UPDATED on July 6, 2021 with new information on code requirements
Rim joists make up the perimeter of the floor system of a wood-framed house. Also known as band joists, rim joists are pieces of lumber—either sawn lumber or engineered lumber—that are installed vertically above the mudsill. On a new house, a typical rim joist might be a 2×10 or 2×12. Older houses have all kinds of different rim joists, including 2x6s, 2x8s, or even square timbers like 8x8s. Some older balloon-framed houses lack rim joists—instead, the bottom plates of the wall framing (or the studs) sit directly on the mudsill.
If you live in an older house, it’s important to check whether your rim joists are insulated. In a house with an unfinished basement or crawlspace, inspecting your rim joists should be easy. If your basement has a finished drywall or plaster ceiling, however, you’ll probably need to cut some inspection holes in the ceiling to inspect your rim joists.
Two-story homes usually have another ring of rim joists above the first-floor ceiling. If you need to insulate this type of rim joist, it’s best to hire a cellulose-insulation contractor. (To learn more about the “grain bag” method that cellulose contractors use to insulate rim joists, see “How to Install Cellulose Insulation.”)
Why do we insulate rim joists?
If your rim joist is uninsulated, the only layers between the rim joist and the outdoors are the sheathing, which is typically between ½ in. and ¾ in. thick, the asphalt felt or housewrap, and the siding. Rim joists are above grade, so it makes sense to insulate them to the same level as above-grade walls—a minimum of R-13 in climate zones 1 through 4, or a minimum of R-20 in climate zones 5 though 8.
These days, rim-joist insulation is…