Insulation under a new garage floor
RBWI
| Posted in General Questions on
I will be having a new garage floor installed in August. I would like to put insulation underneath it. Does anyone have a recommendation of either extruded polystyrene or expanded polystyrene, should the seams be taped, and should I put a vapor barrier down first? The concrete company has no thoughts.
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Replies
If you're insulating and heating the garage, insulate the slab.
You still want top start with 3"-6" of compacted crushed stone. Use recycled EPS or XPS if you can, new EPS is you can't find recycled. Don't bother taping the seams. 6 mil - 10 mil poly film on top of the insulation.
Thank you both. The stuff I’ve read online said to put the poly down first, then insulation. You guys both think that the vapor barrier should go on top of the insulation?
Hi RBWI.
I agree with Peter's advice. EPS is the more environmentally friendly option. And definitely don't skip the poly vapor retarder. If you need help deciding what R-value to shoot for with your insulation, read this: Determining Sub Slab Rigid Foam Thickness.
Brian, in the article that you make reference to, they state to put the vapor barrier on the gravel and the insulation on top of it. That is consistent with everything else I have read. Has that changed since that article was published in 2013?
The vapor retarder should be in direct contact with the concrete, on top of the insulation.
Another reason for doing this is that pouring concrete directly onto rigid foam could cause the foam pieces to move around and float up. This is not something that you want to be fighting as the concrete hardens.
References at Building Science include BSI-059 (Slab Happy) and BSI-003 Concrete Floor Problems.