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Thermomass vs subslab foam Mixed Humid

GBA Editor | Posted in General Questions on

I’m planning a (monolithic) slab for my house in Zone 3 (2800 HDD; 1800 CDD) where frost maxes out at 8” but the termites are as plentiful as pollen. So, external insulation on a slab won’t work. But Robert referred to ThermoMass and that seems perfect for this area since the foam is isolated by the concrete.

I’m wondering if I can do away with subslab insulation board in this climate – if the side slab insulation will keep the slab the same temp (60-65 degrees) all the way across or if the temperature/range will be significantly improved by adding ~$3000 worth of rigid foam.

Thanks for the input
Joe W

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  1. GBA Editor
    Martin Holladay | | #1

    Your question confuses me. Thermomass is used for walls, not floors. You are designing a house with a slab-on-grade foundation. Do you want to use Thermomass for above-grade walls?

  2. J99aAMQzYo | | #2


    Check out the <a href= Foamglass review over at Building Green. Alex Wilson just named it as one of his Top 10 Products of the year at Greenbuild this week.

    One of the reasons he likes it so much is for application like yours. In addition to the "green" advantages that it offers vs. typically board foam, it is also not a food source for termites and has very high compressive strength. So it is perfect for below grade, external insulation in high termite zones.

  3. J99aAMQzYo | | #3

    Sorry, faulty formatting in the link above :-(

    The link is correct, but the product name was omitted. The product is called Foamglass.

  4. Armando Cobo | | #4

    CZ3 covers a lot of areas (Dry, mixed and Humid from CA to SC), but even the 2009 IECC does not require insulation on a slab. However, if you still want to insulate, try laying 1” or 2” rigid foam under the whole slab and between 2-10 mil under it for protection, and treat the soil. A second option would be to “sandwich” rigid insulation in two slabs, but the bottom slab would be 24”-48” wide around the perimeter only. A third option would be no do a 3-pour foundation and do the wall in Thermowall.

  5. Riversong | | #5


    I don't like monolithic slabs precisely because there is no thermal break (or capillary break) between the exterior air/soil and the interior floor. That's less problematic in your relatively mild climate, but still both a comfort and energy issue.

    You could pour an independent perimeter grade beam and sandwich some slab-edge foam between that and the slab, which would require two pours. But you could use the ThermoMass system for a slab as well. In their industrial applications, they sometimes pour a wall on the ground and then raise it as a unit. There's no reason you couldn't pour a slab the same way, though you'd have to isolate the foam at the edge from termites, perhaps with a perimeter grade beam.

    You might call ThermoMass and discuss it:
    Linda Conaway
    Cast-in-Place & Residential Systems
    Thermomass Building Insulation Systems
    Boone, Iowa
    Office: 1-800-232-1748 x 102
    Fax: 515-433-6088
    Email: [email protected]

  6. 5C8rvfuWev | | #6

    Response to Martin:

    I'm able to use it, per Bob Long (head at Thermomass who was kind enough to speak with me) as a vertical inclusion 4" inside of the concrete (and under the wall plate). The slab I want is monolithic and this use of rigid foam makes it so I can insulate the edges w/o running afoul of local inspectors or providing a home for the local termites.

    I am wondering if the slab edge insulation reduces my need for under-slab insulation in this climate zone. Or if I should still spend the $$ for underslab insulation even though the results may be muted.

    Sorry if I was unclear and caused confusion. Thank you for your reply.

    Joe W

  7. 5C8rvfuWev | | #7

    Reply to Andy Ault,

    I've read Alex on that, but you're right -- I should check Foamglass out. Much appreciated.

    Joe W

  8. 5C8rvfuWev | | #8

    Reply to Armando

    I take it that you feel the sub-slab insulation is a matter of preference. Given that's the case and the climate info I provided, how could I get a grip on the measurable difference between insulation/no insulation?

    Thanks for your ideas

    Joe W

  9. 5C8rvfuWev | | #9

    Thank you Robert,

    As I explained to Martin, Linda Conaway gave me Bob Long's phone # and he explained there would be no problem in using the ThermoMass in a VERTICAL application at the slab edge which allows me to isolate the cement from termites. This would provide the thermal break, as you say, "between the exterior air/soil and the interior floor." Im asking because, as you say it is "less problematic in your relatively mild climate, but still both a comfort and energy issue."

    My question was an attempt to determine just how much of an issue it would be to leave out the underslab insulation, given that I've introduced slab edge insulation.

    I'm sorry for the confusion. You're often good for an equation or two, or a different approach, so I'm holding out hope ....?

    Thanks for your patience.
    Joe W

  10. Riversong | | #10


    It's still not clear what you're speaking of. A monolithic slab is a thickened-edge slab that is both footing and floor and upon which the framed wall is built.

    If you're talking about pouring a frost wall or grade beam inside of which a slab will be poured, that's not a monolithic (single pour) slab foundation.

    Will there be footings, then a ThermoMass perimeter wall and then a slab inside of that?

  11. 5C8rvfuWev | | #11

    I'm sorry I'm not being clear, Robert. Yes, I understand what a monolithic (single pour) slab is as it is frequently used in this area. What I asked Bob Long about is a thickened edge on the slab sufficiently wide enough to contain the slab edge insulation ... 10" or so overall with 2" of EPS contained in it ....

    The slab would provide the footing for walls and the (polished concrete) floors inside.

    If I use his system slab edge, would under slab insulation provide a cost-effective benefit?

    Joe W

  12. Riversong | | #12


    I think we're all still confused here, including perhaps Bob Long.

    The ThermoMass wall system is designed to sit on top of a monolithic footing. A footing cannot be split vertically by non-structural foam, and neither can the thickened edge of a monolithic slab (which is the footing).

    The ThermoMass wall system is ideal as a frost wall, sitting on top of a spread footing, with the inside wythe of concrete dropped to form a step to receive the slab and sub-slab insulation, so that the slab abuts the midline XPS of the wall and the subslab insulation is continuous with the wall insulation.

    I just used this detail on a walkout basement for a super-insulated house with radiant slab here in VT.

  13. armando Cobo | | #13

    Installing a full 1” or 2” rigid insulation under the slab does immensely reduce the energy load, and a way to calculate is by hiring a HERS rater to do an energy analysis on your project. 24” flat rigid insulation around the perimeter is good, 48” is goodder, and full insulation under the slab is goodest!!! :-))
    In CZ 3 and CZ4, under slab insulation is very cost effective way to lower your HERS score. That's one out of my bag o' tricks learned over the years...

  14. Riversong | | #14


    Ground temperatures in CZ 3 are between 62° and 70°. How do you figure that subslab insulation, not required by IECC, would "immensely reduce the energy load"?

  15. 5C8rvfuWev | | #15

    Thanks again, Robert. The design you describe is what I had in mind originally. I'll mention your concern that I was unclear when I speak with him as I get closer. Even if it requires a footing and stem wall, I would prefer the Thermomass; as I said, it's important here to get insulation inside and away from the bugs.

    Meanwhile, I'll pursue a way of determining the impact of sub slab insulation in a zone 3 application with rigid foam outside the foundation.

    Thanks again!

    Joe W

  16. Armando Cobo | | #16

    If you read my #4, I said the 2009 IECC does not require insulation, but just in case Joe wants to insulate his slab there could be some options. But I was thinking more of ABQ, NM where is T24"=40°F in CZ4 and in ABQ it make a big difference there. However, in Tulsa OK (CZ3) T24”=42°F, Little Rock, AR (CZ3) T24”=45°F and Dallas, TX (CZ3) T24”=47°F. Since I don't know where Joe lives, that could be some help. So as you can see Robert, your 62°F-70°F maybe in a more southern CZ3 and that’s why I was asking Joe before where does he lives.
    On another note Joe, I want to be clearer, the HERS rater does not calculates the loads, he calculates the HERS scores with or without rigid insulation, with 1” or 2”, and he can give you reasonable cost and payback information as well.

  17. Armando Cobo | | #17

    By the way, those temperatures are at the two coldest monts of the year....

  18. Riversong | | #18


    That soil temperature range is for deep soil (which is very close to annual mean air temperature) in the entire CZ3 range.

    Shallow ground temperatures are meaningless, since the soil under a heated slab or foundation will quickly equilibrate to something between deep soil temp and indoor temp - and that will determine the steady-state heat loss.

    Measured soil isotherms, for instance under and around a conditioned but uninsulated basement in Saskatoon CAN in March were 70°+ and 50° under a slab on frostwall.

  19. Riversong | | #19

    The deep year-round ground temperature here in VT (CZ6) is 42°-44° (which, as a caver, I've datalogged). When I do my heat loss calculations downward I use the average of indoor setpoint temp and deep ground temp, which is probably conservative.

  20. jklingel | | #20

    Joe: Why not dig up John Siegenthaler's slab heat loss equation and calc it yourself? The equation is on page 24 of his Modern Hydronic Heating, 2nd Edition book, or perhaps online somewhere. If not, using Q = UA (delta T) will at least get you ball parked. First, you need to ask a local engineer or soil researcher what your soil temp is, naturally. Remember, that insulation is going to be working for you for 50 yrs; right now, it is cheap. A retro ain't. j

  21. 5C8rvfuWev | | #21

    John, thanks. That's pretty much what I was hoping to get as reference. I appreciate it.

    Armando, thanks for the encouragement and direction. Sorry I was away when the climate conversation took off -- in the OP I gave the local info: "Zone 3 (2800 HDD; 1800 CDD) where frost maxes out at 8”

    I also appreciate knowing that a HERS rater will be able to give the specifics. That should give me good guidance.

    Best wishes,
    Joe W

  22. 5C8rvfuWev | | #22

    Robert, I'm sorry to be so thick on this and it well may end up that I need a footing. But after your last patient explanation to me I got off the computer and went back to find the source for my original idea which led me to follow up on your recommendation for ThermoMass. Here is what started me off, if you have the Building Science books.

    Joe Lstibureck, the Design section of "Builder's Guide to Mixed-Humid Climates" has a section on monolithic slabs. I'm sorry I can't scan it post here. On p.206 (Figure 7.38, p 206) he shows a design for a "monolithic slab with brick veneer above. Here's his description:

    "Protective membrane acts as termite barrier and acts as flashing at base of brick veneer.
    "Grade beam for brick veneer cast simultaneously with monolithic slab

    He shows vertical rigid insulation between the grade beam and the main slab. The slab and grade beam are connected by a "fiberglass tie."

    I realize that a drawing is no guarantee it will work, and I can be very thick when I don't "get it" which I don't.

    Just for reference, the drawing does not show subslab insulation, but as y'all have pointed out that will be a decision to come when I get that equation figured out and have a conversation with the HERS rater I'll be using. And once I can speak with ThermoMass again, I'll see if I confused them, too.

    Meanwhile, thanks for your patience and thought.

    Joe W

  23. 5C8rvfuWev | | #23

    Since y'all asked: Average soil temp 4 years for the area (2-4-8 inches) = 65 degrees

    Yesterday's average (4") was 45 degrees.

  24. Armando Cobo | | #24

    Shallow ground temperatures are meaningless…

    Not to beat a dead point, but…. 1. We are not talking about basements and the ground temperature 8’ below. 2. In CZ4 (by code), when using radiant heat you must install 2”-R10 rigid insulation to avoid heat loses; as in heat transfers from hot to cold. It’s just pure physics… a la Lstiburek. I thought I had saved a BSC paper on this, but I can’t find it…. If I do, I’ll post it…. Or maybe someone else has it and posts it.

  25. homedesign | | #25

    I don't think radiant heat was ever mentioned
    I wonder if this is similar to your thinking?... trying to insulate the slab edge..not the horizontal slab......

    BSC and Greencraft attempted to integrate slab insulation at the Lewisville house. The typical design of exterior insulation with a covering at the slab edge was not allowed. Local code forbids the covering of the exterior slab edge such that termite activity can be observed easily. An attempt was made to integrate 2” of vertical XPS embedded in a monolithic slab design, but there is a need for extensive reinforcement through the XPS due to expansive soils. BSC recommended replacing typical reinforcing bars with fiberglass ties in order to reduce thermal bridging losses through the vertical XPS. Another problem is that the structural design did not allow for the XPS to extend from the top of the slab down to the bottom of the grade beam. Time and budget constraints prevented resolution of this technology barrier
    but BSC and Greencraft will be implementing slab insulation on the next project.

    from this page

  26. homedesign | | #26

    Do you think Chris Miles is trying this in his 2010 project house?

  27. homedesign | | #27

    I think some type of Termite Tolerant Slab EDGE Insulation is what is needed in the Hot and Mixed Climates
    Below the Slab insulation could work against us instead of for us (mixed & hots) depending on ground temperature

  28. homedesign | | #28

    OOPS Tolerant was not a good word ... Termite Not-So-Friendly ?

  29. 5C8rvfuWev | | #29

    Yes, John, that's the design I had in mind, but I had no idea is was an "experiment." (Good grief. The mention of insulation around here leads to great head-scratching. I think I'll just pass, lol.) Thanks for the link to the paper.

    Joe W

  30. 5C8rvfuWev | | #30

    Another reply to John

    "I think some type of Termite (in)Tolerant Slab EDGE Insulation is what is needed in the Hot and Mixed Climates"

    Right! The Foamglas website says it would work, but local code is absolute in prohibiting such, so I'd want to begin there.

    Joe W.

  31. homedesign | | #31

    Be Wary of Foamglas and claims by manuf.
    check this out
    my take is it sounds risky to me

  32. 5C8rvfuWev | | #32

    As should be very apparent from the conversation above, I don't know enough to be able to evaluate risk vs return. So if you have time to put your concern into words I'd appreciate it.

    My ignorance is pretty frustrating -- I come from a family of contractors and tradesmen and have built four houses (for myself) ... they were well enough done for the time, but I finished the last one in 1984! I don't need just a new skill set; I need a new frigging dictionary! Sheesh.

    I feel better now,

    Joe W

  33. homedesign | | #33

    Maybe there is a way to incorporate Foamglas?
    I am only saying to be wary.
    I don't plan to use slab edge insulation anytime soon.
    I think there is a need ... but not a track record

    Do you know how to recognize the Pioneers?

    By the arrows in their backs ;--)

  34. homedesign | | #34

    My comments were in context to Termite country

  35. 5C8rvfuWev | | #35


    Lol, I get it. Thanks. Over on this side of the river they have 'gators on their britches'

  36. Armando Cobo | | #36

    Joe, I would think most building officials would be receptive to new products and new technology; I’ve been successful several times over the last few years. I would try to get them all the information I could about Foamglas and perhaps more documentation from the manufacturer could help you out. You have nothing to loose.
    I do like this product as a good option based on what I read in their website; and probably more research is warranted.

  37. Riversong | | #37

    In CZ4 (by code), when using radiant heat you must install 2”-R10 rigid insulation to avoid heat loses; as in heat transfers from hot to cold. It’s just pure physics

    But physics requires the correct inputs to get the correct results. We're talking CZ3 and an unheated slab within conditioned space. But even in your example, the subslab R-10 does not "avoid" heat loss but merely slow its rate. The slab will still be heating the ground and the sub-slab ground temperature will equilibrate within the first year to something between indoor set-point temp and deep ground temp, which will depend on soil type and moisture content.

  38. Riversong | | #38


    I wish Lstiburek showed a cross-section of that slab design.

    I think it was Dr. Joe who is confusing all of us. A monolithic (literally: single stone) slab is, by definition, an uninterrupted slab of concrete that forms both footings and floor. But apparently he's using that term because "the structural design did not allow for the XPS to extend from the top of the slab down to the bottom of the grade beam", so the slab is not entirely separated from the footing by insulation. I'm not sure that is going to offer the thermal break that he was hoping for since concrete is so highly conductive and the heat will short-circuit around the foam. But probably better than nothing.

  39. homedesign | | #39

    what would you suggest for insulating slabedge in termite country?
    Monolithic slab is almost a given in North Texas.
    I will email Greencraft (Chris Miles) and see if he is doing the Lstiburek detail this year.
    Maybe I can get a copy of the detail.

  40. 5C8rvfuWev | | #40

    Joe L's drawing is very clear; it's my inability to explain it that's the issue -- and the fact I don't have scanning capability. John's quoted passage from BSC (#25) is better than mine.

    Joe W

  41. Riversong | | #41


    The only time I built in termite country was in the hollers of Tennessee years ago. One house was on an uninsulated block foundation (insulation in the floor framing) and the other one was on piers.

    The termites weren't as much of a problem there as the poison ivy, which blanketed the building site. I can't remember how they resolved the issue of eradicating it. Since it was a community land trust organized by a Nun, they debated whether poisoning the ground was appropriate, but I think they ended up spraying something.

    If I was unfortunate enough to build again where the termites thrive (which may be Vermont before long), I would probably investigate Roxul, FoamGlas, borate-treated EPS and Termimesh. And I might play with ways to use the ThermoMass system to contain the insulation.

    Other than the fast and easy aspect of a monolithic slab, I see no advantage in it. I would always want to separate the foundation from the floor.

  42. Riversong | | #42

    Can someone else scan the image or get Dr. Joe to send a digital version to Martin?

  43. Riversong | | #43

    Let's be glad we don't live in THIS ( termite paradise.

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