What Is a Rim Joist in Construction?

February 20, 2024

Rim joists play a critical role in the floor frame of homes. They carry the weight of walls and floors and work in tandem with other joists to form a band around the structure’s foundation. They also provide support for the exterior sheathing, siding and trim boards of a building or home. In addition, rim joists cover the ends of other joists to prevent exposed cavities and stop moisture from freely entering your home, damaging your foundation in the process.

The conventional material for rim joists is solid framing lumber, typically 2 x 10s or 2 x 12s. They may also be made of engineered wood products, such as oriented strand board or laminated veneer lumber (LVL). Rim joists are usually the same size as other joists to ensure they are flush with joists above them. They are connected to the sill plates with joist hangers, which are metal straps that hold them together.

Insulating the rim joists in a new house or in an older home is a smart way to minimize energy loss. It is best to use an air impermeable insulation, such as spray foam, which creates a tight seal and keeps heat in and moisture out.

In crawl spaces or unfinished basements, the rim joists are often exposed to humid air. Without insulation, the rim joists’ surfaces can become colder than the interior air’s dew point. This causes water vapor to condense on the surface, which can lead to mold problems and other issues. If the rim joist is covered with insulation, however, it warms to a higher temperature and water vapor will diffuse at a slower rate.

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