What is a rim joist?
A rim joist is the structural member that forms the outer edge of a floor system, connecting floor joists to the house frame or foundation at the perimeter.
The rim joist (also called rim board or band board) is the horizontal structural member installed at the perimeter of a floor system, running along the outside edge where the floor joists meet the top of a foundation wall or house frame. It ties the individual joists together and braces the ends of the floor system against the foundation or band. In typical wood-frame construction, it sits perpendicular to the floor joists and is secured to them with nails or screws.
For homeowners in Greater Toronto Area, the rim joist is a critical detail in both additions and retrofit work. When contractors add a new storey or extend the floor system, they must size and install a rim joist that matches code requirements for the span and load. Because the rim joist often sits at the exterior perimeter, it is exposed to temperature swings and moisture, making it a thermal weak point and a target for air leakage.
Insulation retrofits frequently address the rim joist as a priority. Uninsulated or poorly sealed rim joists can be a major source of heat loss and cold drafts in older homes. Contractors wrap or fill the rim joist cavity with foam board, spray foam, or batt insulation, and seal gaps with caulk or weatherstripping. This work is often combined with basement or crawlspace finishing projects. Proper rim joist treatment improves interior comfort and reduces energy costs, which is why builders and renovators planning home additions or basement work include it in their scope.