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Radiant sub-floor – how to make an air space with foam?

lookloan | Posted in General Questions on

Hi – Speaking with the manufacturer of Thermal Fin C plates and it was mentioned best to leave a inch or two air space below plate attached to sub-floor.  I have floor trusses that are spaced 12, 14, 16 inches wide so I was going to simply spray foam for an R-30 to unconditioned full basement below.  Talking 70 bays to figure this out for.  How would you create an air space if you were in this stituation or would you simply foam locking it all in? Thanks

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  1. Expert Member
    BILL WICHERS | | #1

    I’d tack some 1x2s or 1x3s into the floor trusses to make guides, then put some 1/2” polyiso under those. That would mean the 1x boards would also act as spacers to define the depth of your air space, then the polyiso would seal off the air space. I’d apply spray foam underneath the polyiso to get to whatever depth is needed.

    If these are open web trusses, spray foam is probably the only practical way to really seal everything for a continuous layer of insulation though. You could notch the polyiso around the trusses, or just stuff some mineral wool in the gaps to provide a backing for the spray foam to be sprayed against. You’ll have an air gap under MOST of the subfloor this way, and I don’t see that being any problem for your radiant floor.


  2. Expert Member


    This may be a regional thing, but unconditioned basements would be extremely difficult to build under our code.

  3. Expert Member
    Dana Dorsett | | #3

    In most basements it's easier & cheaper to insulate the basement walls to whatever the IRC is calling out than to install R30 foam between the joists/trusses. That also helps protect the basement from wintertime freeze-up or summertime mold issues.

    With an insulated basement you'd only need R10-R13 or so under the radiant, and it need not be air-tight expensive foam.

    What's your IECC climate zone?

  4. lookloan | | #4

    Hi Dana - zone 5. - the problem is the full basement floor was not insulated under the slab on this new construction. I asked building inspector a few months ago and he said I could insulate walls as you suggested, but since the floor was never insulated I have to do R-30 underneath. (I think the code is R30 for floor and R49 for roof. This is a single family 84x26 - full basement- walk up attic that will be formed under roof deck. 2x6 exterior walls.
    Since ceilings are 12 feet tall, need floor radiant heat to focus upwards thru sub-floor. Due to floor trusses, can’t do the weight of concrete so looking to do 24x24 tiles that look like cement over 1 inch Advantech

  5. Expert Member
    Dana Dorsett | | #5

    >" the problem is the full basement floor was not insulated under the slab on this new construction. I asked building inspector a few months ago and he said I could insulate walls as you suggested, but since the floor was never insulated I have to do R-30 underneath."

    I'm calling BS on the inspector. In the IRC there is NO code requirement for sub-slab insulation (unless the slab itself is the heating radiator), and as long as there is the equivalent of R15 continuous insulation on basement walls it will meet code without R30 between the joists, and it won't be a huge energy hog with just R10 for room/zone isolation for the radiant floor. Even for slab-on-grade construction in zone 5 the IRC only calls out R10 c.i.down to 2' below grade for unheated slab with no sub-slab insulation.

    See Table N1102.1.2:

    Given that going with R30 sub-floor will result in LOWER mold & freeze resilience of the basement this is worth pushing back on with the inspector.

    Since there is no sub-slab R specified in the IRC, does the inspector presume it needs to be R30 under the slab, since it's a floor?

    Deep subsoil temps in most zone 5 locations are in the 50s F, not colder, and in an unheated basement that's allowed to drop into the 50s or even 60F the heat loss from the slab is pretty tiny. If the ceiling is insulated to R30 and the walls are not insulated, with a lot of above-grade foundation exposure the slab can't reliably source enough heat from the dirt to keep the basement above freezing.

  6. lookloan | | #6

    Thanks Dana. I always respect and look forward to your informative, detailed feedback.
    The architect who designed my house said the same thing to me as the town inspector regarding needing R30 under sub-floor or basement roof. The basement is a walkout if that changes things. With the basement being 84 feet long and 26 feet wide, that is a lot of walls to cover which I am OK with, but then since it is foam is flammable, don't I have to also cover with like also sheetrock? If I do just under the sub-floor, I can put a layer of rock-wool board to cover the foam. Thoughts are appreciated.

    1. Expert Member
      BILL WICHERS | | #7

      Are you perhaps confusing subfloor (under the floor that sits on the joists and forms the ceiling of the basement), with subslab (under the concrete slab that forms the floor of the basement)? There is a very big difference, and very different insulation requirements.

      It’s also possible to put a subfloor directly (or nearly directly) on a slab (drycore, etc), but that shouldn’t be treated differently from subslab insulation requirements as far as codes go.


    2. Expert Member
      Dana Dorsett | | #17

      >"With the basement being 84 feet long and 26 feet wide, that is a lot of walls to cover which I am OK with, but then since it is foam is flammable, don't I have to also cover with like also sheetrock? "

      Yes- it needs a thermal barrier against ignition to meet code.

      >"The basement is a walkout if that changes things. "

      Yes, it changes things. A walkout is at even GREATER risk of freeze-up (or even frost heaving the slab~) than a mostly-bel0w-grade basement if the subfloor is insulated and walls are not.

      The walk-out side also needs to be insulated to R10 down to a depth of 2' in zone 5. (The slab edge insulation requirement.)

      >"The architect who designed my house said the same thing to me as the town inspector regarding needing R30 under sub-floor or basement roof."

      By "basement roof" do you actually mean "basement ceiling"? Roofs have sky a above them, not living space.

      Is it possible there is some local code (or local mis-interpretation of the IRC) driving that?

  7. lookloan | | #8

    Bill - not confusing anything. I wrote the sub-floor of the 1st floorr or basement ceiling as being the same and the slab poured concrete floor of the basement being different. The poured concrete basement floor has no insultation under it and inspector and architect both said under the 1st floor sub-floor needs to be insulated R30 as a result.

    Dana is saying what I have been told is not correct. I find it hard to believe the inspector and a popular architect in CT got it wrong, but I am open to listening as I would have preferred no to insulate the rafters under the 1st floor, hence basement ceiling. I have a feeling I have to but I am open to hearing if the basement walls, without the basement concrete slab floor would be enough rather than having to do under the ceiling. Right now the architect's plans are insulate under the rafters in the basement ceiling - and leave the basement walls bare. Rim joists get insulated.

    1. Expert Member
      MALCOLM TAYLOR | | #9


      Both your architect and the inspector are right in that if you don't insulate the basement walls you need to insulate the floor above, but they have confused the requirements for floors with those concerning slabs. The relevant sections of the code are in the links Dana provided - and this isn't one of those situations where there are two interpretations - they are flat out wrong.

    2. Expert Member
      Dana Dorsett | | #18

      >"...I would have preferred no to insulate the rafters under the 1st floor..."

      First floors don't have rafters under them- they have joists.

      Ask the architect to dig out the relevant code and make the case. Sticking R5- R10 under the slab would make it an insulated slab, but accounts for only a tiny fraction of the energy use of the house. Insulating the walls to R15 c.i. with only R10-R19 isolating the radiant from the basement would do more for the house than insulating with R30 under the radiant.

      Is the basement going to be used for anything (such as housing the boiler, water heater, laundry, plumbing, etc.)?

  8. lookloan | | #10

    Hi guys - I don't mean to be dense, but I am getting confused by the last post where Malcom said the architect and inspector are right saying you need to insulated the basement ceiling, but the architect and inspector are wrong. So in my case, the architect is saying insulate the basement ceiling under the first floor R30.

    "Both your architect and the inspector are right in that if you don't insulate the basement walls you need to insulate the floor above."

    Insulating the basement ceiling closes the envelope up thru the 1st floor walls and the attic.
    I was told the attic has to be R49 and floor R30. I have to look at the plans but I think it's R30.
    The plan was to leave the concrete walls (8 feet) plain 10 inch thick cement with no covering.
    The furnace or boiler will be in this basement.

    I think what Dana is saying is if I insulated the basement walls, then I don't have to do R30 in the basement ceiling and I can do R13 or R15 or something to cover the radiant plates.

    I don't like fiberglass by the way in the basement ceiling if mice get in and nest in them.

    I did what I think is a nice job sealing the exterior sill plate to the foundation. I started with Seal Sil between foundation and Sill plates. Then used liquid Zip to seal the crack between the foundation and sill plate which gave a solid seal. Then I put a layer of Siga Tape 6 inches and 9 inches, sealing an inch of the cement foundation, pressing and molding the tape underneath sheathing over hanging the foundation, and finished up the sheathing. The stuff I used was like a black felt on the outside. This tape is amazing and sticks solid to the cement foundation perfectly. No need to prime and was told by Siga it was a newer tape.

    1. Expert Member
      MALCOLM TAYLOR | | #13


      I was trying to illustrate the thinking that got your architect and inspector to the conclusion they reached. Once they had made the mistake of confusing the requirements for slabs with those of floors, the conclusion that the floor above had to be insulated followed logically - even though it's incorrect.

    2. Expert Member
      BILL WICHERS | | #15

      Basically you have to insulate the floor (basement ceiling) UNLESS you insulate the basement walls. You have to do one or the other, but not both.

      Most of your basement heat loss is through the upper part of the basement walls where you can have frozen ground. As you go down deeper, towards the depth of the slab, the average soil temperature will be higher and then less of an issue in terms of heat loss which makes insulation less of a concern. Insulating the walls is usually the better option, and is much easier to do well compared to insulating the basement ceiling.

      Note that if you do a good job sealing the rim joist, you’ll have less issue with pests getting into your insulation since they won’t be able to get into the basement. That’s something of an added bonus you get when you seal your rim joist :-) I eliminated my basement mouse problem that way, to the disappointment of my two cats.


    3. Expert Member
      Akos | | #16

      Up here in the great white north, you always insulate the basement walls and never the basement ceiling.

      Even the latest energy codes that really pushed new builds to close to what would have been considered high performance a decade ago does not require any insulation under the basement slab (because it doesn't make much difference in energy use).

      Both your architect and inspector are misinterpreting the building code.

      Besides being expensive to do, insulating the basement ceiling only is a recipe for having a moldy basement.

  9. Expert Member
    NICK KEENAN | | #11

    If the boiler is in the basement it has to be protected from freezing.

    I like insulated basement walls and a sealed basement. The question is what to do in the floor between the basement and the first floor. With an unheated but insulated basement the basement will tend toward the soil temperature, in which case it should be insulated (slightly) from the heated part of the house. But with the boiler down there it might even be warmer than the heated part of the house, in which case I wouldn't insulate and let it contribute to heating the first floor.

    1. Expert Member
      MALCOLM TAYLOR | | #12


      If as the architect suggests the basement is left uninsulated, It's not just the boiler that needs to be kept warm, it's the water supply and drains, which somehow have to get from below ground up to the insulated floor. Our code doesn't allow you to have a service panel in an-heated space either.

  10. jjk066 | | #14

    What is the purpose of leaving an air gap below the fins? Heat transfer from the fins directly to the subfloor will be more efficient than from the fins to the air to the subfloor.

    1. Expert Member
      NICK KEENAN | | #19

      It's to prevent striping. With an air gap the air is heated to a uniform temperature which makes the floor more uniform. It's a stretch to call this "radiant" heat because most of the heat transfer is being done by conduction and convection.

      As to efficiency, so long as no heat is being lost to spaces you don't intend to heat it's 100% efficient. This is probably going to require hotter water than a radiant floor which may mean the boiler is going to be running less efficiently.

    2. lookloan | | #20

      I spoke with the creator of Thermal Fin C plates over the phone and the space was a something he said I should so. I gather from the conversation the warmth stays in that small 2-3 inch space and warms more area of the sub-floor so heat spreads to a larger area away from the plates.
      Regarding the freezing of pipe, electric panel, I am putting the mechanicals on one side of a large
      26x84 long basement. That part is 8 feet under ground and there is a sheering wall separating that part with one door opening. This area makes up 1/3 of the basement that is almost closed off from the other walk out half. The other half would be PEX water lines run through insulation under the sub-floor thru the open floor trusses. I broke my ankle so I have not been to the job site in 7 weeks so I plan to look at the 2x4 trusses as I may be able to use the top trusses 2x4 plate for space that was automatically built in.

      1. Expert Member
        MALCOLM TAYLOR | | #21


        Sorry to hear about your injury. That''s both painful and dispiriting when you are in the middle of a build. Good luck with the rehab.

  11. tommay | | #22

    Along the lines of what Zephyr said......the easiest and cheapest way to do it is to purchase some rigid insulation with a foil face...1/2 - 1". Cut some 1/2 -1 " strips and secure them using caulk/adhesive to the upper corners of the bay where the sub floor meets the joists. Then cut pieces that fit snugly into the bay and carefully push them up to the strips, basically forming a U shape channel under the plates within the bay. Once installed you can go ahead with spray foam or fiberglass insulation.

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