How Much Insulation Under Floor?

Jeff Retke asks,

We live in an older 1949 home. There is no insulation under the floors, limited original insulation in the attic. My “crawl space” is bigger/taller than most and often feels relatively warm. I’m wondering if I wanted to start adding insulation (we are on a very limited budget) would it be better to add insulation in the attic, or to start putting insulation under the house?

Our Answer

Insulation is one of the most effective ways to reduce home heating costs, so investing in insulation always makes sense. Start with attic because that is where the most heat loss is. Optimally you want 15-20 inches of insulation in the attic, either fiberglass batts or blown fiberglass.

Once that is done, you can move onto the floor of the crawlspace. In that area you want to use the deepest batt that the floor joists can handle – so if you have 2 x 10 inch joists, then use 10 inch batts. For this area you want to use unfaced batts, with no vapor barrier.