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Can Foam Insulation Be Too Thick?

Determining the best thickness for sub-slab foam

Posted on Aug 21 2009 by Martin Holladay, GBA Advisor

In the U.S., designers of cutting-edge superinsulated homes generally recommend 2 to 6 inches of rigid foam insulation under residential slabs. For builders who use extruded polystyrene (XPSExtruded polystyrene. Highly insulating, water-resistant rigid foam insulation that is widely used above and below grade, such as on exterior walls and underneath concrete floor slabs. In North America, XPS is made with ozone-depleting HCFC-142b. XPS has higher density and R-value and lower vapor permeability than EPS rigid insulation.), the most commonly used sub-slab insulation, that amounts to R-10 to R-30.

As Alex Wilson recently reported, “Building science expert Joe Lstiburek … argues that for any house north of the Mason-Dixon Line we should follow the ‘10-20-40-60 rule’ for R-values: R-10 under foundation floor slabs; R-20 foundation walls; R-40 house walls, and R-60 ceilings or roofs.”

For reasons that are somewhat murky, however, Passivhaus builders install much thicker layers of sub-slab insulation than most superinsulation nerds.

Passivhaus buildings have very thick sub-slab foam

Passivhaus designers use an oft-praised software package developed with the help of German physicists — the Passive HouseA residential building construction standard requiring very low levels of air leakage, very high levels of insulation, and windows with a very low U-factor. Developed in the early 1990s by Bo Adamson and Wolfgang Feist, the standard is now promoted by the Passivhaus Institut in Darmstadt, Germany. To meet the standard, a home must have an infiltration rate no greater than 0.60 AC/H @ 50 pascals, a maximum annual heating energy use of 15 kWh per square meter (4,755 Btu per square foot), a maximum annual cooling energy use of 15 kWh per square meter (1.39 kWh per square foot), and maximum source energy use for all purposes of 120 kWh per square meter (11.1 kWh per square foot). The standard recommends, but does not require, a maximum design heating load of 10 W per square meter and windows with a maximum U-factor of 0.14. The Passivhaus standard was developed for buildings in central and northern Europe; efforts are underway to clarify the best techniques to achieve the standard for buildings in hot climates. Planning Package (PHPP). The PHPP software helps designers determine how thick insulation needs to be for a house to achieve the Passivhaus standard. Among the key requirements of the standard: the house must have a maximum annual heating energy use of 15 kWh per square meter (4,755 BtuBritish thermal unit, the amount of heat required to raise one pound of water (about a pint) one degree Fahrenheit in temperature—about the heat content of one wooden kitchen match. One Btu is equivalent to 0.293 watt-hours or 1,055 joules. per square foot) and maximum source energy use for all purposes of 120 kWh per square meter (11.1 kWh per square foot).

To meet the standard, Katrin Klingenberg, the founder of Passive House Institute U.S., installed 14 inches of expanded polystyrene (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.) insulation — 7 layers of 2-inch foam (a total of R-56) — under the slab of her home in Urbana, Illinois. The Waldsee Biohaus, a Passivhaus language institute in Bemidji, Minnesota, has 16 inches of EPS under its foundation slab.

What’s the explanation for these differing recommendations?

I recently approached engineer John Straube in hopes of satisfying my curiosity on the surprising disparity between the sub-slab insulation recommendations of North American physicists and Passivhaus advocates. John Straube is a colleague of Joe Lstiburek at the Building Science Corporation, a professor of building envelopeExterior components of a house that provide protection from colder (and warmer) outdoor temperatures and precipitation; includes the house foundation, framed exterior walls, roof or ceiling, and insulation, and air sealing materials. science at the University of Waterloo, and a very smart guy.

I asked Straube whether the differing recommendations resulted from uncertainties related to soil temperature measurements. No, Straube answered, the reason for the disparity lies elsewhere.

Is there a cheaper way to do it?

As it turns out, the PHPP software never considers whether the incremental cost of thicker and thicker insulation is greater than the cost of an alternative method of meeting the home’s energy needs — namely, a photovoltaic(PV) Generation of electricity directly from sunlight. A photovoltaic cell has no moving parts; electrons are energized by sunlight and result in current flow. (PVPhotovoltaics. Generation of electricity directly from sunlight. A photovoltaic (PV) cell has no moving parts; electrons are energized by sunlight and result in current flow.) array.

Straube explained to me that Passivhaus designers have only a few dials to turn when adjusting a home’s specifications. (Straube gave credit to energy consultant Marc Rosenbaum for the control-panel metaphor.) The Passivhaus standard does not allow the use of site-generated PV to help meet the 15 kWh per square meter and 120 kWh per square meter goals. Once the designers have specified the best available windows, for example, the window dial can’t be turned down any further. These houses are just about as airtight as buildings can be built, so the airtightness dial has basically been bottomed out. Once the easy dials have been turned, the only remaining variable under the designer’s control is insulation thickness — so, to make the house work, that’s what gets adjusted, even when the resulting insulation thickness is illogical or uneconomic.

Sometimes PV is cheaper than thicker foam

It’s easy to understand why many Passivhaus advocates disdain PV: the electricity produced by a PV array is very expensive. “In most of climate zones 5 and 6, PV (at $8 per peak wattUnit of rated power output, for example from a photovoltaic (PV) module in full sunlight, as distinct from its output at any given moment, which may be lower. for the total system) will generate electricity at around 50 to 60 cents per kWh, if financed by a 25-year 6% mortgage,” Straube said recently. That’s why many builders have concluded that PV arrays are toys for the wealthy.

Figures provided by Dr. Wolfgang Feist, the director of the Passivhaus Institut in Darmstadt, Germany, are very close to those provided by Straube. “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,” Feist told me in 2007.

What Feist failed to mention — but Straube went on to prove — is that the logic of the Passivhaus standard drives cold-climate designers to use sub-slab polystyrene that is even more expensive than an unsubsidized PV array.

The cost of PV-powered heat

Before making his back-of-the-envelope calculation, Straube studied soil-temperature data. “A slab on grade insulated to R-32 in Finland had an average heating season soil temperature of 12.5°C (55°F),” Straube wrote in an e-mail. “Hence, during the heating seasons the average temperature difference between soil and indoor air is about 15°F.” In other words, the delta-TDifference in temperature across a divider; often used to refer to the difference between indoor and outdoor temperatures. across an insulated slab is much less than the delta-T across an insulated wall — at least in cold northern climates.

If a home uses electricity to supply heat, a heat pumpHeating and cooling system in which specialized refrigerant fluid in a sealed system is alternately evaporated and condensed, changing its state from liquid to vapor by altering its pressure; this phase change allows heat to be transferred into or out of the house. See air-source heat pump and ground-source heat pump. is obviously more efficient than a resistance heater. “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, biomassOrganic waste that can be converted to usable forms of energy such as heat or electricity, or crops grown specifically for that purpose., concentrating solar thermal, etc. So this seems like the high end of electric production costs.”

Once this number is known, it becomes a simple matter to calculate whether the incremental cost of very thick foam insulation is cheaper or more expensive than PV. Most builders would agree that it makes little sense to invest in foam when a PV array is cheaper.

It makes sense to stop at R-15 to R-25

Straube wrote, “The cost of insulation becomes more than the cost of generating energy for the walls in a typical house in a 7,200-HDDThe difference between the 24-hour average (daily) temperature and the base temperature for one year for each day that the average is below the base temperature. For heating degree days, the base is usually 65 degrees Fahrenheit. For example, if the average temperature for December 1, 2001 was 30 degrees Fahrenheit, then the number of heating degrees for that day was 35. 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). Basements have less heat loss [than slabs on grade], so the cut-off point is more like R-15 to R-20 for a basement slab. Heating a slab with radiant tubes increases the temperature of the slab from around 68°F or so to 80°F or so on average, so the insulation levels need to be increased by about 50% over this for radiantly heated slabs.”

Building Energy Optimization (BEopt), an energy modeling program developed in 2004 at the National Renewable Energy Laboratory in Golden, Colorado, shows designers the “least-cost path” to building optimization by performing calculations similar to those made by Straube. The idea behind BEopt is that no building should have a PV array until the designer has implemented every building envelope measure cheaper than PV. Once this point is reached, an investment in PV may make sense. Certainly such an investment would make more sense than spending additional dollars on insulation — since the insulation would provide even less benefit per dollar invested than PV.

What about maintenance costs?

Those who have been following the discussion this far may have noticed a flaw in the “PV is better” argument: PV equipment and heat-pumps have a shorter life, and require more maintenance, than sub-slab insulation. In fact, this point may be enough to convince some builders to choose 14 inches of foam over a PV array. It’s a defensible position, but it’s one that should only be made after considering the fact that the homeowners would get more bang for their buck from a PV array than from the last 10 inches of foam.

Last week’s blog: “The History of the Chainsaw Retrofit.”


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Image Credits:

  1. Edwin Dehler-Seter

1.
Fri, 08/21/2009 - 07:03

Reply
by Katrin

Helpful? 1

Hi everybody,

I have posted a reply to this article on the Passive House Institute bulletin board.


2.
Fri, 08/21/2009 - 08:11

Can Foam Insulation Be Too Thick?
by Jon Vara

Helpful? -1

Great piece, Martin. One question: in the first part of the story, you're having a conversation with John Straube, then partway through you shift to saying "Straube wrote." It's not clear where he's writing. An article or paper on the subject? Personal emails to you? If the former, it would be useful to know the source.
JV


3.
Fri, 08/21/2009 - 08:20

He sent me an e-mail
by Martin Holladay, GBA Advisor

Helpful? 1

Jon,
After I spoke with John Straube on this subject at the Westford Building Science Symposium — a.k.a. "Summer Camp" — we had an exchange of e-mails on this topic.

Thanks for your comment. I'll clarify the ambiguity in the text.


4.
Fri, 08/21/2009 - 09:15

Excellent
by John Brooks

Helpful? 1

Martin,
You are writing about the things that I am struggling with.
The problem that I have with PV is that it is ugly and it usually involves holes in the roof.
And what about when it is time to replace the roof cladding?
I prefer the German approach.
My clients do not want to see solar acne on their homes and neither do I.


5.
Fri, 08/21/2009 - 09:53

Ouch!
by Jon Vara

Helpful? 0

"Solar acne?" The horror! And get rid of those damned outdoor clotheslines! We're trying to look at nice houses here!


6.
Fri, 08/21/2009 - 10:00

One Comment - Two Questions
by Mike Guertin, GBA Advisor

Helpful? 0

I struggled to speculate why Passivhaus case studies typically used such thick slab insulation. Now I get it; great investigation of the rational and sorting out alternative positions.

Two questions for you Martin: In one calculation you wrote ” …the cost of PV-powered heat is no more than 60/2.5 = 24 cents per kWh.” I’m presuming the ‘60’ is from the “50 to 60 cents per kWh” you noted earlier?
And what is the best / most common way to express cost per R of various insulations? You used “…EPS (which costs around 10 cents per R per board foot).” Is it appropriate to use ‘Cost / R / board foot’ or ‘Cost / R / square foot’? Since different insulation materials have different R values per inch thickness and a board foot measures volume (12”x12”x1”) I’ve used the Cost/R/ square foot (area (12”x12”) but that may be wrong. I get confused because the resulting number (144) is the same but the unit changes.
[Editor's note: In response to Mike Guertin's perceptive question, the reference to "cost / R / board foot" in the original article has been edited to read "cost / R / square foot."]


7.
Fri, 08/21/2009 - 10:02

warped perception of science
by John Brooks

Helpful? 1

Martin & Jon Straube,
Forgive me....I am an Architect ... not a Scientist.
It is difficult for me to imagine that heat transfer is exactly the same from House to Air as from House to Earth when DeltaT is the same.
I visualize the Earth as an easier "path" than the air....
Maybe the difference is not very great?
Are they really the same?


8.
Fri, 08/21/2009 - 10:07

Ouch!
by John Brooks

Helpful? 0

Sorry Jon,
I have seen homes with pv integrated into the design ....and it can be attractive.
Or at least not offensive.
More times than not it is less than attractive..


9.
Fri, 08/21/2009 - 10:11

Cost of insulation
by Martin Holladay, GBA Advisor

Helpful? 1

Mike,
John Straube wrote "10 cents per R per board foot." I agree with you that the units we should be discussing are the cost per R per SQUARE foot. If an insulation product costs 10 cents per R per SQUARE foot, then R-10 of insulation costs $1 per square foot, or $100 for 100 square feet. Or R-20 insulation would cost $2 per square foot or $200 for 100 square feet.

Before I change a direct quote from John Straube, however, I'll give him a chance to respond.


10.
Fri, 08/21/2009 - 10:15

50 to 60 cents per kWh
by Martin Holladay, GBA Advisor

Helpful? 0

Mike,
You're right that the number 60 in the calculation 60/2.5 = 24 cents per kWh comes from the high end of the range of PV prices discussed earlier by John Straube. He figures that PV electricity costs between 50 and 60 cents per kWh.


11.
Fri, 08/21/2009 - 11:14

PV means you have one more trade to schedule
by Andrew Henry

Helpful? 0

Hi Martin,

One thing that got left out in your PV versus more sub slab insulation discussion is that if the designer chooses the PV route, then the project manager has to fit the electrician/ PV panel installers in to the schedule. These are highly skilled trades, and electricians that know how to install a PV system are still not all that common, which may make the schedule less flexible. In the end it's not simplifying things.

Whereas the greater sub slab insulation option of Passive House shouldn't necessarily complicate the schedule, either way sub slab insulation will be placed down, it's on the schedule and it doesn't require a highly skilled electrician.

That said if the designer is planning to design to Passive House and also to install a PV system for the small energy load remaining then my above argument may, or may not, be valid. Still, I have to say that for the small remaining energy load, grid supplied electricity should be cheaper (at the moment).

The tech.view columnist at Economist.com wrote a column
last week reminding us of the virtue of simplicity which is worth reading with respect to this posting.

Finally, is the homeowner going to have the necessary knowledge to maintain the PV system. That's not an issue with insulation. Let's not forget about the occupants!

Cheers,

Andrew

P.S. Martin is there a way we can suggest story/ blog post ideas to you?


12.
Fri, 08/21/2009 - 11:47

Simplicity, maintenance, and blog ideas
by Martin Holladay, GBA Advisor

Helpful? 1

Andrew,
1. Your point about simplicity is absolutely right. Many designers and homeowners will prefer the simplicity (in construction scheduling as well as equipment longevity) of thick sub-slab foam to PV.

2. There really isn't any maintenance required for PV. My oldest PV module is 29 years old, and has had absolutely zero maintenance. It just sits on the roof, exposed to the weather. While it's true that I regularly remove the snow from my PV array in winter, most people don't have to — because they live somewhere where the snow is less frequent, and because they don't depend on every kWh of electricity from the sun in January and February like I do. (I'm off-grid). Since the days are very short and the sun is very weak during the winter, you don't lose much electricity if you don't clear the snow from your array. Finally, inverters are the weak link in a PV system. But I bought my inverter in 1985, and it's still working fine.

3. Yes, I'd be very happy to entertain any and all suggestions for future blog topics.


13.
Fri, 08/21/2009 - 11:50

Blog topics
by Andrew Henry

Helpful? 1

Hi Martin,

It's ok to put the blog topic into a comment on one of your posts? I don't want to clutter up the conversation thread.

Cheers,

Andrew


14.
Fri, 08/21/2009 - 11:54

Or you can e-mail me
by Martin Holladay, GBA Advisor

Helpful? 0

Andrew,
Post it here if you want, or e-mail me directly at
martin@greenbuildingadvisor.com


15.
Fri, 08/21/2009 - 14:18

Ground temperature
by John Semmelhack

Helpful? 1

A heating season ground temperature of 55F in Finland sounds pretty outrageous to me. I calculated an average ground temperature for the heating season (approx. Oct. to April) for the Helsinki, FInland climate data set from the Passive House Planning Package (PHPP) and came up with 41.3F. I also used the classic rule of thumb for average ground temperatures: "average annual ground temperature = average annual ambient air temperature" to get 40.2F (again using the PHPP data set for Helsinki, Finland).

Where is John Straube's data from?


16.
Fri, 08/21/2009 - 14:21

The temperature difference was measured
by Martin Holladay, GBA Advisor

Helpful? 0

John,
According to John Straube's e-mail to me, he based his calculation on measured data. Here's what he wrote: "The prediction of heat loss through slabs is notoriously inaccurate. I recently did a literature survey of measured temperatures and heat loss of slabs and basements, looking for real measurements of insulated cases. As an example of one of the few results I found, a slab on grade insulated to R-32 in Finland had an average heating seasons soil temperature of 12.5 C (55 F)."

I'll try to get in touch with him to track down a reference for the study.


17.
Fri, 08/21/2009 - 15:33

Ground temperature
by John Semmelhack

Helpful? 1

If the information from John Straube is accurate, Martin, then you should have written a different article! Instead of "Hey! You're using too much insulation below your slabs!" you should have written "Hey! The ground temperature in your climate data is dead wrong!"

That's mostly to say: I'd love to see an in-depth follow-up on the ground temperature issue.


18.
Fri, 08/21/2009 - 15:52

More data
by Martin Holladay, GBA Advisor

Helpful? 1

John,
Although you imply that John Straube's reference to 12.5°C soil under an insulated slab in Finland was surprisingly warm, I found a reference to a study that refers to soil temperatures that are even higher -- between 15°C and 17°C -- under insulated slabs in Finland.

I haven't read the study; just the abstract. I don't know the R-value of the insulation, so the data may not be relevant. If anyone is interested, here's the link:
http://scitation.aip.org/getabs/servlet/GetabsServlet?prog=normal&id=PPS...


19.
Fri, 08/21/2009 - 18:24

NONSENSE!!!
by Graham Irwin

Helpful? -2

This is complete nonsense. I am torn between ignoring this stuff and spending time addressing inaccurate and poorly researched conclusions, but here we go:

1) Katrin's house exceeded the PH heating limit by almost 50% because she opted to use electric heat rather than fossil fuels and faced a source energy penalty due to inefficient generation.

2) Mr. Straube attempts to justify his hypothesis by assembling a hypothetical PV/heat pump assembly to supply heat that might otherwise be preserved with insulation. He does acknowledge that the COP of the heat pump goes down with temperature, but fails to provide analysis on the seasonal variation in PV output. Presumably, the PV output is far lower during heating season than other times of the year, or it wouldn't be heating season!

One might then turn the argument toward "net metering," which really means that the PV system overproduces in summer (helping to cool inefficient commercial buildings) and in winter, this "good deed" is magically repaid with fossil fuel produced electricity.

If one wanted to truly operate at an equivalent level of sustainability by supplanting insulation with a heat pump, what is required is clean power in winter. The PV array that powers the heat pump should be installed in Arizona (Chile even better) with dedicated transmission lines to the heat pump. Even if the PV panels are cheaper than foam, things ain't so rosey now...


20.
Sat, 08/22/2009 - 03:57

Not really nonsense
by Martin Holladay, GBA Advisor

Helpful? 1

Graham,
As I pointed out in the last paragraph of my article, choosing 14 inches of foam over PV is "a defensible position," so I have already agreed with you.

That said, it's important to note the point at which thicker and thicker insulation becomes more expensive than PV. To do so is not nonsense. Relying on net metering is not nonsense; it is a routine fact for tens of thousands of homeowners in Germany, the U.S., and many countries around the world.

By ridiculing net-metering, Graham, you seem to be implying that using the grid to distribute PV electricity is somehow inappropriate. But all Passivhaus buildings use electricity; all Passivhaus buildings use the grid. Since they are already grid-dependent buildings, what's wrong with PV-equipped buildings using the grid to distribute excess electricity in the summer, and deliver needed electricity in the winter?

If it's OK for a Passivhaus building to buy electricity from the grid whenever the building needs it, then we should all be willing to consider a distributed-generation model that includes the use of the grid to share PV-generated electricity. Net metering is the way that happens.


21.
Sat, 08/22/2009 - 04:02

#1 of three e-mails from John Straube
by Martin Holladay, GBA Advisor

Helpful? 1

I have received three e-mails from John Straube responding to questions posted here.

#1 concerns Mike Guertin's contention that Straube intended to refer to "cost / R / square foot" when he mistakenly referred to "cost / R / board foot." I responded to Mike by saying I thought he (Mike) was right.

John Straube wrote, "I agree, Martin."

In response to Mike's perceptive question, I have edited the article to reflect the intended units (cost / R / square foot).


22.
Sat, 08/22/2009 - 04:06

#2 of three e-mails from John Straube
by Martin Holladay, GBA Advisor

Helpful? 0

E-mail #2 concerns John Brooks' question. Brooks wrote, "It is difficult for me to imagine that heat transfer is exactly the same from House to Air as from House to Earth when Delta-T is the same. I visualize the Earth as an easier 'path' than the air."

John Straube wrote, "The difference between earth-slab and wall-air heat transfer is encompassed in the 'surface film' coefficient. This coefficient (fudge factor) accounts for the insulating effect that a still layer of air near the surface provides to a wall. It is not that important for well insulated walls, as its values is less than R-1. Another possible difference is the dynamic nature of the above-ground air temperature variations, but for lightweight structures this mass is not that important. Otherwise, the physics of heat flow above and below grade ARE the same."


23.
Sat, 08/22/2009 - 04:14

#3 of three e-mails from John Straube
by Martin Holladay, GBA Advisor

Helpful? 0

E-mail #3 concerns John Semmelhack's challenge of Straube's sub-slab soil temperatures.

John Straube wrote, "Mr Semmelhack's question points to the root of the confusion: the lack of good models because of a lack of measured field data. I agree that PHPP gives too low a temperature, and that the 'average annual ground temperature = average annual ambient air temperature' also misses the target (mostly because it ignores the fact that there is a heat source above the soil — the house — providing heat for 12 months of every year). This is the reason that I was basing my numbers on measured data, even though it is sparse.

"The closest example I have is the Finnish paper 'Thermal and Moisture Conditions of Coarse-grained Fill Layer Under a Slab-on-ground Structure in Cold Climate' from the Journal of Thermal Envelopes and Building Science, Vol 28, no. 1, July 2004. The coldest temperature measured was 45°F, rising to 68°F by the end of the summer, before falling again.

"A more recent paper, 'Heat, Air, and Moisture Control in Slab-on-ground Structures,' Journal of Building Physics, Vol. 32, No. 4, April 2009, shows even higher temperatures, with averages of 15°C/60°F in the winter, except near the edge where they dropped to 10°C/50°F (winter average over 15°C though).

"In Norway, another cold climate that builds lots of slab on grade, a paper in the 7th Nordic Building Physics Conference on slab heat loss reported, 'In Norwegian climatic conditions, with a yearly mean soil temperature varying from 2° ~ 7°C, we can use 12°±1°C as a default value for the inner zone reference soil temperature". 12°C is 53.6° Fahrenheit.

"In Britain, a study of a commercial building is reported in 'Temperatures in and under a slab-on-ground: two- and three-dimensional numerical simulations and comparison with experimental data,' in the journal Building and Environment, Vol. 35, 2000, found temperatures (in a milder climate near Cardiff, Wales) in the 14°-16°C (57°-61°F) range during winter periods. I also have some papers from Sweden, and basement measurements from Canada.

"Suffice it to say, 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).

"I would be interested in hearing about any published measured data to add to my small but growing collection."


24.
Sat, 08/22/2009 - 09:13

Sub-Slab Temperatures
by Stephen Thwaites

Helpful? -1

I like John Straube's comment about slabs warming up the soil below the slab. It makes sense.

If that is the case, then aren't increasing amounts of slab insulation somewhat self-justifying?

John Straube's comment would also suggest there is a region of soil that is acting like a buffer, dare I say insulator, between the slab and the 'ambient' soil temperature.

As a window person I can see an analogy between window analysis, which looks at Frame, Edge of lass (2 1/2”) and Centre of Glass regions. In this case we might have a Slab, Soil near Slab and Ambient Soil regions. I suspect the distinctions between these regions isn't nearly as distinct as with windows. Clearly there is no foundation analogy to the highly conductive metal spacers found in most windows. Nevertheless, I think there's something similar going on.


25.
Sat, 08/22/2009 - 10:15

thick insulation
by reese mcclure

Helpful? 0

Just like most green technologies out there foam is over priced. Build with straw bale and you dont have these kind of issues. If your structure works in harmony with the its environment the costs are insignificant. A typical straw wall will have an R-value of R-55. I have built a home that uses straw bale insulation,solar/wind power,methane harvester,radiantant heating,solar hot water,grey water collection,rain water collection for flushing toilets,responsible watershed,reclaimed building materials,and many other obviouse green technologies. And it will come in under $125sqft. I would love to know what they spent on that foam insulation, and will they ever recoup the costs. The costs need to need to make sense. By the way solar can be purchased for under $4 bucks a watt and by the time the rebates kick in its under $2 bucks a watt, so why are people still trying to demonize solar. If they just did a little shopping around. If you can find it cheap call me or if you need a straw bale home built , I will help. reese 951-453-3899, And for all you skeptics out there call me and we can talk about it.


26.
Sat, 08/22/2009 - 13:22

Straw bale walls have an R-value of R-27.5
by Martin Holladay, GBA Advisor

Helpful? 1

Reese,
The most thorough R-value testing of straw-bale performance was done at Oak Ridge National Labs. According to these tests, a wall built from 19-inch straw bales has an R-value of 27.5 — not 55.

To read more about the testing, see "Refining Straw Bale R-Values," from the March/April 1999 issue of Home Energy magazine.

Needless to say, straw-bale homes still need foundation insulation. If the straw-bale home has a slab-on-grade foundation, it should certainly include a continuous layer of XPS or EPS foam under the slab. Foam under slabs makes sense.


27.
Sat, 08/22/2009 - 22:40

Testing info out dated (straw rocks)
by reese mcclure

Helpful? 0

Hey Martin thanks for the info, if you google straw bale r values you will get several sources for r values. 99% are pretty constant on the r value. Only the one you sent. So I would wonder who funded the test, maybe a foam manufacturer or a lumber company. I would take the info with a grain of salt and put an actual home constructed of both materials and see the results. I have always believed that the foam is over rated. As far as need insulation under the slabs, I would look to the old adobes and say wheres the foam. I think this foam craze we are on is just that. Man once again trying to out do mother nature. I am a green builder in southern california that loves new technology, I have looked at spray in foam for a long time and I think its over priced and will fall to the side in the next 5 years. We will always have a use for it ,just not as big as those invested in that industry thinks. Hope your not a foam guy. Any how thanks for the info feel free to call and share any other info you might have. reese 951-453-3899


28.
Sun, 08/23/2009 - 05:47

Straw Bale Discussion
by John Brooks

Helpful? 1

Hey Reese,
You should probably try to start up a straw bale discussion in the Question Section.
I think that you caught Martin wearing his cold climate glasses ;-)
He forgot to ask you where you live.
Perhaps most climates should be thinking about slab edge and maybe slab perimeter insulation.


29.
Sun, 08/23/2009 - 06:27

Heat Store or Heat Sink? Storing or Bleeding?
by John Brooks

Helpful? 0

I remember a discussion between Robert Riversong and Thorsten Chlupp.
Two very wise builders.
Robert said that we are storing energy below or slabs and Thorsten said that we are "bleeding" energy.
Jon Straube and Wolfgang Feist are a couple of very wise Scientists.
Why is there such a major difference here?
Martin... have you tried to get a comment from Wolfgang Feist?


30.
Sun, 08/23/2009 - 08:36

Who is Right?
by John Brooks

Helpful? 0

The Physics here is over my head.
I hope that the two teams will duke it out for everyone's sake.
We need good answers and "Control Panel Design" sounds like a good concept to me.

What I see is a difference in priorities.
We (N. Americans) are making our decisions based on dollar costs and Return on Investment.
The Germans have set a High Bar for their Enclosure Performance and they seem to be achieving their goals.
By the time 2030 gets here.....Who will be ahead?
Who will have the most Low Energy Homes Built?
What would be wrong with damn good enclosures with Design integrated PV on top if you like.
Why is it one or the other?


31.
Sun, 08/23/2009 - 09:06

oops
by John Brooks

Helpful? 0

Sorry John Straube... I knew it was John but typed Jon (twice)
Jon Brooks


32.
Sun, 08/23/2009 - 12:05

Who is right?
by Martin Holladay, GBA Advisor

Helpful? 2

John Brooks,
I don't think this is a situation where we have to say that either Wolfgang Feist or John Straube is right, and the other guy is wrong. Both are right.

To sum up:
1. The Passivhaus people, who I respect very much, have a stringent energy goal that they have decided must be reached WITHOUT photovoltaics. As I have already noted, this is a defensible position, since envelope improvements are more foolproof and long-lasting than PV modules.

2. To reach this goal in cold North American climates appears to require very thick insulation levels. When it comes to foam beneath slabs, the last 6 or 10 inches of Passivhaus foam are quite expensive in terms of energy saved per dollar invested -- more expensive than PV. But to Passivhaus proponents, the money is well spent, because of whole-house benefits. They're willing to make the investment.

3. The PHPP software APPEARS to assume that subslab soil temperatures are colder than shown by monitoring studies using subslab thermocouples. However, I stand ready to amend this statement if anyone provides data to show me to be wrong.

4. PV is expensive, but relatively long-lived and maintenance-free. Some U.S. homeowners with access to net-metering agreements may prefer to invest $4,000 in rooftop PV, and save $4,000 on sub-slab foam by stopping at R-20 per Straube's recommendations.


33.
Sun, 08/23/2009 - 12:30

Investing in the enclosure first
by John Brooks

Helpful? 0

I don't think that anyone has mentioned that it might be easier to add PV in the future than it would be to add insulation below the slab.


34.
Sun, 08/23/2009 - 15:00

Data confusion?
by Dark Lad Slim

Helpful? -1

I'm not certain, I would need to consult EN standards and the calculation method used by to John Straube, but John Straube’s suggestion that the ground temps in PHPP are to low could be flawed. The reason for this relates to standards and methodologies and the way in which they use data sets.

*John Straube is using a seasonal average measured *beneath* a slab conditions (in accordance with some standard or method that is unclear).
*PHPP is using the seasonal average based upon ambient ground conditions (in accordance with the EN standard) i.e. not beneath the slab.

Now consider that:
* The Cardiff paper that John Straube cites finds that the calculation method in the EN standard shows a good correlation with real heat losses.
* The PassivHaus Institute studies determined that for super insulated buildings the EN standard did not consider basements and the proximity of ground water adequately. The building physics has been written up in a Protokollband. Such refinements to the EN standard are contained in PHPP. Also consider that PHPP can accept input based upon dynamic calculations.

So in conclusion to this section the EN standard is generally good for insulated building but could be refined.

Data and stanards confusion:
I fear that John Straube may not have considered that the methodology of the EN standard may differ from the one that he is using. So his warmer ground temps, which relative to external ambient conditions would reduce the rate at which heat is lost, is considered by the EN calculation used by PHPP but not in the same manner. (Also it is wise to remember that as the U-value is improved the heat losses are reduced and therefore the under slab ground temperature becomes closer to ambient ground conditions). To account for the fact that the under slab temp relates to the U-value the EN standard calculates a weighting factor that is applied during the calculation.

Perhaps this would adequately explain the difference between the figures in PHPP and the measured ground temps that he cites. If John Straube is using the measure below slab temps without considering the fact that the temp will fall as the insulation is improved then this may also suggest that the point at which the cost effective limit for under slab insulation could be beyond that which is currently being considered.

Any thoughts on this issue?


35.
Sun, 08/23/2009 - 17:37

REALLY NONSENSE!!!
by Graham Irwin

Helpful? -1

Actually, what you said was "For reasons that are somewhat murky, however, Passivhaus builders install much thicker layers of sub-slab insulation than most superinsulation nerds."

The reasons are not murky in the slightest. They are the result of precise calculation of the net and incremental benefit of a given thickness of insulation, balanced with solar heat gain, etc., and they are figured quite precisely on a project-by-project basis.

Further, I am not ridiculing net-metering, I am speaking to the ridiculous notion that net-metered PV power is carbon-free power. AGAIN, while one may produce a surplus of power in summer, that gets sucked up by inefficient commercial buildings. In winter, there is no surplus of PV electricity for heating because the sun isn't out much, or you wouldn't need heating, because lack of sun is what causes winter. The power we get in winter comes mostly from fossil fuels, unless we're in an area of the country that has a surplus of hydro, wind, nuclear or geothermal.

With me so far? If this is the case, then the calculations suggested have to be rejiggered to account for the low output of PV during heating season - you can't assume the average daily output in the middle of winter. Contrast this with a proper level of insulation - it is effective at precisely the times it is needed most because it is designed for precisely the times it's needed most.

Do I think PV is bad? No, but it is not the panacea it is held up to be. Yes, we need power for buildings, and Passive Houses need power too, because it is recognized that insulating to zero energy is not cost effective. That said, it is recognized by PH that grid electricity produced with fossil fuels represents consumption of fossil fuels. The net metering argument is not much different than buying carbon credits (i.e., paying to plant trees in Sri Lanka or whatever.) Perhaps your next article could discuss how buying carbon credits is cheaper than insulation... ;-)

My point is that putting PV on a roof does not make the building more efficient, and that PH is the most cost effective point of efficiency, because it is calculated precisely to reward the builder with mechanical system reductions in exchange for increased efficiency. This precise and project-specific calculation enables one to determine exactly how much energy is being saved by one thickness of insulation over another - how is that "murky?"

To suggest that "any house north of the Mason-Dixon Line ...should follow the ‘10-20-40-60 rule’ for R-values: R-10 under foundation floor slabs; R-20 foundation walls; R-40 house walls, and R-60 ceilings or roofs." because we can make up the difference with PV assumes that there are no differences in orientation or solar exposure among ALL houses north of the Mason-Dixon line. Yet you call the site-specific calculations involved in Passive House "murky." Don't you see how COMPLETELY ASININE this assertion is?


36.
Sun, 08/23/2009 - 20:06

Asininity is hard to assess
by Martin Holladay, GBA Advisor

Helpful? 1

Graham,
I prefer to discuss the technical issues on their merits; I'll leave it to others to determine my asininity. I can assure you, however, that I'm doing my best to avoid asininity.

You assert that "Passivhaus is the most cost effective point of efficiency," but your statement is only true if you assume that it makes more sense to invest in sub-slab foam that is expensive (considered on a basis of energy saved versus money invested) compared to PV, which yields more energy for the money invested.

You disdain PV because PV systems produce most of their energy during the summer, when heating is not needed, rather than during the winter. Since I have lived off-grid for 34 years, and since I have been watching the meters on my PV system every day for 29 years, I can assure you I am very familiar with this fact.

I understand your disdain for the concept of net metering, because you have explained it twice. I nevertheless believe that I would rather live in a house that obtains much of its electricity from PV -- even if the electricity is seasonally unbalanced -- than to live in a house (like most Passivhaus buildings) that obtains all of its electricity from the grid. PV is expensive, but it is part of the solution we need as we make the transition from fossil-fuel-generated electricity to renewably generated electricity.

This morning I spent several hours stacking firewood gathered from a clearing on a ridge near my house. Once I've completed work on the clearing, I hope to erect a wind turbine there. My hope is that the wind turbine will generate electricity during the dark days of winter.

As I wrote in my essay, the decision to include very thick sub-slab foam under Passivhaus buildings is entirely defensible. I'm not ridiculing the position, and I certainly believe, Graham, that your position is not asinine.


37.
Mon, 08/24/2009 - 02:22

Holy Lord of the Foam
by Brett Moyer

Helpful? 1

I may sound like a building science neanderthal, but isn't there a point at which one has to step back and say.... "MAN, THATS A #$@% LOAD OF FOAM!!!!"
Furthermore, shouldn't we be directing our efforts towards changing the way the american home is built?
Shameful, horrific homes are being built this very moment throughout the country that are far from energy efficient. Lets address these homes first, and then worry about the validity of PHPPs and yearly mean soil temperature calculations. I can appreciate a healthy debate, and I understand the need for progressive thinking, but come on...
AAAAAND to Graham Irwin,
You sir, are way out of line calling Martin asinine!! (sorry Martin, I'm sure you can defend yourself, but I had to say something)


38.
Mon, 08/24/2009 - 08:58

#4 of four e-mails from John Straube
by Martin Holladay, GBA Advisor

Helpful? 1

In response to Dark Lad Slim's comment that "John Straube’s suggestion that the ground temperatures in PHPP are too low could be flawed," Straube writes the following:

"There must be some confusion. I am not 'following a standard' other than the stated laws of physics. I explicitly DID NOT consider the PHPP/ EN standard as this approach appears to give the wrong answers. The whole point of my investigation was that the 'standard' approaches seem not be getting us to numbers that match real measured data.

"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."


39.
Mon, 08/24/2009 - 09:41

Not-So-Friendly
by John Brooks

Helpful? 1

I think that Martin Likes to see "The Fur Fly"
http://www.passivehouse.us/bulletinBoard/viewtopic.php?f=4&t=182
He likes to stir things up.
He brings up some darn good topics.
This has been a great discussion ... but Not-So-Friendly..

Maybe there should be a Beer Summit .. sometime around October 16-18
John Straube,Wolfgang Feist,Katrin and Martin
German and Canadian Beer......not sure about Vermont beer?
I think that PH & BSC would make a good team and should be sharing knowledge.
Let's lighten up a little.


40.
Mon, 08/24/2009 - 09:50

Trying to be friendly
by Martin Holladay, GBA Advisor

Helpful? 2

John,
I sent an e-mail to Katrin Klingenberg this morning. I said, in part, "I would be happy to interview you for an upcoming blog; or, if you prefer, to provide you the opportunity to write a guest blog on the GBA Web site. As you probably know, you are always free to post a comment on our Web site. I have always valued your input. ... I hope that we can continue to work together and keep the channels of dialog open. I'm not sure you realize how often, in my writings and conversations, I have spoken highly of the Passive House standard and its accomplishments."


41.
Mon, 08/24/2009 - 13:13

Correction
by Graham Irwin

Helpful? 1

Martin,

If I led you, or others, to believe that I was calling you "asinine" I apologize, but also point that my writing has been misread. Specifically, I asserted that a single prescriptive level of insulation for every home from Maryland to Minnesota is absurd if one is concerned with cost effectiveness, as is the assumption that PV output and cost is uniform across this area. Further, I do not disdain PV, but the notion that net-metering = carbon neutrality = PV is dollar for dollar as sustainable as insulation does not hold.

Of course it is better to use PV than fossil fuel electricity, but since PV output is minimal when heating load is maximal, the solution is not to assume that you're getting an average daily output of electricity at the depth of winter and compare that cost to insulation, the solution is to reach the most cost effective point for efficiency, then add renewables after (ENDING with PV.) Passive House is designed to do this, and the insulation benefit is carefully evaluated and traded off with other measures. The building in Bemidji is heavily insulated because it is REALLY cold there in winter, but the thickness was carefully calculated and justified.

I tend to lose my patience with this forum because I believe that you know these things and are attempting to stir up controversy, conversation, web traffic with sensationalist statements. Trouble is, as much as they may draw traffic to your site, they do not result in any greater insight, nor do they help with the fledgling Passive House movement in the US - they just suck up and waste time from people who already have to much to do. We either have to let these ridiculous assertions stand or work to dispel them, and we have enough to do already. If you want to explore these issues, please do so objectively and responsibly, avoid subjective words like "murky," and devote equal effort and time to researching and presenting both sides of an argument. If you are unclear about how the Passive House standard is derived or justified, please consult with those who know BEFORE your write these articles. This is the road to responsible journalism vs muckraking, IM(H)O.


42.
Mon, 08/24/2009 - 14:22

What's murky and what isn't
by Martin Holladay, GBA Advisor

Helpful? 3

Graham,
I'll try to address the issues in the order that you raised them.

1. "A single prescriptive level of insulation for every home from Maryland to Minnesota is absurd." You are referring to my quotation of an Alex Wilson blog that reports on Alex's understanding of Joe Lstiburek's recommendations. I used the quote as a short-hand reference to U.S. recommendations for cold-climate insulation levels. It's fair to say that Lsitburek's understanding of the cost-effectiveness of insulation is more subtle than your parody implies. By focusing on the Alex Wilson quote, which was never intended as a location-specific recommendation, you distort Lstiburek's sophistication as well as my own.

The intent of the quote was to illustrate the fact that U.S. builders following superinsulation principles generally use less sub-slab foam than do cold-climate Passivhaus designers. That fact remains — whether or not the sentence from Alex Wilson was well crafted or poorly crafted.

2. I never claimed that "net-metering = carbon neutrality = PV is dollar for dollar as sustainable as insulation." I'm not sure if anyone knows whether the manufacture or use of either polystyrene or photovoltaic modules is sustainable. Probably, neither practice is sustainable in the long run. That's why I try to avoid the use of the word "sustainable."

3. You assail the assumption that "you're getting an average daily output of electricity at the depth of winter and compare that cost to insulation." I never made that assumption; as I pointed out, I'm well aware that the daily output of a PV array in winter is less than the daily average output on an annual basis. However, in the U.S., the calculation of the return on investment for a PV array takes the value of the electrical output of the PV array over its lifetime, and compares that value to the cost of the investment. I'm not the only one to analyze PV investments this way; it's the standard way it's done.

4. You write that "the solution is to reach the most cost-effective point for efficiency ... Passive House is designed to do this." But Straube shows that Passivhaus designers end up overinvesting in sub-slab insulation -- that is, going beyond the point of cost-effectiveness. You reject that by challenging the standard way that PV investments are assessed. I'm afraid your position is rather lonely.

5. "I believe that you know these things and are attempting to stir up controversy." I can assure you that I approached John Straube with a technical question because of a single motivation: curiosity. When Straube answered my question, I learned something new. I am attempting to share my new knowledge with any interested readers who care to follow my blog. There was no controversy until various people posted fairly vociferous responses on this page — your post that "this is complete nonsense," Katrin's assertion on her blog that I (Martin) "prefer to talk to other people, 'experts' and smart guys, who I bet have little idea about the PHPP," your memorable line, "Don't you see how COMPLETELY ASININE this assertion is?," and your latest characterization of my words as "ridiculous assertions." To the extent that there is a controversy, I haven't been fanning the flames. I'm trying to focus on technical issues.

6. Finally, you advise me to "avoid subjective words like 'murky.' " Your advice on vocabulary selection is interesting, coming as it does from a writer I might charitably call "unrestrained." Evidently you are unhappy that I wrote, "For reasons that are somewhat murky, however, Passivhaus builders install much thicker layers of sub-slab insulation than most superinsulation nerds." It is a simple fact that the reason for the disparity discussed in my essay was murky to me. The reason was evidently also murky to Jon Vara and Mike Guertin, who were enlightened by my investigation into the topic. Clearly, the reason was not murky to you. I don't doubt you're a step ahead of me. But my intention as a journalist has always been to try to shine light on murky areas — including areas that are murky to me — and to share what I learn with my readers.


43.
Mon, 08/24/2009 - 14:30

The conclusion of the Cardiff
by Dark Lad Slim

Helpful? 0

The conclusion of the Cardiff paper -
Thermal transmittance values have been calculated from the measured data and compared with the values determined from current design guides. ….. the U-values calculated using the CEN draft document were found to be in reasonable agreement with the measured value for normal weight concrete and in excellent agreement with the light weight concrete.

....hmmmm - Does this sound as though the real-world measurements simply do not match the standard approaches and assumptions? Okay, maybe not 100% correlation but certainly within realistic parameters? Yes, very much so.

"…heat flow across an insulated slab is due to the temperature difference across it" This simple fact is obvious to anyone that has spent a few minutes considering heat flow. So where is the surprise? Where is the physics?

A singular measurement is just a snap shot at a given geometric location - even it this is a seasonal average it is still specific to a location. In so far as I appreciate John Straube's average under slab temp accommodates the average of seasonal fluctuations and so it would seem that he is using a steady state model rather than full three dimensional dynamic modeling. Fine. A steady state model is what's used in a number of energy design tools.

I wonder, has he has applied this average to the whole of the floor slab? And, what was the geometry of the buildings in question? Was the average at the center of the slab, across the whole are of the slab?

The reason for these queries stem from the fact that that the temperature under the slab is not consistent i.e. an average fails to consider geometry and the fact that heat loss is greater at the perimeter (there are greater temperature differences at the edge of the slab than at the center of the slab due to greater heat losses to ambient air.) The zone affected by this thermal bridging could be up to 1-2m from the perimeter edge - for a house that is 10m deep and 15m long (is that okay for McMansion?) that is accounts for a substantial proportion of the floor area (about 30% or so) This fractional area will only increase as houses get smaller.…If he has just used one temperature, and has applied it to the whole slab, then this could result in an error. On this basis further clarification on the calculation method for "following ... the stated laws of physics" would be appreciated.

DLS


44.
Mon, 08/24/2009 - 15:15

DESIGNING WITH TOY BLOCKS
by Jesse

Helpful? 0

The 10-20-40-60 rule is fun! You have your different shaped, colored blocks. Flat blue blocks for the floor, big red blocks make the roof. But Passive House is not so fun. Germans in lab coats, beakers, mysterious humming machinery, holistic systems. Too serious!
And you know what's cooler than a really low electric bill in winter (boring!), is watching the meter turn backwards in summer ( Awesome!)


45.
Mon, 08/24/2009 - 20:01

Passivhaus slab
by Elana

Helpful? -1

I can't believe the grief Martin is getting for arguing the technical merits of insulation versus PV! It seems that some of the people reacting here have "drunk the Kool-Aid" (an American reference) and are uncomfortable with scientific analysis of energy questions. Keep up the awesome work, Martin.


46.
Tue, 08/25/2009 - 08:25

#5 of five e-mails from John Straube
by Martin Holladay, GBA Advisor

Helpful? 0

In response to Dark Lad Slim's questions about "the conclusions of the Cardiff paper," John Straube responded:

"Why is Dark Lad Slim hiding behind a fake name?

"The standard approach worked in Cardiff because the slab was almost uninsulated in a mild climate, exactly the type of scenario the standards were developed for and where they work. I did not reference it for that reason. I referenced it for the soil temperatures. Read all of the papers, especially the ones in cold climates with 4" to 8" of insulation to get the whole picture, e.g., the one that I was talking about. There are several reasons why the standard approach works for such slabs, but that is a whole article in itself.

" [Quoting DLS] ' "…heat flow across an insulated slab is due to the temperature difference across it." This simple fact is obvious to anyone that has spent a few minutes considering heat flow. So where is the surprise? Where is the physics?'

"This was a response to the query as to why I would use temperatures below the slab. Obvious to me. Confusing to many apparently.

"[Quoting DLS] 'A singular measurement is just a snapshot at a given geometric location — even if this is a seasonal average it is still specific to a location. In so far as I appreciate John Straube's average under slab temp accommodates the average of seasonal fluctuations and so it would seem that he is using a steady state model rather than full three dimensional dynamic modeling. Fine. A steady state model is what's used in a number of energy design tools.'

"Read the papers. I did not use a snapshot. 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).

"[Quoting DLS] 'I wonder, has he has applied this average to the whole of the floor slab? And, what was the geometry of the buildings in question? Was the average at the center of the slab, across the whole are of the slab?'

"Read the papers. Inform yourself.

"[Quoting DLS] 'The reason for these queries stem from the fact that that the temperature under the slab is not consistent i.e. an average fails to consider geometry and the fact that heat loss is greater at the perimeter (there are greater temperature differences at the edge of the slab than at the center of the slab due to greater heat losses to ambient air.) The zone affected by this thermal bridging could be up to 1-2m from the perimeter edge - for a house that is 10m deep and 15m long (is that okay for McMansion?) that is accounts for a substantial proportion of the floor area (about 30% or so) This fractional area will only increase as houses get smaller.…If he has just used one temperature, and has applied it to the whole slab, then this could result in an error. On this basis further clarification on the calculation method for "following ... the stated laws of physics" would be appreciated.'

"You act as if you know something about the topic, yet your questions imply that you have little understanding of the basics. 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 may be considered too large by DSL but 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. Back to the reason for my looking for Delta T.

"It would be nice to give a whole course on basic sub-grade heat flow, but I am not going to do this by e-mail."


47.
Tue, 08/25/2009 - 13:19

interesting idea, but I'll stick with Wolfgang & Katrin
by Dave Brach

Helpful? 0

Martin, thanks for bringing up this idea. This discussion made me re-evaluate the merits of large quantities of insulation, but I think Graham ultimately make some very good points about comparing insulation with electricity generation. Insulation saves heat in the winter, when it is most precious. If I'm paying a bit more to save a Btu in winter than I would to produce it insummer, the simplicity and logic of the more pasive technology make this a more attractive choice. The PHPP is a design tool for seriously reducing loads through a systems approach, not for calculating energy production, and if it turns out that eschewing PV in lieu of thicker insulation is not 100% cost effective, the simplicity, rigor, technical elegance, and usefuleness of the tool itself far outweighs this consideration.


48.
Tue, 08/25/2009 - 13:49

Not all HDD climates are made the same
by Mark

Helpful? 0

Given the low internal gains within a Passive House, and the assumed 20C internal temp, it is worth remembering that not all HDD climates are made the same. HDDs are dependent upon internal temperature and internal gains (and solar gains). On this basis one mans 7,200 HDD climate may be another mans - lets say - 8,900 HDD climate (this is a rhetorical number rather than anything founded on calculation, but I'm sure that you get the point.) Unless the same HDD convention is used the for Passive House buildings and Building Science buildings are the same then the cost per kWh saved could be distorted.


49.
Tue, 08/25/2009 - 14:22

Hmmm... Murky
by Dan Whitmore

Helpful? 0

Martin,
Initially I was intrigued by tyour article, especially since I'm currently turning the dials on the control board for insulation on a project using PHPP. Mr Straube has some interesting scientific analysis. I can generally understand the possibilities of 'banking' a small amount of heat sub-slab so one's not dealing with the same Delta T below the structure as one is sub-soil 15' away. Attending a presentation by him would most likely be an excellent learning experience. Unfortunately the way the article presents his conclusions and conclusions others have drawn from his analysis I can only think to describe as 'murky'.

Dr Straube is 'a very smart guy.' OK, I can accept that. However when he's presented in a sentence such as: "...the surprising disparity between the sub-slab insulation recommendations of North American physicists and Passivhaus advocates..." there seems to be a good bit of murky subjective journalism going on. Dr Feist, Ms Klingenberg, et.al. are merely 'advocates?' Please.

PHPP is founded on years of scientific analysis and site studies. Certainly it needs more. I've never heard anyone who's got a genuine interest in it's application say otherwise.

As for the question of utilizing PV instead of insulation? I wish this were separated from the discussion about sub-slab insulation needs because i see too many holes in the argument.

- One of the large benefits of a Passive House is limiting the heat load to a point where an expensive heat source isn't needed. (If you want PV, scrap the pricey heat pump, put up a few panels & some foolishly inefficient baseboard heaters. Oh, right the sun doesn't shine in winter... back to Carbon trading.)

- As for the assertion that PHPP has no balance for site-generated electricity. Check near the back for the Primary Energy sheet. At the bottom where it says:

Solar Electricity
Planned Annual Electricity Generation

Specific Demand
PE Value: Conservation by Solar Electricity
CO2-Emissions Avoided Due to Solar Electricity

-Yes 14" of foam is a lot, but as someone earlier pointed to, it's rather difficult to retrofit more. It's installation costs are very low and can replace some of the expense of groundwork/backfilling. (Back of the envelope calculations for the Smith house: using 5" of foam instead of 14" for the interior area of the slab (not incl. the perimeter thickened edge which had 4"(?))
600s.f. x 56/s.f. x $.10=$3,360
600s.f. x 25/s.f. x $.10=$1,500

Please tell me where I can get a PV system of effective size for $2000.

-PV in nearly all of it's phases is a complete add-on using delicate equipment and skilled technicians (yes I've had a few costly interactions with delicate PV systems). Retrofitting PV is a huge part of that industry and will likely just decrease in cost as huge amounts of public $ are thrown at it. In the cost comparison game, PV may win down the road but in my calculations it's not there currently. Sadly/ disturbingly the idea of conservation has long been counter to the capitalist growth model which is captaining the ship we all inhabit.

"the calculation of the return on investment for a PV array takes the value of the electrical output of the PV array over its lifetime, and compares that value to the cost of the investment. I'm not the only one to analyze PV investments this way; it's the standard way it's done."

Hmmm... Thought the idea of Green Building was to attempt to alleviate the the problems created by the building industry/built infrastructure not just analyze costs. I guess that's just me & my rosy glasses.

New building methods and techniques definitely do need to be analyzed & critiqued and it's to be expected that one that challenges so many traditional methods will take a lot of shots. Passive House is new on this side of the Atlantic so it's getting it's share. Martin, I wish that this article & the one regarding the Boston seat-of-the-pants superinsulated retrofit (clearly not a Passive House) were more interested in analysis than what seems to be just poking holes at the parts you haven't grasped. The science used isn't murky, it's just involved.

Dan Whitmore


50.
Tue, 08/25/2009 - 14:44

Response to Dan Whitmore
by Martin Holladay, GBA Advisor

Helpful? -1

Dan,
Thanks for your thoughtful comments. I agree with many of your points.

1. Straube is not advocating that anyone should plan to "bank" heat in the soil. Rather, he has calculated the heat loss through an insulated slab, and determined at what point further insulation thickness becomes uneconomical.

2. I meant no offense by the use of the word "advocates." The word was intended to refer not to any specific individuals, but to people who advocate in favor of building to the Passivhaus standard. I don't think there are any negative connotations to the word "advocates." I'd be happy if anyone wanted to refer to me as an "energy-efficiency advocate" or an "energy conservation advocate."

3. Every Passivhaus building I have read about includes a heat source. If there are some that have no heat source, I'd be interested to know the climate where they are located and their internal loads.

4. I don't think the cost of the HVAC equipment installed in the typical Passivhaus building -- usually including a HRV, an air-source heat pump, and a water storage tank connected to the air-source heat pump, which scavenges heat from outgoing exhaust air -- is less expensive than the HVAC equipment in an American superinsulated house, which can be as simple as a Panasonic exhaust fan with a timer (or an HRV if you prefer) and a small gas space heater with through-the-wall venting. Another option is a Mitsubishi Mr. Slim ductless minisplit for heating, as was used by Carter Scott in Mass. In other words, I'm not convinced that Passivhaus HVAC equipment is cheap.

5. Dr. Feist told me, "From what I have seen, most builders I have talked with in North America still think that increasing insulation is an expensive thing. … I’m surprised, because insulation is the cheapest thing you can do." He's right — up to a point. But as Straube shows, installing more than about 4 or 5 inches of polystyrene is very expensive. The insulation ends up costing more than 60 cents a kWh. That's more than most of us can afford.

6. You ask, "Please tell me where I can get a PV system of effective size for $2,000." Well, my first PV system was a $275 PV module hooked up to a $35 car battery. The system was effective. I powered a radio and one small fluorescent light. Of course, most people want more energy than I was satisfied with back in those days. Here's the point: I can easily design a PV system for your roof that will cost $2,000. The percentage yield for the dollars invested will be very close to the same yield that one would get for buying a $20,000 system (with a few minor adjustments due to economies of scale for the larger system). You invest one-tenth of the capital, and you get one-tenth as much electricity. But in either case, your meter turns backwards on sunny days. And the yield per dollar invested is identical.

But Straube isn't saying that you have to buy the $2,000 PV system. He's simply saying that 60 cents per kWh is probably a good place to stop when it comes to insulation investments. Once these investments are made, you're done. You end up with a cheaper house -- or perhaps with $2,000 more to contribute to your window budget.


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