Video answer · Seal Tech Insulation
Why Is My First Floor Always Cold, Even With the Furnace Running?
The most common reason is air leaking in at the rim joist, where your house framing sits on top of the foundation.
The short answer
The most common reason is air leaking in at the rim joist, where your house framing sits on top of the foundation. Cold outside air gets in along the whole perimeter of the basement or crawl space and chills the floor above it.
What the rim joist is
The rim joist (or band joist) is the framing that runs around the top of your foundation wall, holding the ends of the floor joists. In many homes it was left with no insulation, or with a fiberglass batt stuffed in that does not stop air at all. Outside air leaks in through the joint between the sill plate and the foundation and through gaps in the sheathing.
Why closed-cell spray foam is the go-to fix
- It bonds to the wood and concrete and seals the gaps completely.
- It adds high R-value in a small space.
- It resists moisture, which matters in a basement or crawl space.
- Unlike fiberglass in this spot, it does not hide condensation or give mice a place to nest.
Other things to check
- Uninsulated crawl spaces under part of the house
- Floors over an unheated garage
- Leaky basement windows and doors
Sealing the rim joist is often one of the most noticeable comfort upgrades for the money.
Further reading: Where to Insulate in a Home (U.S. Department of Energy).
Want warmer floors this winter?
We will check your rim joists and crawl spaces for leaks.