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Q&A Spotlight

How to Insulate a Foundation

Can the R-value of a foundation wall really exceed the R-value of the materials used to build it?

Insulating concrete forms are one way of combining a structural concrete wall with rigid insulation. What about building a house whose exterior walls are concrete with all of the insulation on the exterior? Photo by Roe Osborne.

Peter Fusaro is building a high-efficiency house on spec, and his plans include insulating the foundation walls. The question is how.

One option is to apply rigid foam to the outside of the foundation. That would leave roughly 1 ft. of insulation above grade, and Fusaro is concerned about how durable the foam would be.

Another possibility is to use a sandwich of 2 in. of foam between two outer faces of concrete, each 4 in. thick, making an assembly with both structural and thermal properties. He’s been told a wall built that way would have an effective R-value of 19.27.

In a Q&A post at GreenBuildingAdvisor, Fusaro looks for advice. That’s the subject of this week’s Q&A Spotlight.

R-value is inflated

To GBA senior editor Martin Holladay, what Fusaro is describing sounds like the Thermomass Building Insulation System . These panels consist of Dow Styrofoam polystyrene insulation and two outer layers of concrete held together with a grid of fiber-composite fasteners. According to the company, the system is designed to minimize thermal bridging that would degrade the performance of the insulation.

The problem is that 2 in. of extruded polystyrene foam has an R-value of only 10. Concrete contributes very little thermal insulation to the assembly, so the R-19.27 Fusaro cites would seem to be little more than wishful thinking.

“The claim that the thermal mass of the concrete wall increases the effective R-value of the wall is false, although you hear it often,” Holladay writes.

“In some climates, especially with dependable nighttime temperatures below 72 degrees and dependable daytime temperatures above 72 degrees, walls with a lot of thermal mass can lower the energy used for heating or cooling. However, let’s not confuse thermal mass effects with R-value.

“In most parts of Climate Zone 5A [where Fusaro is building], you won’t be getting any thermal mass benefits from a Thermomass wall – especially if the wall is mostly below grade.”

Yet R-19.27 is exactly what Thermomass claims for its wall system. “Due to the mass effect created by Thermomass, a system’s performance R-value can be two to three times greater than the material R-value,” the web site says.

ICFs are an alternative

Insulating concrete forms (ICFs) are also a combination of foam and concrete. They consist of hollow-core foam forms that are stacked in place, reinforced with steel and filled with concrete. They are, in effect, the mirror image of Thermomass walls: concrete on the inside, foam on the outside.

Andrew Lennox suggests them as an alternative for Fusaro, adding that the Canadian Mortgage and Housing Corp. recently completed a study showing that ICF walls “provide a 5-day thermal lag due to thermal mass that contributes to thermal performance over and above the R-value in the assembly.” Plus, he adds, the R-value in a typical ICF wall can reach 28, depending on the particular product.

Gaining a higher R-value than the insulating material alone would provide sounds like the claims Thermomass makes. And, says Thorsten Chlupp, the five-day time lag on ICF walls is “certainly different” than a CMHC study of 2006 would suggest. “The ICF industry is very good about coming up with wonderful claims for their walls,” Thorsten writes. “The only one confirmed in independent testing is their inherit airtightness. Which is, in my humble opinion, the only reason that ICF homes perform well.”

What does thermal mass really do?

Chlupp (whose super-efficient home in Fairbanks, Alaska, called SunRise House, has been featured on GBA in the past), believes thermal mass is a great asset, but only when it’s put to use correctly. “Insulation on both sides of your mass defeats the purpose,” Chlupp writes. “In a heating climate, any thermal energy which travels through the foam layer into the concrete mass has no ability to travel back into the conditioned space. At least I don’t know of any scientific study that showed this differently.

“I was never able to reproduce it in my humble tests … Heat simply does not travel back into the building … which is the case if the thermal mass is not insulated to the interior. So you get some decrement delay within the mass layer, but it is very much limited by its design and cannot find its full potential. If you are after heat storage capacity, decrement delay and absorption capacity, build a concrete wall and insulate on the outside in a heating climate or opposite in a cooling climate. Or use a medium-weight assembly with a thick layer of dense-packed cellulose with a fraction of its embodied energy.

“We shouldn’t forget that ICF walls have about the highest embodied energy there is … and they are expensive.”

John Klingel suggests that Fusaro try to visualize how heat energy will escape the house, and base his insulation plan on that. “Your design will determine where to insulate.” And in regards to ICF construction, Klingel adds this: “The wind blows, the (manure) flows, and where it stops ain’t nobody knows.”

Our expert’s opinion

Here’s GBA technical director Peter Yost’s take:

My favorite way to insulate a basement is still to place all the insulation on the outside of the wall, for two reasons. One, it gives the highest probability that the insulation will be continuous throughout; and, two, it pulls all of the mass of the wall to the interior, giving the best opportunity for the concrete mass to buffer temperature extremes — although this is more about thermal comfort than it is about increased energy efficiency due to thermal mass effects.

Exterior insulation is not without its issues; it is difficult to protect during and after construction, and it can lead to sticky planar transition details as the top of the foundation wall meets the above-grade wall assembly.

Per usual, Martin has nailed this: few climates yield significant added thermal mass benefits (higher thermal mass “effective” R-values), and with most of basement walls below grade, thermal mass effects are further dampened.

Oak Ridge National Laboratory has done quite a bit of research on thermal mass effects in homes. Take a look at this report from ORNL . You can see that the whole-house impact of significant thermal mass is quite modest, even in climates such as Santa Fe, New Mexico, and all of the research is for above-grade assemblies, never for basements.

Additionally, it is telling that in this BSC Building America report, only one high-performance foundation analysis even mentions thermal mass effects (Case Study 12), and even in this case thermal mass effects are mentioned only in passing.


  1. Keith Gustafson | | #1

    I dunno, but I have a different take on the ORNL report, it seems to point out 'significant' savings from thermal mass effects.

    Martin as usual completely correct that mass does not increase r value. because r value is a static measurement. If it was exactly 20 degrees 24/7 all winter long, thermal mass [in a wall assembly] would be meaningless. The world however is dynamic, so a high mass wall really only has to fight the average over 'x' time rather than peak delta T, or so I would guess. The ORNL report link seems to call that worth at least 10 percent.

    ICF and other manufacturers should not lie about R value. There is however value that can be expressed.

    I think intuitively that it is easier to heat a brick structure than a tent, although they have similar r values.

  2. User avater
    Armando Cobo | | #2

    Moisture management is 1st...
    With basements is more important to have the best moisture barrier and details that the kind of insulation and where is installed. As long as R10-R15 min. is installed on the foundation wall (inside or outside) and under the slab, it makes small difference in energy efficiency; so it becomes an issue of budget, preference of insulation and workability. I would spend all my money and effort on the moisture management on the basement walls and slab.

  3. User avater
    Elden Lindamood | | #3

    Tongue-in-cheek effectiveness
    Yesterday I was driving my car at 90 miles per hour! Okay, actually I was going 65 miles per hour, but it was into a 25 mph headwind, so my EFFECTIVE speed was 90 mph. So now what I can't figure out is why, if I was going 90 miles an hour, it still took me 2 hours to travel 130 miles.
    I mean, yeah the ground was only passing under the car at 65 mph, but if my effective speed was 90mph, then I would think I would at least have averaged 77.5 mph after adding in the speed lag.

  4. Roger Anthony | | #4

    Foundation Insulation?
    Surely we insulate to either keep warm or cool at lower cost, with the benefit of improved comfort?
    Therefore my question is. Why insulate a crawl space when it is not used?
    Surely the logic is to insulate as close to our comfort zone as possible?
    With this in mind placing the insulation over the floor, as a floating floor and on the inside of the walls and ceilings, means using less insulation and saving years of costs heating the mass of the building.

  5. User avater GBA Editor
    Martin Holladay | | #5

    Response to Roger Anthony
    There are lots of reasons to insulate a crawl space. Here in Vermont, insulating a crawl space is a necessity, not an option, because plumbing pipes will freeze if the builder forgets to insulate the crawl space.

    In warm climates, builders often install ducts in a crawl space, so those crawl spaces need to be insulated to keep the ducts within the home's conditioned space.

    There is one more reason to insulate and seal crawl spaces: in humid climates, vented crawl spaces don't work. They become soggy nightmares.

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