installation method and the add. labor renders the operation worthless as far as ROI is concerned. So we need to find ways to install more insulation , more efficiently labor wise.

Lastly, about PHIUS and PH in NA . I do not agree that it is a matter of penalizing areas or climates, but more of goals. By using the same energy target for all of NA climates, the PHI standard pushes the designs to overuse of insulation that defeats the basic goals of reducing Co2 ,
and severely limits the financial possibility for mid-class families to build with this standard .
There is no "saving the planet" with a handful of buildings.
We need to modify ALL of the buildings for it to work.

hope i am making sense :p

Thermal mass does not make you magically absorb more SHG energy, it just disposes of it in a diffrent way , and helps regulate the temperature inside the rooms toward the user set point.

Jun 29, 2014 12:45 PM ET

by user-417066

It's interesting, if you are familiar with my first PH featured in the Houses issue of FHB you would see that I had cast a typical frost wall / slab above grade assembly. Atop that I framed a 2x8 wood floor approximately 16" above the slab. After 4" of xps on the slab, I filled the rest of the cavity with cellulose. Unfortunately PHIUS would not allow that assembly to be repeated, contrary to my beliefs.

My insulation contractor that I use regularly consistently strives for improvement, just recently he and his employees developed some techniques for cellulose installation that decrease installation time by about 25%.

We are all learning, it justs takes time........

Jun 30, 2014 10:38 PM ET

by jinmtvt

i am not familiar with it, care to link me up ?

Why would PHIUS not like the setup ? Why do they have something to say about this ?

25% is quite alot of labor saved!!

I will be working on balancing labor and insulation systems in my near future projects
( just acquired more land for more high efficiency projects!! :)

Jun 30, 2014 11:34 PM ET

by user-417066

The article is in Fine Homebuilding - Summer 2012 "Houses" issue #227

PHIUS denied are repeated use because they had a fear of a leaky plumbing waste pipe in the cellulose under the floor. I told them we would pressure test the whole system as well as just run vertical risers thru the cellulose and do the horizontal runs under the slab - NO GO!! They wouldn't pre-certify the house with that detail. Personally they should concern themselves with the requirements of their program and leave us to solving for the requirements.

Yes the insulator is rockin!!! He's a good guy wanting to do the right thing!

What part of the world are you located?

Jun 30, 2014 11:35 PM ET

by user-417066

Page 76 on the article

Jul 1, 2014 11:06 AM ET

by jinmtvt

I do not have access to fine homebuilding, but it looks interesting
will go for their trial and look at your stuff soon :)

I am located in the "paradise of the extremes "
31c and 80%hum today , but we had -30c all winter !
Vive le Quebec :p

i am a bit east of Montreal, north shore of the St-Laurent river.

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