If you’re building a new cold-climate home with a basement, you’ll probably want to install a continuous horizontal layer of rigid foam under the basement slab. Even though the rigid foam won’t save enough energy to justify its cost, it’s worth installing for another reason: it will reduce summertime condensation and mold accumulation, and will therefore help your basement smell better. (To learn more about sub-slab foam, see “All About Basements.”)
What if you’re living in an older house that has an uninsulated basement slab? If you want the benefits of floor insulation, you’ll have to install the rigid foam on top of your existing slab.
Before we discuss slab insulation, it’s worth reviewing the three basic mechanisms that can make a basement slab damp:
If you install carpeting on an uninsulated slab, you can end up with moldy carpeting. The cold slab is a condensing surface, and the indoor air (especially during the summer) provides the moisture. That’s why you want to install rigid foam on your basement slab before you install carpeting.
Here are the basic steps to installing rigid foam above an existing slab:
Ideally, your basement slab was installed over a layer of 6 mil polyethylene. If you’re not sure whether your slab has poly underneath, you can always drill a hole through your slab in an inconspicuous area of your basement. If the diameter of the hole is at least 3/4 inch, you should be able to tell (with your finger, a probe, or a flashlight) whether there is any poly under the slab.
If there’s no poly under your slab, or you’re worried that there may not be any poly under your slab, it’s probably a good idea to install a layer of 6-mil poly between the…