How to Insulate a Floor over a Crawlspace
Use a combination of batts, continuous rigid foam, and a vapor barrier to safely insulate this tricky space.
We have a home with a finished basement under most of the house and a vented crawlspace under the remaining 500 sq. ft. The crawlspace is completely sealed off from the basement, and there’s no ductwork in it. The entire first floor is framed with I-joists. When the house was built, we installed two 6-in. R-19 batts into each joist bay in the crawlspace and stapled chicken wire to the bottoms of the joists to hold it in place. The batts don’t fill the spaces between the joists because of the narrow webs, so their performance is less than optimal. We would like to correct this and are thinking of thickening up the joists with sheets of foam, then reinstalling the fiberglass and chicken wire.
Since this is a pretty amateur solution, we would appreciate your opinion on this method and are open to suggestions for a better way.
—ARLAN AND BARBARA ZASTROW, Roseburg, Ore.
Associate editor Matthew Millham responds: Your plan should improve your floor’s thermal performance, but there are things you could do to make it even better.
Let me start by saying that crawlspaces can be tricky. Some assemblies work in some climates but not in others. Two popular approaches are to seal the crawlspace and make it part of the conditioned space, or to seal it off from the conditioned space and vent it. Building scientists tend to like the first option, but research has found that either option works in the maritime climate of the Pacific Northwest as long as there aren’t ducts in the crawlspace, and you’re okay there.
Assuming that you want to stick with your vented crawlspace and that you’ve already removed the fiberglass batts, start by air-sealing the rim joist to keep outside air from leaking into the floor insulation. Use high-quality caulk or canned closed-cell spray foam to seal any gaps between the mudsill and the rim joist and between the rim joist and subfloor, as well as any similar gaps around end joists.
Your plan to pack out the I-joists with rigid foam so the batts fit snugly between them is fine, but it’s a lot of work for a workaround. Look into fiberglass or mineral-wool batts designed for use in steel-frame buildings. These batts are slightly wider and should fit snugly between your I-joists without the need for foam filler strips. Whether you keep the old batts or get wider ones, make sure they’re in contact with the subfloor as the energy code requires for optimal thermal performance.
Once the batts are back in, add a continuous 1-in. to 2-in. layer of rigid foil-faced polyiso foam insulation to the bottom of the joists. This will reduce thermal bridging through the joists, provide an air barrier, and add to the R-value of the whole floor assembly. The polyiso can also help keep the joists and insulation warm and above the dew point so they don’t get wet and moldy.
Seal the seams between polyiso sheets and between the polyiso and walls with high-quality housewrap tape to create a good air barrier, and use canned closed-cell spray foam to fill any gaps between the foam and that drainpipe. As long as you aren’t using the crawlspace for storage and don’t have any fuel-fired appliances in there, you shouldn’t need additional fire protection on the crawlspace ceiling, but check with your local code official to make sure. If you want or need further protection from pests, add a layer of OSB beneath the foam.
One last thing: If you don’t have one already, install a vapor barrier on the crawlspace floor to keep the space from getting damp from below. Cover the entire crawlspace floor in the vapor barrier (usually 6-mil or thicker polyethylene sheeting) and run the edges at least 6 in. up the stem walls. Attach and seal the sheeting to the walls with horizontal furring strips and mastic, and lap any seams at least 6 in. and seal or tape them as well.
Drawings: Dan Thornton