Glossary · Insulation & windows
Rim joist (header)
Also called: rim joists · basement header · band joist · rim board · rim joist area · joist header
The band of framing on top of the foundation wall around the edge of the floor; it's prone to air leaks and often poorly insulated.
What it means
Called the header in Canada and the rim or band joist in the US, this strip runs around the basement ceiling where the floor framing meets the foundation. It has many joints, is often left uninsulated and is where the stack effect draws cold air in. Sealing the joints and insulating with rigid foam or spray foam is a common audit recommendation.
Why it matters in an audit
It's a small area with outsized air leakage, which is why audits so often flag it.
In Canada
NRCan notes that stuffing batts into the header doesn't stop air leaks; seal joints with caulking, and cover spray foam with a fire-resistant material if the basement ceiling doesn't cover it.