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Insulating a concrete slab floor

We are converting our garage to living space. We plan on sealing the concrete with a polyurethane sealer, using pressure treated sleepers as floor joists, covering this with 3/4" plywood subfloor, and then installing hardwood flooring. We want to maximize the floor insulation. The floor slopes so that the sleepers will be about six inches near the outside wall and only about two inches on the inside wall. We would like a recommendation on the best type of insulation (ie closed cell polyurethane sheets, spray in closed cell foam, etc). We are also curious if we should leave an air gap either above or below the insulation to allow for circulation. Any help would be appreciated.

Re: Insulating a concrete slab floor

Hi, John, I’m Doug Walden and new to This Old House Discussions. I’m the product manager of air sealing at Owens Corning, so I wanted to try to help answer your question.

First of all, applying polyurethane sealant in the concrete floor is probably not necessary. Assuming that you are doing this to control moisture from the slab, 6-millimeter plastic sheeting would be a simpler and more effective solution. Next, you can place the sleepers on top of the plastic sheeting.

After that, I would lay down extruded polystyrene rigid foam insulation and snugly fit in the spaces between the sleepers. This type of insulation will provide R-7.5 thermal resistance for the 1.5” thickness of the sleeper cavity and has very good compression strength. You should be able to find this type of insulation at your local home center (you could even try our FOAMULAR product). No need to leave a gap for circulation.

Finally, the plywood sub-floor could go on top on the insulation, followed by the hardwood floor.

Hope it works out – let me know if you give it a try.

Re: Insulating a concrete slab floor

I agree with Doug's reply but would add the following. 1.5" is more than enough and it should be placed at the bottom of the sleeper cavity, the air gap should be above the insulation. This isn't so much for circulation as there won't really be any, but the air will stratify in the cavity with the warmest air at the top against the bottom of the floor. This still air will also insulate, because it is not moving, it will be worth about an R-3 or R-4 per inch.

If you use a closed cell foam and put it at the top of the cavity, it will create a problem. The closed cell foam will be a vapor barrier. You really do not want two vapor barriers with a cavity between them, they will trap any moisture that might get in there. You do need the 6 mil plastic on the concrete to protect the sleepers from moisture wicked up through the concrete.

Re: Insulating a concrete slab floor

Hi, Keith. My apologies for any miscommunication, but thank you for raising a very good point. Hopefully I can help clarify.

I just want to make sure I’m correctly interpreting how the sleepers would be positioned. As I understood, the sleepers would be lying on their side and therefore be 1.5” tall. This would then create a 1.5” cavity that could be filled with extruded polystyrene rigid foam insulation in the spaces between the sleepers, thereby leaving no air gap above or below the insulation.

However, it sounds like you may be envisioning the sleepers on their edge, which would create a 3.5” cavity.

John, would you be able to clarify based on your project?

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