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Community and Q&A

Under-slab foam: How thick? Spreadsheet for review

John Klingel | Posted in Plans Review on

I just built a spreadsheet to look at the savings and cost of various thicknesses of foam under a slab. In another thread, I had questioned the PassivHaus use of 14” as being excessive, so I thought I’d take a simplified look for myself, ignoring inflation, etc. This is for zone 8, 14,000 HDD, and $60 for a 4’x8’x4” sheet of 15 psi EPS.

Column B has calcs for my house. Column C is how much money I’d save using each thickness instead of 2”. Column D is the cost of the extra foam (beyond 2”). Column E shows what the money for the extra foam would generate if it were instead invested at 6% APR. This indicates that somewhere around 12” I’d be just as well off to invest the money as put it into foam.

Column F is the savings generated by adding foam in increments of 2”. This is the “diminishing returns” column. Column G shows the cost of each additional 2”(always $3,323) divided by the annual savings for that 2”. If I am thinking right, this indicates that for 4” of foam, I will invest and additional $5 to save $1 annually, and for 14” I’d invest an additional $107 to save $1 annually.

In Row 4 I copied the calculation entry from the last entry in each column, just to indicate whence cometh the entries in that column.This spreadsheet is rough, approximate, and based on certain assumptions, but what I garner from it is “stick w/ about 8” under the slab”.

Let me know what you see, or what/ if I screwed up. If you see a different angle from which to look, let us know.

Thanks. john

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  1. John Linck | | #1

    How many square feet of floor john? Seems like about 3000.

  2. John Klingel | | #2

    3216, including 840 in garage.

  3. Mike Eliason | | #3

    a. don't insulate slab under garage.
    b. in the projects we've migrated on zone 4-6, 4-6" of high density EPS seems to be the sweet spot. but all those projects are very compact (2400sf on one floor probably doesn't qualify) and have glazing over 0,6SHGC
    c. 14" of foam for zone 8 may or may not be excessive. for anything in zones 6 or below, the approach i use for PH, this would be entirely too much.

  4. Michael Blasnik | | #4

    I'm wondering how you calculated the annual heating cost column. The values look more like you assume that the slab is floating over the outdoor air rather than being in the ground.

  5. John Klingel | | #5

    Mike E: I think I just hit the wrong button; hope this does not double post. Why no insulation under the garage slab? The slab will be continuous over the whole place, and the garage will be kept about 50 F. No insulation there would kill me. Looking at the SS, the 14" is not as bad as I originally "felt", but, yes, I still think it is hard for me to justify. With a two-story, that may be another story.

  6. Michael Blasnik | | #6

    Hmmm The numbers still seem very large -- even for your extreme climate. You don't provide many details -- what fuel cost are you using? how many hours per year of a 40F dT? The ground will warm up -- especially with lower insulation levels.

  7. John Klingel | | #7

    Michael B: The garage heat loss was calc'd two ways, and the numbers are within 15% of each other. You are right: With one method, I chose to look at the slab as a wall, but used a delta T of 28 degrees instead of 115. The ground is fairly steady, roughly, at about freezing, though certainly with a warm building on it that will change (However, no one knows by how much). The second way I calc'd the slab losses was by using John Siegenthaler's method, as described in his book Modern Hydronic Heating. For that, I had to extrapolate one constant, as his chart does not go up past R20 for slab edge insulation. (Had earlier listed delta T of 40; fixed that.)

  8. John Klingel | | #8

    MB: I used a sub-slab delta T of 28, not the 40 I reported above (now fixed). HDD = 14,000 above grade. After I calc'd the hourly usage w/ dT = 28, I threw all the hourly's into one equation and calc'd the annual that way, so this will be a tad high. Two reasons for that: (1) I'd have to guess at the sub-slab HDD w/out a lot of research; not worth it. (2) The slab will never warm up like the air will. Garage sf = 840. R30 under the slab, R50 at the edges. Propane, $3.70/gal. Boiler efficiency about 94% (theory). I didn't mean to get into all the details, but if you are interested, here ya go. If you see anything upon which I fumbled, let me know, but I think this is pretty accurate. The SS for my existing house predicts a tad low, but I never have thrown in anything for leakage (intentional, or not) into that SS, so if I did we'd get pretty good agreement. Heat loss ain't rocket science.... but you still have to throw the right info at the equation. c ya. j

  9. TJ Elder | | #9


    You're evidently a numbers guy, so have you done any analysis of two story massing and heat loss? You may add some volume to accommodate stairs, but you can reduce exterior surface area. Less skin area means less heat loss, of course.

  10. John Klingel | | #10

    TJ: A cube is pretty close to ideal, since it minimizes the surface area to volume ratio, without getting into curvie, wasteful-inside shapes. Aesthetically? Debatable. No doubt about it on the heat loss, though. We are moving, after 30 yrs, from a 2-story that uses 4.5 btu/hr/sf, including heating water. The new place is for medical reasons; bad joints and stairs make a poor mix, and if a wheelchair is ever in the picture, this will be "better" (for us) than installing an electric chair lift here. I don't think my wife would go for the hand-over-hand type. So, we bite the bullet and build on a slab. The plus side is, Cricket will have a long haul (in the hall) to chase her ball, and with 4" legs she'll get a good workout.

  11. TJ Elder | | #11


    About geometry and circulation: there could be an architectural solution here, but not necessarily a simple one. Here's a house with a ramp between two levels:

    This approach might not ultimately improve the efficiency of the design if it added a lot of volume. The key to integrating a ramp would be using the space around it. Frank Lloyd Wright's Guggenheim in NY comes to mind--the ramp is the gallery / the gallery is a ramp. You set the ball rolling and Cricket runs downhill.

  12. John Klingel | | #12

    Quite an interesting design. That just shows where the mind can go if you let it. A well-done concept. As for Cricket, I think I'll stick w/ a flat house and the ramp up to the big bed. j

  13. John Semmelhack | | #13

    Just a note for others reading - nowhere does "Passivhaus" recommend 14" of sub-slab insulation. Some Passive House (PH) designers have used very high levels of sub-slab insulation to help meet the PH requirements. Others have used more moderate levels to do the same.

    There's more than one way to skin a cat.

  14. Matt Black | | #14

    Why EPS? The IRC says only XPS for horizontal applications below grade because of its higher moisture resistance.

  15. John Klingel | | #15

    Ahh. The EPS/XPS debate again, huh? I wish I had an answer for you, Matt. I asked Ms Klingxxxx, who at one time anyway headed the PassivHaus US group, and she said the feeling of EPS retaining more water was a myth, so she was quite comfy w/ it. Thorsten Chlupp told me he is tired of pulling up wet XPS. EPS is more environmentally friendly than XPS. A small and almost meaningless anecdote: I sunk a piece of each over a year ago, and a couple months ago pulled them up to see how they were doing. Both floated like corks, fwiw. I have not read of any independent studies that show either one being significantly better than the other and moisture absorption, and so I think if you keep them reasonably dry you'll be fine. That is all I know on the topic. If you know of any studies, please advise us all. Thanks. john

  16. Kevin Dickson, MSME | | #16

    According to this report, you should use a sub-slab delta T of more like 10F after a year or so when the ground temperature hits steady state.

    I've had some sopping wet EPS after two winters on top of a slab. The XPS didn't take on any water at all.

  17. GBA Editor
    Martin Holladay | | #17

    Are you talking about Type IX (high-density) EPS, or the less expensive lumberyard stuff?

  18. John Klingel | | #18

    Kevin: I read the report quickly, and did not see a recommended delta T for soil temp. For the time being, I will keep the delta T at 28 F, as our ground is cold; typically near freezing, and with 10" or 12" of foam under the slab I suspect it will stay pretty cold. That is purely a hunch. I have not seen if CCHRC has any data yet on Thorsten's house, where they are monitoring temps all over the place. Either way, "the right delta T" does not make a huge difference to me within the range we are talking. Lacking definitive information specific to Frb, I'm going to have to make a call on foam thickness and live w/ it. If hard data for Frb ever materializes, I can always adjust. Thorsten is using 12" of EPS, so there is likely a good reason. I will ask him if he cares to dive into this debate. I would also like to know if the Type IX EPS used in the HUD study (linked above) in the early '90's is the same Type IX of today. I have no clue, but all I saw in the study was a comment about how the frost snuck in farther than expected on the EPS-insulated house. There is apparently a reference in the footnotes to further detail that, but I have not explored it yet. So, I still don't know anything for positive. Sigh....

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