Rim Joist: Purpose, Insulation & Common Mistakes



Rim joists run around the outer edge of wood-framed houses, providing critical structural support to a home’s floor frame.

But before you break  out the toolbox, let’s  look at the purpose of  rim joists and the benefits  of proper insulation. Later we’ll get into the best materials to use when insulating rim joists.

What Is a Rim Joist?

A rim joist is a board that runs perpendicular to the floor joists and end joists. Rim joists close off the open cavities of the floor joints, creating the outer edge of the floor framing.


Builders typically construct rim joists from the same material as the other joists. This material is most often traditional or engineered lumber, but builders can also use steel or a combination of steel and lumber.


Rim joists run around the entire perimeter of a house. Engineers typically run floor joists across the shortest possible length to prevent flexing. Since rim joists run perpendicular to floor joists, rim joists usually run parallel to the long side of a house.

Rim Joist Insulation

- Preventing Condensation: Condensation promotes mold growth, which can negatively impact the health of people occupying the building. - Maximizing Drying Potential: Vapor diffusion is the primary method of drying wet walls,  with the layers on both sides of the rim joist affecting its ability to dry.

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