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Community and Q&A

Insulating the underside of precast concrete slab garage

Nathan_Kipnis_FAIA | Posted in Green Building Techniques on

I am designing a garage using precast concrete slabs to allow for the space underneath to be used as conditioned living space.

The garage floor will consist of a waterproof urethane top layer, 2″ concrete topping for the drainage slope, and 8″ hollow core precast concrete slabs. 6″ metal studs will be secured to the underside of the precast concrete slabs.

The question is what insulation to use. I do not want to introduce closed cell foam as this is a renovation and we are trying to avoid closed cell foam’s outgassing issues in homes that are inhabited during renovation work. 5/8″ gypsum board would be used to seal the insulated cavity.

I am concerned that cooler conditioned air would get up to the underside of the precast planks with open cell foam. We are tight on the way the stairs work, so I am not inclined to make this system thicker.

Any informed thoughts on this would be appreciated.

Nate Kipnis

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  1. GBA Editor
    Martin Holladay | | #1

    Well, all you want is a miracle. You want to add insulation, but you don't want to make the floor assembly any thicker. So how are we going to do that? Maybe with insulating paint?

    All joking aside... You tell us that there will be "6 inch metal studs" attached to the underside of the pre-cast concrete floor system. Are you sure these are studs (vertical framing members)? Or perhaps you mean ceiling joists?

    Ideally, the underside of the concrete will be insulated before any steel framing members (whether studs or joists) are installed. Once you attach steel to the concrete, you've got a thermal bridge that defeats the purpose of the insulation.

    Closed-cell spray foam would be best for this application. Of course, spray foam insulation has thickness. The thicker, the better.

    If you don't like spray foam, you could attach one or more layers of rigid foam to the underside of the concrete floor assembly.

  2. Nathan_Kipnis_FAIA | | #2

    Martin, sorry for the confusion. The insulation would be in the 6" cavity under the precast slab, which is formed from securing studs to the underside of the planks. The precast planks are the full structure and the 'stud cavity' is for the insulation only. They are not structural. I could use treated 2x6's in lieu of the metal for a slightly better thermal break.

    I suppose I could use a 2x4 on the underside, fill it with foam, and run a 1" or 2" layer of foam board on the underside of the 2x4 to thermally break the structural and then seal it with the gypsum board.

    My concern is using open cell foam in this situation. I am thinking that vapor will makes it way to the underside of the precast plank and form condensation. So, thinking this though, maybe the best way is what I just said in the paragraph above. This is a building condition I haven't run into before and haven't seen addressed before.

  3. rocket190 | | #3

    If your topping is non-structural you have more options. 1. Install and grout plank; 2. Install 6" of 25psi foam over plank. ; 3. Pour non-structural topping and waterproofing layer.

    The loss of headroom is the same. You will get continuous r-value at about 50% less than spray foam. The ceiling could be strapped and rocked or painted as is. You will probably need to bump up top pour to 4" , which will cost you 2" of headroom.


  4. GBA Editor
    Martin Holladay | | #4

    If you are talking about horizontal framing members that support the ceiling, I still think you're talking about ceiling joists, not studs. But that's a side issue. Ideally, the insulation needs to go between the concrete slab and the framing members -- especially if they are made of steel. If you switch to wood 2x6s, the thermal bridging problem still exists, but it won't be as egregious.

    Rick's suggestion -- to put the insulation above the structural floor rather than under the structural floor -- is certainly worth considering.

  5. Nathan_Kipnis_FAIA | | #5

    Rick and Martin, thanks for the input. I appreciate it.

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