Support for Sagging Insulation
Propping up fiberglass batts beneath the floor
Our 1963 home has floor beams that are 48 inches apart. The fiberglass insulation under the floor is sagging, and I don't know how to hold it in place. Any suggestions?
—Dave Nixon, Madras, Oreg.
Tom Silva replies: Your type of floor construction is quite common for houses built over crawl spaces in the Northwest. Instead of joists, the floor rests on a tongue-and-groove lumber subfloor, which is supported by widely spaced beams on posts and piers.
I'd support the fiberglass batts by stapling chicken wire or a zigzag network of nonstretch twine to the underside of the beams. Then I'd fasten a layer of 1-inch-thick foil-faced foam against the beams with galvanized roofing nails. The foam adds R-7.5 to the floor's insulation value. Make sure the fiberglass is unfaced and tight against the subfloor before you install the foam. After it's up, seal all its seams with foil tape.
This is not a fun job—you'll be spending a lot of time on your back wearing a mask and gloves,but the payoff will be a more comfortable house that costs less to heat.