musingsheader image
0 Helpful?

Can Foam Insulation Be Too Thick?

Determining the best thickness for sub-slab foam

Posted on Aug 21 2009 by Martin Holladay

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.”

Tags: , , , , , , , ,

Image Credits:

  1. Edwin Dehler-Seter

Aug 25, 2009 3:53 PM ET

Response to Dave Brach
by Martin Holladay

Thanks very much for your thoughtful response. I agree with you completely when you wrote, "If it turns out that eschewing PV in lieu of thicker insulation is not 100% cost-effective, the simplicity, rigor, technical elegance, and usefulness of the tool itself far outweighs this consideration."

Right! That's what I tried to get across in the last paragraph of my essay: choosing thicker insulation over PV is a defensible position, even if the insulation is rather expensive. That's why I'm surprised at the vociferous objection to what I wrote.

Aug 30, 2009 11:02 PM ET

Glad to see someone else working the insulation / PV issue
by m_sev

Martin / John -

It's amazing to me how few people are willing to run a few simple hand calculations to understand the diminishing value of increased insulation versus the somewhat diminishing costs of PV.

I made the observation about diminishing value of insulation while working at BSC; the difference in yearly utility cost on one project taken from a R-40 roof to a R-60 roof was ~$20/yr in the Boston area. If insulation is free and easy to install, have at it, but if it costs any significant money, check your assumptions, and ignore emotional appeals to the 'some is good, more is better' philosophy. Plotting U-value vs R-value yields a log function, and highlights that addressing the lowest R-values like air leakage and windows tend to be higher priorities than taking a slab from R-20 to R-40 or a roof from R-40 to R-60 Changing from R-1 to R-2 cuts heat flow by 50%, R-2 to R-3 cuts heat flow an additional 17%, R-3 to R-4 an additional 8%, changing from R-40 to R-60 reduces heat flow by ~1%. What's the payback on that and when did this stop making sense?

My view is that cost effectiveness of insulation probably crosses the cost effectiveness of PV somewhere in the R-20-40 range (walls/roofs), but doubtful many people would agree. It's very easy to quantify the cost of PV (~$8/watt), but quantifying costs of increasing insulation thickness in new buildings gets lost and distributed through a whole series of decisions and trades from the foundation to framing to insulation to siding to finish carpentry, and therefore is not as easy to compare. In retrofits, however, it's much easier, and I think PV is probably cost competitive down in the R-10-20 range, since the costs of retrofit tend to be so much higher.

On the other hand, the value of PV is relatively constant, since every next PV watt purchased yields 1-1.25 kWh/yr if it's on a roof (flat or facing between SE and SW), not in the shade in most parts of the US. In fact, the cost per watt of PV goes down slightly as the system gets larger, since overhead and mobilization costs are distributed by a larger number of watts. Oh, and PV installations aren't usually complicated holistic affairs like insulation upgrades, and have very precitable results, so after the basic air-sealing and retro-insulating techniques are likely to be a more reliable performer than way-thick insulation.

My question for the PH people is: How did someone arrive at 15 kWh/m2? Why not 22, or 7? If there is no consideration for economics, how about 0 kWh/m2?, especially since the whole of the PH standard allows 105 kWh/m2 for non-heating loads for a total of 120 kWh/m2/yr. If there are ~6 months of heating, essentially, PH allows 67 kWh/m2 for heating (105/2 +15), since almost all of that energy input is keeping the place warm (excepting things like clothes dryer exhaust that cross the building enclosure). And what about different climates - it doesn't really make sense to have the same standard for sunny southern California and for northern Minnesota. I have to say I think the PH standard could use some work - I think the media attention it has gotten is great, but technically, it seems like a 'one size fits all' approach with enough complications that the media doesn't really understand what they are essentially promoting. I'm all for energy efficient buildings, but how about a more holistic standard like the USDOE Building America Benchmark, why can't that get more headlines?

On the durability of PV or heat pumps versus insulation, I wouldn't assume that foam insulation is a forever product, since most of them have no critter repellent, and the thermoplastic insulations (EPS, XPS) seem to shrink over time (2% is allowed, ~2" on an 8' board). Although Straube wasn't concerned about insects and foam at my last interchange with him, I know of enough problems in surprising places that I'm certainly not convinced - in fact, my point is from the other side - why wouldn't insects move right in? - foams make some good livin' for insects. I don't think foam's problems are difficult to address, there just needs to be motivation to do so from industry experts...ideally before those long term payback foams are being torn off buildings due to infestations of insects. Pennies to the manufacturers now, thousands to homeowners and insurers later. That would change the PV cost question, now wouldn't it?

Anyway, thanks for getting people to think harder about the relative value of options for reaching lower energy consumption. The math says that eventually this will become common knowledge, but it's a surprisingly emotional issue at this point and few people seem wiling to pick up a pencil or calculator to double check their math.


Aug 31, 2009 8:15 AM ET

On the origin of 15 kWh per square meter
by Martin Holladay

Thanks for your detailed and thoughtful comments. One of your questions — "How did someone arrive at 15 kWh/m2? Why not 22, or 7?" — gets to the heart of the Passivhaus discussion. It is a question often raised in the U.S.

I raised the same question when I interviewed Wolfgang Feist in 2007. His answer was vague and unsatisfying, but it did offer a clue. Here is the exchange:

MH: "What do you say to critics who question the 15 kWh per square meter goal, calling it arbitrary?"

Feist: "The definition of a Passivhaus doesn’t need any number. As long as you build a house in a way that you can use the heat-recovery ventilation system — a system that you need anyway for indoor air requirements — to provide the heat and cooling, it can be considered a Passivhaus."

Of course, Passivhaus designers sweat buckets to achieve 15 kWh per square meter, so it's a little flippant to say that "the definition of a Passivhaus doesn’t need any number."

Feist's insistence on the centrality of an HRV provides an important clue to the origin of the "Magic 15" number. Here's what I wrote in the introduction to my interview (Energy Design Update, January 2008):

"In Central Europe, the vast majority of Passivhaus designers choose to deliver space heat through a home’s ventilation system. This method imposes certain limitations; in Passivhaus buildings, ventilation airflow is usually in the range of 0.3 to 0.4 air changes per hour. Obviously, ventilation air cannot be delivered at unacceptably high temperatures; Feists’s Passivhaus Institut advises that ventilation air should be no hotter than 122°F. These criteria establish limits to the amount of heat that can be delivered by a Passivhaus ventilation system."

So, it APPEARS that this was the reasoning behind establishing the number 15:

1. Space heat must be delivered through ventilation ducts.
2. The ventilation rate shall be 0.3 to 0.4 air changes per hour.
3. Air temperatures of delivered air shall be no higher than 122°F.
4. The best windows in Europe are U-0.14 windows; the best achievable air tightness is 0.6 ACH @ 50 Pa; with these limits specified, the best houses in a central European climate will need 15 kWh per square meter for heating.

Once established, the 15 kWh per square meter is now being applied in Minnesota, where it is much more difficult to achieve than in Germany, which has a milder climate.

Sep 2, 2009 1:55 PM ET

#6 of six e-mails from John Straube
by Martin Holladay

John Straube e-mailed:
"The conversation on slabs was useful. Got me thinking, looking up papers, etc., etc.

"All that I have uncovered has strengthened my argument. People have sent me their 2-D and TRNSYS calculations off line. I found several new papers, I ran the BaseSim model and reviewed all the old Canadian basement research. And over R-20 is simply impossible to justify. R-10 under basement slabs seems like a safe number for today's understanding of the future, but someone who is really paranoid of the future might consider R-20. Slab-on-grade not much different, assuming you insulate the vertical frost walls to protect against frost getting under the footing."

Sep 3, 2009 8:53 AM ET

everyone is right :-)
by Marc Rosenbaum

WARNING: Content is potentially asinine, read at your own risk!

When I first learned about PH over 10 years ago, one of the most baffling things was the amount of sub-slab insulation. Now that I have been through the PHC training and have worked with PHPP, I think I understand the answer a bit better.

1 - As Martin quotes Straube quoting me, there are only so many "dials" one can adjust to meet the PH heating criterion of 15 kWh/m2/year. In North America, we have less advanced windows and doors (esp. doors) and ERVs/HRVs, so those dials get maxed out quickly in northern climates. You may be able to hit a lower ACH50 rate (easier if the house is bigger) and pick up some more. You've optimized the south facing glazing. What's left is the opaque surface thermal conductances. The lowest marginal cost in most situations is below the slab.

2 - I don't necessarily think PHPP is far off in calculating the heat loss to the ground. Please recall that this is a 'simple' spreadsheet but it has been based on much more sophisticated simulations and well vetted with energy measurements - i doubt we have anything even close in North America in terms of vetting predicted vs. actual building energy use. PHPP calculates a Ground Reduction Factor to reduce the degree days used for losses to the ground. It uses a single factor for below grade walls and slabs. I typically see factors in the range of 0.5 - 0.55 in my calcs for upper New England. Using Burlington VT climate data in PHPP I got a GRF of 0.55 for a slab on grade house with an R-55 slab. The average ground temperature used (data is monthly) is between 52-53F. I calculate that the modified HDDs across the slab is 205 days x 15.5F, or 3178 HDDs (205 days is the predicted length of the heating season). That's about 41-44% of the annual HDDs for Burlington. Recognize that the losses near the edge are higher, and the GRF of 55% seems fine. I have done similar calcs for a heated basement building in Boston climate.

Finally, as to whether PV or sub-slab foam is more cost effective - there is certainly a point where load reduction should hand the baton over to renewable generation. I don't think that the costs we use for PV include depreciated output over time, maintenance (small), and replacement of failed components (like my inverter that died). Same is true for heat pumps. The cross-over point between the two needs to take this data into account. And if you think that in the future our ability to manufacture large amounts of hardware might be affected by peak energy, then all the more reason to do something once that never needs re-doing.

I look forward to the visit in October of Dr. Feist and hope we can discuss some of these issues with him directly. In the meantime, the reason I have gotten involved with PH is that I think it's rigorous targets are in the ballpark of what I think is needed for our building stock. It has some flaws when imported into North America IMO, and discussions such as this can help push the process forward if we keep our wits about us. In full disclosure i need to say I'm on the Board of PHIUS, I've taken the PHC training, and have also begun to teach in PHC trainings, so I'm in pretty deep, yet not without a critical view of the process.

Sep 3, 2009 9:51 AM ET

A question for Marc
by Martin Holladay

Thanks for posting. Your voice of reason is appreciated; like you, I tried, in my August 23 post, to affirm (too late, evidently) that both Feist and Straube are right.

For me, the most interesting sentence in your post was this one: "There is certainly a point where load reduction should hand the baton over to renewable generation." Yet (perhaps wisely) you remain silent about pin-pointing when that baton passing should occur.

Am I correct that the PHPP software (unlike the BEOpt spoftware) never considers the question of whether a proposed building envelope measure is more expensive than PV? In other words, there is no warning that pops up when a Passivhaus designer goes too far in the direction of non-cost-effective insulation?

Sep 3, 2009 10:32 AM ET

by Marc Rosenbaum

Martin's question:
Am I correct that the PHPP software (unlike the BEOpt spoftware) never considers the question of whether a proposed building envelope measure is more expensive than PV? In other words, there is no warning that pops up when a Passivhaus designer goes too far in the direction of non-cost-effective insulation?

Yes - there is no cost-effectiveness analysis going on in PHPP. I'd hate this if it didn't let me toggle the assumptions beneath the calcs.

Sep 3, 2009 7:51 PM ET

Compared to what?
by John Brooks

Non-cost-effective insulation?

Cost effective compared to what?
Compared to the way we have always done things?
Compared to doing it wrong?

I can understand comparing to other less costly measures that we could take to improve the enclosure.....

I do not understand comparing to the cost of adding PV.
Most of the neighborhoods in North Texas do not allow PV... Or solar clothes dryers ............
It is prohibited in the Deed Restrictions.

Sep 4, 2009 3:50 AM ET

Yes, John — compared to PV
by Martin Holladay

I'm surprised that comparing the cost of insulation to the cost of PV seems such a stretch to so many people, especially those following the Passivhaus guidelines.

For those of us who have been following (over the past 7 or 8 years) the discussions surrounding the design of net-zero-energy houses, this is not a new concept. It has been standard operating procedure.

If anything, PV has been criticized as too expensive. PV-generated electricity is so costly that many designers say, "Wo! Why go that far? Why burden the homeowners with expensive electricity? I'll just stop short of adding PV, and they'll get the least-cost house."

Now Passivhaus advocates are strongly defending an even more expensive option — more expensive than PV. As I've said repeatedly, it's perhaps defensible. But it's surprising.

Sep 4, 2009 2:27 PM ET

Whole systems design?
by Mark Siddall

Apologies for the length of this one – should have read all the posts before putting finger to keyboard.

Mark (M_Sev),
As Martin indicated the 15 kWh/m2.yr ties in with the ventilation system but in truth with derived from two factors (Dr. Fiest confirmed this to me in person earlier this year). These factors are:

1) the ability to heat a home using an full fresh air heating system i.e. it does not re-circulate the air, and
2) passive solar gains.

Item 1, along with avoiding the pyrolosis of the dust in the air (occurs at 50C) gives rise to a peak load requirement of 10W/m2 (30m3 per person per hour on the basis of 30sqm per person). Once this target is achieve this allows central heating systems to be removed and the ventilation system to be to supply both fresh air and any heating requirement. The capital saving from omitting the central heating system allow this same money to be reinvested in the building envelope i.e. with good design you are able to improve the envelope at no additional capital cost.

The target of 15 kWh/m2.yr has been developed based upon whole systems design and life cycle analysis. It is fair to say that the 15 kWh/m2.yr was developed for a central European climate. More challenging climates can, and have, been able to satisfy the standard a little or no marginal cost (in Sweden PassivHaus schemes are being built at no marginal cost.)

Depending upon the size and geometry of the building the 10W/m2 may result in energy consumption of <15 kWh/m2.yr but in some cases it may be up to ~20 kWh/m2.yr (say smaller homes of 70sqm). This is energy performance is irrespective of orientation. By making use of passive solar gains, item 2, the 15 kWh/m2.yr target becomes much more easily achievable – optimise orientation and glazing areas.

Achieving the 15 kWh/m2.yr target is a careful balancing act so it is worth recognising that there is a point in some climates, say northern climates where daylight is not always in abundance it may not be possible to exploit the solar gains during the winter, the 15 kWh/m2.yr target can push you towards increasing the glazing area beyond the point at which the 10 W/m2 can be achieved - this is why in Sweden attention is paid to daylighting and orientation rather than full blown passive solar and why the PassivHaus Institut also allows certification using the 10W/m2 peak load ( in some cases this means an annual energy consumtpion of about 20 kWh/m2.yr).

0 kWh/m2.yr has been explored in Germany this was found to require technologies that resulted in 1) significant capital costs 2) many new products 3) resulted in a whole life energy demand (including embodied energy) that was greater than that of a Passivhaus. This included PV and SHW etc.

I fail to understand where you get you 67 kWh/m2.yr from. 15 kWh/m2/yr is for space heating. 120 kWh/m2.yr primary energy is about 50 kWh/m2.yr delivered energy for the whole home much of which will at the end of the day result in useful internal gains (I understand that best practice Passivhaus design achieves about 70 kWh/m2.yr primary energy).

Given the whole systems analysis and life cycle costs it does make sense to have one energy standard. Why? Because the MVHR and the airtightness are the backbone of the concept as a consequence when you design in different climate the building envelope - U-values/ R-value and the efficiency of the MVHR for that matter - that adapts to the climate.

It strikes me that focusing upon the elemental approach - a wall, a roof, a floor, or a window - squanders the any ability to engage with whole systems engineering - by taking this elemental approach you can 'pessimise' the whole.

The whole systems approach that is in many ways inherent to PassivHaus has been shown to be cost effective in Europe, and recently there was an interesting study from the USA in the journal "Energy and Buildings" that looks at Net-Zero Energy Homes and Katrin's PassivHaus - with a theoretical PV array to achieve "zero". The study found that PassivHaus was one of the cheapest ways of achieving net-zero energy (though it did recognise that at this time there may be climatic limits to the economy).

Obviously there will be a cost limit to what is affordable but this has to be viewed on a project by project basis over a realistic time frame, say 25-30 years, and you have to choose the correct datum - i.e. say an uninsulated building or the regional/national average stock rather than current regulations (if you don't do this you can't compare really apples with apples). With this in mind I recently tweaked a copy of PHPP so that I can calculate the net present value of the energy saved - and then the cost /kWh saved (as opposed to bought) for that period. As long as the NVP for the whole building remains within acceptable cost boundaries then you have an affordable solution. [ I've not managed to use my PHPP bolt-on for a project yet but one of the great things about PHPP is that you can make these tweaks and alterations (though I should note that if you want a Certified home you can't use a copy that you've tampered with!) ]

So all in all I agree with Marc R that there is a point a renewables may prove to be more cost effective than insulation and I would add that this point would need to be climatically considered (there is now one size fits all) over the whole life cycle cost of the building.

So where does this leave EPS? If you made a whole house, PassivHaus or not, using EPS insulation it may turn out with a NPV that is to high - to me this just suggests that the type of insulation should be refined to address whole life costs (this may be a result of climatic considerations requiring a certain resistance, due to cost/supply issues or some other factor). Here you could optmise sub-components, such as any EPS forming a slab on grade, by improving the U-values elsewhere and/or using cheaper insulation and by reducing that of that used under the slab. To my mind this is just good value engineering practice and has nothing to do with PassivHaus.

I hope this contribution is recieved as constructively as it is intended.

Kind regards,
Mark S

P.S. Martin, the average pressure test result in a European PassivHaus is about 0.37 ach/hr @50pa. The world record - as I understand it - was set in Canada 0.15 ach/h2 @50pa way back in 1981.

Sep 4, 2009 2:59 PM ET

by Martin Holladay

Mark Siddall,
Thanks very much for your informative post.

A couple of points that Passivhaus advocates often mention seem surprising to U.S. ears. One of these is the idea that if you deliver all of a building's space heat via ductwork, then you have "omitted the central heating system." You haven't, really -- you're just delivering the heat through ductwork instead of hydronic pipes.

It also seems a little odd to stress the advantages of a heating system that "does not re-circulate the air." It's all fine and good to use 100% fresh air in your ventilation ducts, but to U.S. ears it doesn't seem such a crime if an HVAC system includes some recirculation. Most of the arguments against recirculated air (or "scorched" air) are unscientific.

What I admire about the Passivhaus standard is the fact that Passivhaus buildings have very low energy requirements. However, Passivhaus designers sometimes exaggerate the negative consequences of building a building that uses a little more energy than a Passivhaus building, or delivers the heat through a method that is different (for example, not via ventilation ducts). It's good to have a goal. But U.S. designers who design buildings that require 22 or 27 kWh per square meter for heating in very cold climates have not designed a bad building. They just haven't designed a Passivhaus building.

Sep 5, 2009 10:51 AM ET

Martin, I don't know how it
by Mark Siddall

I don't know how it is in the USA but here in Europe hydronic heating is the principle means of heat delivery. In houses with poor airtightness heat recovery and heating via air is very inefficient. For heating the specific heat capacity of water being that much greater than air, and for heat recovery a leaky building severely impairs the efficiency of the heat recovery.
On this basis if an incremental approach is taken to a traditionally conceived building one improves insulation standards and airtightness. When the airtightness is at about 3-5 ach @50pa whole house mechanical ventilation becomes necessary for indoor air quality (VOCs, moisture, condensation, mould etc. i.e. the usual IAQ issues). Once you get to about 1.5 ach @50pa heat recovery starts to become economically viable - the cost effectiveness increases as the airtightness improves even further. In a low energy building, or even moderate or poorly insulated building, a decent airtightness leads to the inclusion of both hydronic heating and a ventilation system (examples include Swedish homes and the German Low Energy standard - say 55-65 kWh/m2.yr.) By pushing the envelope further, to PassivHaus standards of performance, the need for hydronic heating systems can be removed.

I appreciate that the recirculation of air for heating is much more common in the USA. Much of the theory behind the PassivHaus standard is based addressing comfort standards by low energy or passive means. With regard to this technique there are issues relating to comfort that arguably need to be considered (drafts caused by air movement at to great a velocity and acoustics i.e. noise from fans etc.)

With regard to the energy consumption - you say that "U.S. designers who design buildings that require 22 or 27 kWh per square meter for heating in very cold climates have not designed a bad building." I have to say that I agree. As noted above if energy performance is actually in this territory (when assessed using PHPP) then the designs are almost to PassivHaus standards if the all air heating approach is utilised - see the 10W/m2 element of my post above (PassivHaus buildings in Nordic countries above 60 latitude have heating energy demand 20 - 30 kWh/m2 according to location,

Personally I have no interest in exaggerating claims about "lesser" buildings. I am interested in discussion of comfort criteria particularly with regard to ISO-7730 - the comfort standard that underpins much of the PassivHaus concepts. If other low energy performance standards can demonstrate comparable performance, and cost efficiency, then I would be interested in hearing about them.


Sep 5, 2009 1:33 PM ET

Most U.S. houses deliver space heat by ducts
by Martin Holladay

In the U.S., most houses get their space heat via ductwork. In that way at least, they resemble Passivhaus buildings. Only a small minority of U.S. houses have hydronic heating systems. In the U.S., a house that delivers heat through ductwork is considered to be a house with a central heating system; whereas in Europe, many Passivhaus builders insist that their homes (which deliver heat through ductwork) have no central heating system.

Those of us who advocate superinsulated construction have, since the early 1980s, been striving to reduce the air leakage rates of our buildings to an absolute minimum. Heat-recovery ventilators have been sold and installed in Vermont homes since the early 1980s. The superinsulated approach is not incremental. It advocates taking extreme measures to air seal houses.

However, there is much more diversity in heating systems in the U.S. than among Passivhaus builders. Some superinsulated houses are heated with small sealed-combustion gas space heaters with through-the-wall venting; some are heated by a single hydronic baseboard on a loop from the water heater; some are heated by small electric resistance baseboard heaters; some have a woodstove. We aren't as fixated on delivering heat only through ventilation ducts that deliver only 100% outdoor air; this rule seems somewhat arbitrary and unnecessarily limiting.

Sep 5, 2009 3:39 PM ET

Space heating strategies
by Mark Siddall

PassivHaus does restrict the technical means of providing space heating, all the means of heating that you mention can be used. The all air heating offers the most cost effective strategy (avoided labour, materials and maintenance) this is the only reason for a preferance - indeed in the UK some of those that support the PassivHaus approach like the idea of retaining some hydronic heating to enable greater zone and user control (personally I must confess that I am skeptical about this theory/ approach as the studies that I've read suggest that, over time, any the internal temperature differentials will tend to iron themselves out).


Sep 7, 2009 12:35 PM ET

HRV payback?
by John Brooks

Quoting Mark Siddall (from this blog):
"Once you get to about 1.5 ach @50pa heat recovery starts to become economically viable - the cost effectiveness increases as the airtightness improves even further."

Quoting Martin:
(from a GBA question)
"2. Heat-recovery ventilators are NOT cost-effective. In other words, the heat that is recovered is not enough to justify their high purchase price. However, they are the most effective available ventilation systems, and have the lowest operating cost."

My question.....
Is the equipment that much better in Europe...or is Martin being too negative?
Is Heat Recovery really a folly with no economical payback in the US?

Sep 7, 2009 1:27 PM ET

Heat Recovery a folly in the USA? Not as I see it.
by Mark Siddall

I'd say that there are two issues that need to be considered. The efficiency of the HRV and the specific fan power. The efficiency should be >75% which is, I think, available in the USA - though testing methods may differ!

The specific fan power (SPF) may be more tricky to address. The SPF is determined by the fan motor and the design (small ducts make the fans work harder due to friction). If the fan power is too high, i.e. consumes to much energy, then the efficiency of the whole system is degraded - look for electrically commutated fans as these are more efficient. As I see it there is no physical or technical reason why HRV should not be an option.

I should confess that my suggestion for an ach @50pa of less than 1.5 is based upon my assessment of the mild climate UK and UK costs for energy and MVHR systems (i.e. not those of the USA). In a more challenging climate the whole life cost of HRV may be affordable/ suitable at less stringent levels of airtightness (but why would you allow less airtight construction? you'd only end up with drafts and discomfort.) Ultimately you'll have to do your own NPV calcs if you are solely concerned with the fiscal benefits of MVHR.

In terms of airtigthness a starter for ten would be Canada's R2000 scheme, developed in 1984, requires HRV and has an airtightness requirement of 1.5 ach/hr - but why not employ best practice and target 0.6? (It is simply a matter of skills (design and construction) and does not require more technology.)

Smart shopping is what is required, I'm sure that suitable products are out there if you look hard enough (even if they do not quite achieve the same standards of performance as PassivHaus units I'm sure that you can source some half decent products.)

Sep 7, 2009 2:58 PM ET

I Believe
by John Brooks

I believe that we (North Americans) can build airtight
thermal bridge free
Uber-Insulated and well ventilated Low Energy Homes
I Believe...we can do it
But we have to believe we can do it.

We should not wait until 2030

Sep 8, 2009 4:30 AM ET

HRV efficiency
by Martin Holladay

According to a 1998 study by researchers at Lawrence Berkeley National Laboratory (Roberson, Brown, Koomey, & Greenberg), the cost of operating an exhaust-only ventilation system averages 56 cents per day, while the cost of operating an HRV averages 49 cents per day, in a hypothetical average house. (These numbers represent the averages for several climates.)

Let's say that a good Panasonic exhaust fan on a timer costs $400 to install, and the HRV costs $2,000 to install. A simple payback for the $1,600 incremental cost for the HRV would be 62 years.

Of course, I'm sure readers will rush to challenge these assumptions. I'll be pre-emptive and do it myself: Those cost estimates are outdated -- they're from 1998! A good exhaust-only ventilation system costs more than $400 to install! I can install an HRV for less than $2,000!

Okay, okay. You get the idea. One's conclusions depend on one's assumptions. These are some figures -- and they are fairly reasonable figures, but they are open to challenge. Here's the point: one chooses an HRV because it is a great ventilation system -- it provides good fresh air delivery at a low operating cost. One doesn't install it because one hopes that the $2,000 investment will yield a fast payback.

Sep 9, 2009 12:33 PM ET

Back to a whole systems approach
by Mark Siddall

I agree of course that cone's conclusions depend upon the assumptions: Regarding the "cost of operating an HRV averages 49 cents per day" - is this based upon the cost of electricity alone or does this include the energy benefit from heat recovery? What was the efficiency of the heat recovery? What was the specific fan power? Was this whole house MEV or intermitant localised MEV?
Also, and I appreciate that this muddies the discussion regarding be building fabric, but if heating is via an all air system the cost of the heat distribution system can be discounted. (This is why whole systems design is preferable to the discussion of individual components.)


Sep 9, 2009 2:11 PM ET

Ventilation study assumptions
by Martin Holladay

The referenced ventilation study notes, "Ventilation operation costs include ventilation fan energy, the cost of tempering ventilation air, and the cost of tempering infiltration attributable to mechanical ventilation." The study is available online:

"if heating is via an all air system the cost of the heat distribution system can be discounted." I assume that you mean that the blower energy degrades to heat. If the blower is within the conditioned envelope, as it should be, the heat from the blower motor is useful in the winter and detrimental in the summer. The same can be said for a hydronic pump motor, of course.

Sep 10, 2009 4:51 AM ET

Whole systems
by Mark Siddall

What I mean is that you gain multiple benefits from single expenditures - the all air system supplies heat and fresh air, two functions rather than one. This avoids capital cost and reduces payback periods.


Sep 10, 2009 4:51 AM ET

An Interesting Conclusion
by Mark Siddall

I’ve not read this in detail but the conclusion is interesting. Compared to an uninsulated concrete basement (case G47; 35,457 kWh), the largest reduction in the annual sensible heating load (30,667.87 kWh or 86.5%) was achieved using R40 sub-slab insulation, and R50 interior wall insulation (case A15; 4,789.13 kWh). As expected, A15 also has the longest payback period. Although this alternative has the longest payback period (2.3 years), it does show that at heating and construction costs current at the time of the study, even installing a high level of insulation has a short payback when compared to uninsulated basement walls.

R40 = U-value of 0.14 W/m2K
R50 = U-value of 0.11 W/m2K

A payback of 2.3 years is not long, furthermore it is a ROI of 43% per annum – that’s damn good business. There are of course faster paybacks using lesser specifications but this would also have to be balanced against life cycle costs – you pay energy bills for the life of the building.


Sep 10, 2009 5:19 AM ET

Interesting study
by Martin Holladay

Thanks for pointing out an interesting study. (If any readers are confused about what Mark is talking about — as I was at first reading — his comments refer to a study, "Analysis of Basement Insulation Alternatives," that is available by clicking the link at the bottom of his post.)

A couple of points:

1. The study concerns insulation levels in Yukon, Northwest Territories — an extreme climate that is considerably colder than Minnesota.

2. Fuel oil cost is assumed to be $1 per liter ($3.79 per gallon).

Although the researchers calculated the payback period of thick insulation COMPARED TO AN UNINSULATED BASEMENT, they didn't calculate the payback period for adding R-10 additional insulation to a basement that was already insulated to R-30, compared to a basement insulated to R-30. These calculations will, of course, yield very different results.

Here's what I found interesting: "Compared to an uninsulated concrete basement (case G47; 35,457 kWh), the largest reduction in the annual sensible heating load (30,667.87 kWh or 86.5%) was achieved using R40 sub-slab insulation, and R50 interior wall insulation (case A15; 4,789.13 kWh). As expected, A15 also has the longest payback period. Although this alternative has the longest payback period (2.3 years), it does show that at heating and construction costs current at the time of the study, even installing a high level of insulation has a short payback when compared to uninsulated basement walls. Some of the wall and sub-slab insulation options have very similar energy savings. For example, R50 wall insulation and R20 sub-slab insulation (A9) result in an annual heat loss that is only 566 kWh (or 2.7%) higher than the best case A15 with R50 wall insulation and R40 sub-slab insulation."

Sep 11, 2009 3:24 AM ET

Martin, Since when was the
by Mark Siddall

Since when was the discussion about retrofitting insulation to existing floors? We are talking about new buildings (and at best retrofits to poor building stock.) Upgrading an R30 to R40 will indeed result in a very different situation but that was not the central argument of your article. In my view "retrofitting" a theoretical insulated building that had not been built and comparing this to a better insulated one is a very perverse logic indeed. For energy efficiency to be calculated properly, and to avoid developing a false impression of diminishing returns, you have to compare all insulation strategies to an uninsulated building. [Editor's note: text corrected per request of Mark Siddall.] It is the only appropriate datum when looking at whole life costs - or payback periods and ROI if that is the interest.

In my view, with regard to energy efficiency, payback periods are a fools game and ROI is not much better. A building lasts for 50+ years (over 100 in the UK), mortgages last for 25 years (in the UK at least). Provided that the cost of the energy efficiency measures is less than the cost of the fuel over the life span (I choose 25 years) then the efficiency measure is affordable. The best way to think of this is NPV - to get a better grip on the numbers this can be converted into the cost per kWh saved, rather than the cost per kWh bought. (I prefer to use cost per kWh saved rather than ROI as this a more immediate, and relevant, unit that also serves to avoid certain distortions resulting from whole life cost analysis using the NPV alone.)

Indeed other sections of the conclusion are interesting, however, this goes back to the central thesis of payback periods and the nonsensical nature of them with regard to buildings.

Thanks for pointing out the climatic differences, being a foreigner I had little appreciation for the climate mentioned in this report.


Sep 11, 2009 3:30 AM ET

by Mark Siddall

Erratum: "you have to compare all insulation strategies to an uninsulated floor slab" should read "you have to compare all insulation strategies to an uninsulated building"

[Editor's note: correction made.]

Sep 14, 2009 9:01 AM ET

The topic is being discussed elsewhere
by Martin Holladay

This interesting dialog continues ... on a different page:

Nov 20, 2009 12:49 PM ET

British reflections on these issues
by Martin Holladay

BuildingGreen's Mark Piepkorn recently posted a link on his BuildingGreen blog to a British Web site that discusses some of the issues addressed here.

To read Mark Brinkley's thoughts after he toured some Passivhaus buildings in Hanover, German, click this link:

Nov 23, 2009 12:51 AM ET

A more simple insulation case study in action
by Shawn Busse

Here's a link to an article on our project where we attempted to quantify the cost of adding additional layers of insulation.
We're on a limited budget so going from 2" to 4" was a consideration. Achieving the R60 or R80 of the passive homes is way beyond the capacity of our pocketbook, so I wrote an article on how this relates to the "ordinary" homeowner.

Nov 23, 2009 5:53 AM ET

by Martin Holladay

Thanks for showing us your calculations and procedure. Performing such calculations is always fruitful and stimulates thought.

Dec 27, 2009 2:03 PM ET

by Riversong

It's a shame that this "discussion" is generating more heat than light. If only we could harness this output to heat our homes, we would not have to worry about insulation at all.

Ironically, both sides of the dispute use the same premise to arrive at completely opposite conclusions – and the premise is fundamentally flawed.

m_sev ( a pseudonym who says he worked at BSC) argues that "plotting U-value vs R-value yields a log function" when, in fact, it's a hyperbolic function (the one is the inverse of the other). And that "changing from R-1 to R-2 cuts heat flow by 50%, R-2 to R-3 cuts heat flow an additional 17%, R-3 to R-4 an additional 8%, changing from R-40 to R-60 reduces heat flow by ~1%."

This sophistic mathematical exercise attempts to undermine the argument for high insulation levels. But the analysis compares the first incremental reduction in heat loss to the absurd base condition of an R-1 thermal envelope and then measures each successive incremental reduction as a percentage of the initial incremental savings rather than as a percentage of either the base case or the starting point of that incremental change. In other words, while going from R-1 to R-2 offers a 50% reduction in heat loss, stepping up to R-3 offers a further 33% improvement (not 33% of 50% = 17%), and stepping to R-4 offers another incremental improvement of 25% (not 8%). Thus, while there is a diminishing incremental return, it is not nearly as low as m_sev suggests. And making an incremental shift from R-40 to R-60 offers a 33% improvement in thermal performance, not the absurd ~1% of m_sev's faulty math.

It is, of course, true that the last 33% improvement is upon an already very low annual heat load, so the incremental dollar savings is smaller with each equivalent incremental thermal envelope improvement. If one were to compare the energy or dollar savings for each increment to a base case (for new construction), then that base should be a current energy-code minimum home not an R-1 home.

And then PH advocate Mark Siddall, in order to attempt to justify extreme insulation levels, says "you have to compare all insulation strategies to an uninsulated building". If you're considering the benefits of retrofitting a currently un-insulated building, that statement might make some sense. But when comparing the incremental cost and advantage of adding additional insulation to either a new or existing envelope, one must compare each additional incremental improvement to the baseline of that particular increment and to some measure of energy or financial return on investment (which is what Martin has attempted to do in his blog).

This entire "debate" seems more like theater of the absurd than a rational argument. And I continue to find it fascinating that PH advocates (Marc Rosenbaum excepted) get so upset about any challenge that they become verbally abusive. This is the response one would expect from a "true believer" rather than a rational advocate. Is PH some kind of cult?

Dec 27, 2009 2:25 PM ET

Back to the discussion...
by Riversong

I agree that it's difficult, and perhaps misleading, to compare permanent envelope improvements such as slab insulation to shorter-lived mechanical enhancements such as PV. Financial return on initial investment is only one part of the equation. Financial and ecological life-cycle costs have to be factored in (as some here have proposed) - PV production and end-of-life disposal has its own enviromental costs, as does petrochemical foam insulation. And we would do well to follow Amory Lovin's dictum that the cheapest (to society and the world) megawatt is a negawatt - conservation is always to be prefered over production. But that means consuming less by our lifestyles, not using non-renewable materials to reduce the energy impact of a profligate and unsustainable lifestyle.

The whole debate over thermal losses to the ground ignores the thermal mass benefits of a warmed layer of earth surrounding our foundations and underlying our slabs. I've yet to see an analysis of the relative heat loss downward with lower insulation levels and warmer earth compared to higher insulation levels and cooler earth. This is the strategy that Passive Annual Heat Storage and Annualized Geo-Solar systems employ. In most cold climates, there is an insulating snow layer on the ground for much of the winter, and heat loss into the ground does not blow away as it does to the air or radiate to the night sky, but creates a dynamic mass benefit which further undermines the incremental benefit of additional sub-slab insulation.

Dec 28, 2009 5:16 PM ET

two points
by David Whte

I may be too late in commenting here, but looking over the dialog I see a couple of things missing:

1. Dr. Straub's numbers for sub-slab temperatures seem in at least two cases to refer to a central zone of the slab. However, the PHPP analysis effectively applies a UA-averaged soil temperature over the entire underside of the slab, and this is strongly weighted by the colder temperatures near the edge. With adjustment for this, the presumedly too-cold PHPP outputs would match more closely with the research.

2. Is the issue to find minimum life cycle cost of heating the building or maximum cost effectiveness of saving the planet? In the former, the cost must include maintenance, not as a second argument but as part of the basic cost calculation. In the latter, one must include the embodied energy of the PV, which is often left out of discussions because we assume that renewable energy is immune to this. I'm not sure what the embodied energy of PV really is, but note that the PHI uses a PE factor of 0.7 for PV. That's much lower than typical grid electricity, but much higher than zero. This may be one reason that the PHPP does not consider PV a fair strategy in reducing primary energy demand - it does not have equivalent environmental benefit as energy savings using a lower embodied energy strategy.

Apr 7, 2010 11:22 AM ET

comment and simpleton question
by TC Feick

I believe in a pragmatic cost/benefit analysis to any strategy towards sustainable building. In other words, any strategy has to make economic sense over a timeline relevant to the primary user/decision maker to gain widespread acceptance in the marketplace. As one early post pointed out, there is a vast ocean of difference between energy efficiency of stock housing and that of BSC's recommendations and Passivhaus. The latter two approaches will provide very good performance for energy efficiency, agreed? So, how about the delta of the cost between these implementations and the cost savings to take BSC's approach? Seems like there is a good argument to be made for saving some cash in the name of broader acceptance of something approaching an energy efficient home.
Now my question- Why EPS below a slab? I would think that EPS would absorb moisture and affect its R-value?

Apr 7, 2010 11:33 AM ET

Cost versus benefit
by Martin Holladay

T.C. Feick,
Concerning cost/benefit: many people have run cost/benefit analyses for insulation. There are several variables, of course, the most important of which are the climate where the house is located, the presumed future cost of energy, and the expected lifespan of the building.

The colder the climate, the higher one's estimate of future energy costs, and the longer a building is expected to last, the easier it is to justify thick insulation. Most such cost/benefit analyses end up with sub-slab insulation thicknesses in the ranges recommended by the Building Science Corp. The extreme thicknesses advocated by North American Passivhaus proponents fall outside of these analyses.

EPS does not generally absorb moisture; that's why it is commonly used to make coffee cups and dock floats. Of course, you have to specify the right type of EPS — the denser the better. EPS comes in a range of densities; if you want to install it below a slab, don't use the cheap stuff.

May 5, 2011 1:28 PM ET

Edited May 5, 2011 1:32 PM ET.

Under-slab insulation
by Ted Clifton

I appreciate all the good comments on this blog, there is a lot of good, even if seemingly conflicting information here. One thing missing so far, a few people got close....

It is not enough to just insulate under the slab. You must also insulate around the perimeter of the slab to a depth that exceeds the depth of major seasonal fluctuations in the soil temperature, at least to the extent that is cost effective. By doing this, you keep the temperature immediately below the slab insulation closer to 65 degrees, the soil below that 60, etc., and that soil is much less affected by the adjacent soil temperatures that follow the seasonal air temperatures. To my knowledge, nobody has aver done a detailed study to quantify this effect, but we have real-time results that indicate the effectiveness of this strategy.

There are many variables, as each climate zone will have very different soil temperature seasonal variations, and different moisture conditions in the soils will also have a great effect on the transfer rate of energy. It would follow that if you can eliminate the transfer of water beneath your slab, you can also limit the transfer of energy. A closed-cell foam insulation barrier, placed in a vertical position around the slab, would also help with that issue. Our experience suggests that if you are using four inches of foam under the slab, you will receive many times the benefit of an additional two inches of foam under the slab by placing the same two inches vertically around the perimeter of the slab. The amount of foam required is just a small fraction of that required to cover the entire slab area. Since the savings are so great, just go with 4" of foam around the perimeter (in addition to the 4" underneath), and wait for the experts to finish arguing the point as you are saving money in your warm comfortable house, with your PV system supplying energy to your far more cost-effective ductless mini-split heat pump.....

By the way, in many locations around the country, January and February are among the months with the most hours of sunshine! Check it out on Climate Consultant 5.

Jun 1, 2011 7:28 PM ET

Insulation and delta T
by Mike Legge

I am embarrassed to ask this question as it appears so dumb.I understand that delta T is the energy difference across a partition.If
I put 2" of EXP under my slab then the delta T is still the same. Given enough time the slab and foam will come into equilibrium. I will then be in the same position as I was without the foam, but a time interval later.As I see it the insulation delays getting to equilibrium but doesn't prevent it.Is there a factor which denotes the time delay? All vacuum bottles cool with time why not subslab insulation? Cheers and thanks Mike Legge

Jun 1, 2011 8:11 PM ET

Edited Jun 2, 2011 8:35 AM ET.

Response to Mike
by Martin Holladay

If the soil temperature under the slab is the same as the temperature of the concrete slab, then the foam insulation will also be at the same temperature.

However, in winter, a more typical condition is that the soil temperature is at 50 degrees and the slab temperature is at 65 degrees. In this situation, the foam slows the heat loss from the slab to the soil.

The soil is an enormous heat sink, so the heat loss from the slab is unlikely to raise the soil temperature to 65 degrees -- therefore the delta-T is likely to be greater than zero all winter long. In other words, the slab and the soil under the slab never reach thermal equilibrium.

Jun 4, 2011 5:13 AM ET

by Roger Anthony

There is a difference in perception.
In Europe a Passive House is heavyweight and designed with a 400 year life span in mind.
Most homes in Europe are over a hundred years old.
Passive Homes are designed to be heated by the human body and the electrical equipment in the home, freezer, fridge, TV, computer etc.
Fresh air is routed under the ground, and home to benefit from the steady (ish) 12C temperature and is merely topped up with a small in-line heater.
The south facing windows triple glazed probably loose more heat over a typical year than they gain for the home, however they are popular.
From the above you can appreciate that those extra inches of insulation pay for themselves many times over.
In one to four hundred years your typical solar panels will require changing some four to sixteen times.....not at all economic.
Dow brought Styrofoam to market 51 years ago, I started using it 41 years ago, it is in perfect condition, as new, and is still doing the job it was designed for.

Jun 4, 2011 5:37 AM ET

Edited Jun 4, 2011 5:39 AM ET.

Response to Roger Anthony
by Martin Holladay

I agree with your basic proposition, which is why my concluding paragraph states, "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."

A few points about your Passivhaus comments:
1. You wrote, "Passive Homes are designed to be heated by the human body and the electrical equipment in the home, freezer, fridge, TV, computer etc." Actually, all Passivhaus buildings require active heating systems, just like all of the superinsulated homes built in the U.S. over the last 30 years.

2. You wrote, "Fresh air is routed under the ground." Actually, Dr. Feist no longer recommends the use of earth tubes. When I interviewed Dr. Feist in December 2007, he told me, "There were problems [with earth tubes] in northern Europe, especially in Scandinavia. In Central Europe we haven’t
had any hygienic problems so far. Actually, I’m not sure why we don’t have these problems in Central Europe. But I don’t advertise these systems any more, mainly because they are too expensive. If you have a good heat-recovery ventilator, you don’t need it."

3. You wrote, "The south-facing windows triple glazed probably lose more heat over a typical year than they gain for the home." That may have been true in the past, but the best south-facing windows available today can easily gather more heat than they use. To learn more, see Windows That Perform Better Than Walls.

Jul 20, 2011 10:31 PM ET

Thick foam
by Tom Gocze

The diminishing returns on foam is a moving target. Cost of PV's, heating devices and backup fuel are all going to be factors.
My 2 cents worth of comment is that foam is passive. It always works (assuming critters stay out of it!)

PV systems are mechanical systems of a sort and are apt to be down sometime. Storm damage, component failure and other acts of God will leave you with a thermally compromised building, which is antithetical to the Passiv Haus concept.

That being said, as one who has made a living working with foam and foam products, the one place that I would be skeptical of too much foam is in the foundation where insects are most likely to intrude and compromise that system.
Time will sort these system configurations out.

Tom Gocze

Dec 10, 2012 11:18 AM ET

Of course you can have too much foam...
by Skip Harris

When the last cm of foam takes 50 or a hundred or more years for the embodied energy to be saved, let alone the time and money invested, we have left the zone of reason and entered that of religion. There may be other reasons for using that foam: perhaps it allows one to save on mechanical systems or gives extra security and peace of mind for blackouts, but saying that it makes sense from a standpoint of saving energy or money is hard to support.

Dec 12, 2012 7:23 PM ET

And where should the foam be?
by Derek Roff

It's interesting to read carefully through a three-year old article and comments. I didn't see anyone comparing the effectiveness of addition of more insulation to the roof, versus more under the slab. One example cited R-80, another R-60, under the slab, as a way of meeting the Passive House energy use goals. Roof insulation levels aren't mentioned, but I would think increasing insulation in the roof and walls would be much more effective than the second R-40 under the slab. Why are Passive House designers distributing their insulation in the way discussed in this article?

I enjoyed the many interventions from the wise and witty John Straube. I wish he would comment more frequently in Green Building Advisor.

Dec 12, 2012 8:10 PM ET

Edited Dec 12, 2012 8:11 PM ET.

Response to Derek Roff
by Martin Holladay

One reason that more insulation wasn't installed in the roofs of these Passivhaus buildings is that many of them already have R-80 or R-100 insulation specified for the roof -- making the addition of more insulation up there physically difficult, as well as unlikely to return much energy benefit.

I certainly agree with you about John Straube -- it would be great to have him comment on these pages more frequently.

Dec 12, 2012 10:51 PM ET

Too much of a good thing.
by Derek Roff

Thank you, Martin. It's amazing that the energy evaluation software gives any useful credit for such extreme insulation under the slab.

Jan 17, 2013 4:17 PM ET

Storing heat under the structure
by Roger Williams

If I had the money to build, I would build a slab house on a 5 foot frost wall. Insulate the bottom of the 5 feet to R20, the inside walls of the frost wall to R20,backfill and insulate the top(under the slab to R20). Now I have a heat storage capacity of 6000 btu per square foot of storage. A well insulated house of 32 X 60 would require 10M btu for our 8ooo DHD area. 140 sq ft of thermal panel would supply all the heat needed for the house Heat loss to the house would handle the load on the shoulder months and mechanical heat extraction would only be needed in extreme temperatures.

Summer cooling can be handled with nocturnal cooling. PV could supply the rest of the power requirements. Just my 2 cents.

Jan 17, 2013 4:33 PM ET

Response to Roger Williams
by Martin Holladay

Your idea has been tried many times, and each experiment has led to failure.

As I noted the last time this question came up, "To get a useful amount of heat from the sand during the coldest months of the year, the sand has to be hot enough to get water in a hydronic heat distribution system to at least 100°F. And that just isn't going to happen. The sand doesn't get that hot — or if it does, it doesn't stay that hot from early September (when it is likely to be hottest) until mid-November (when you begin to need it). Moreover, the pumping energy is a big energy penalty — parasitic energy that needs to be considered when analyzing possible benefits. Finally, the capital costs of all those extra solar collectors is high — an investment without a significant payback."

Here are two GBA articles on the topic:

From April 2011: Using Sand to Store Solar Energy

From August 2010: Can Heat Be Stored in a Sand Bed Beneath the House?

Register for a free account and join the conversation

Get a free account and join the conversation!
Become a GBA PRO!