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Community and Q&A

Thermal Mass 4/2/4 – anyone use it?

Peter Fusaro | Posted in Energy Efficiency and Durability on

I’m doing a new construction spec high efficient house.

I’m concerned about the overall durability with using the Styrofoam on the exterior wall of the foundation that protrudes the 1′ above grade. I still like the idea of having concrete in that area.

I’m looking at this thermal mass system with there 4″ of concrete sandwiched with 2 ” of Styrofoam and then 4″ of concrete with a overall effect of R-value of 19.27
This seems to me like a great solution. Has anyone used this system?


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  1. User avatar
    Mike Eliason | | #1

    How are you getting an r-value of 19.27.

    It's a little more than half that...

  2. User avatar GBA Editor
    Martin Holladay | | #2

    It sounds like Thermomass. But the R-value of 2 inches of XPS foam is still only R-10.

  3. Andrew Lennox | | #3

    You may want to look into an Insulated Concrete Form system. Canadian Mortgage and Housing Corporation (CMHC) just completed a study that concluded 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.

  4. Andrew Lennox | | #4

    Plus the actual R-Value in a typical ICF wall assembly can be up to R-28, depending on the ICF you use.

  5. Thorsten Chlupp | | #5

    Andrew – can you pls post a link to the new CMHC study which shows a 5 day time lag on ICF walls? That is certainly different then their 2006 study ( and any other study I have seen. I would be very interested to see the study your referred to. Thanks much.

    Martin is very right – the expanded polystyrene insulation ICFs make up 98% of its R-value, and at the usual density used on the forms you get at very best an R-5 per inch.
    The ICF industry is very good about coming up with wonderful claims for their walls. The only one confirmed in independent testing is their inherit air tightness. Which is IMO the only reason that ICF homes perform well. Thermal mass is a great assets and I am a big fan, BUT you have to use it wisely or it either offers little or no use. Insulation on both sides of your mass defeats the purpose. In a heating climate any thermal energy which traveled 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 who 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 it’s 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 an 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.

  6. Peter Fusaro | | #6

    I'm in the North East climate zone 5a, they claim that the R-value combined with the concrete increases the R-value 2 or 3 times greater than the R-value in the 2 " Styrofoam board. I like the idea of keeping the temperature changes balance by having the 2" Styrofoam in between the outside and the inside.
    You bring up a great point about when you say if you insulate both sides of your mass defeats the purpose. This is a basement, down the road the home buyer may decide to finish off the basement and more insulation will be added to the interior side, do you see that being a problem?

  7. Peter Fusaro | | #7

    You are correct the XPS 2" R-10 however they claim that the R-value combined with the concrete increases the R-value 2 or 3 times greater than the R-value in the 2 " Styrofoam board. Were climate Zone 5 A and thought this would be a good solution for foundation walls by keeping the Styrofoam in between. What are your thoughts for when this basement gets converted into a finished environment?

  8. User avatar GBA Editor
    Martin Holladay | | #8

    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.

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

    In most parts of climate zone 5A, you won't be getting any thermal mass benefits from a ThermoMass wall -- especially if the wall is mostly below grade.

  9. Peter Fusaro | | #9

    Thank you for your advice I take it with great respect.
    To give the foundation some continuous insulation with Styrofoam would I be better putting 2" on the exterior or on the interior? The basement will be finished.

  10. John Klingel | | #10

    Peter: Have you looked at the path an erg of energy is going to use to escape your house? The path it would take will help determine where to insulate. As I see it, all you can do is insulate to block the erg head on and from doing an endo. Thus, your design will determine where to insulate. That is how I see it, anyway. BTW: In my limited experience w/ ICF aficionados, remember the expression "The wind blows, the (manure) flows, and where it stops ain't nobody knows."

  11. TJ Elder | | #11

    Peter, more the point is that 2" foam might be too little whether it's in the middle, the inside or the outside. Disregarding questions of thermal mass, you can insulate inside or outside and the choice depends on the overall assembly and how it is waterproofed. If you waterproof the outside of the concrete, the insulation can go inside, or if it's EPS you can place that outside against the soil. Exterior insulation can help protect the waterproofing but it also becomes vulnerable to pests or other damage. Think also about the transition between the concrete wall and the frame wall above, because dimensions change depending on which side of the concrete you insulate.

  12. Roxane Johnson De Lear | | #12

    Hi Peter,
    With the design help of Robert Riversong, I built a superinsulated passive solar home in Vermont. My foundation is Thermomass and I am very pleased with the results. My sandwich is 4 inches concrete, 4 inches rigid blue DOW foam, 4 inches concrete. There are benefits to this system besides the R-value. The thousands of fiberglass rods that end up in the forms provide superior structural reinforcement to the walls themselves. I have not had one crack in the wall since it has been poured-now about a year old; The outside wall and the inside wall can expand and contract independently; I have not fear about moisture ever penetrating these walls-if moisture does come through the exterior wall, it will shed down the foam and drain; the cost was not any greater than if the foam had been placed on the exterior, plastered, and sealed-and now my foam is much more protected and will not decay. My 1 foot thick walls are beautiful and remind me of adobe walls in Arizona; My passive solar works beautifully with Pella solar gain windows on the south and east of my house, last winter on even just semi-sunny days I was seeing temperatures of 74 degrees with my thermostat set to 50. See my video of the process here: and feel free to email me at [email protected]- or check out my building experience at my blg My home is now a top-rated Efficiency Vermont home. Best, Roxane

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