Basements and Rim Joists

Go down to the basement of an old house on a windy winter day and put your hand along the top of the foundation wall. That cold breeze is the bottom half of the stack effect: air being pulled into the house to replace the warm air leaving through the attic ¹.

Where the leaks are

The top of a basement wall is above ground, and the wooden house simply sits on it. The flat board on top of the foundation is the sill plate; the board standing on edge around the perimeter, where the floor joists end, is the rim joist (or band joist). Between every pair of joists is a small pocket along the rim, and each pocket has cracks and gaps to the outside ¹. In a stone-foundation house, the top of the wall may be ragged and full of gaps.

Seal the rim

ENERGY STAR lists three places to seal ¹:

• A. The gap between the sill plate and the foundation.
• B. The top and bottom of the rim joist, where it meets the subfloor above and the sill below, all around the house.
• C. Every pipe, wire, dryer vent, and gas line that passes through to the outside.

Use caulk for gaps up to a quarter inch and spray foam for gaps from a quarter inch to about 3 inches ¹. Around a furnace or water-heater flue sleeve, use high-temperature caulk ¹. Bump-outs such as a bay window hanging off the foundation deserve special attention, since they leak more ¹.

Insulate the rim

Once it’s sealed, insulate each rim cavity. ENERGY STAR suggests rigid foam cut to fit and foamed into place around its edges, or fiberglass batts cut to fit without compression, gaps, or voids ¹. Many people do this at the same time as they finish the basement, insulating the walls from floor to ceiling ¹.

The basement walls

The big decision is whether the basement is inside the heated house or outside it ²:

• Outside the heated house (a cold basement): insulate the floor above, between the joists, and air-seal everything between the basement and the living space, including the rim and the stair door ².
• Inside the heated house (often the case when the furnace, water heater, and ducts are down there): insulate the foundation walls. The Park Service recommends rigid foam on the inside face of the wall, with the joints sealed, only after any drainage problems are fixed ². ENERGY STAR’s retrofit target for cold climates is R15 of foam sheathing or R19 batts in a framed wall ³.

The “drainage first” rule matters: insulation over a damp wall hides the water where you can’t see it. Fix gutters, downspouts, and grading so water flows away from the house before insulating, and watch the walls through a wet season. (The Residential Construction course covers foundation drainage.)

Crawlspaces follow similar logic: the Park Service notes that venting crawlspaces doesn’t keep them dry in humid summers and suggests sealing the vents and bringing the crawlspace inside the insulated envelope instead ².

Try it

With a flashlight and a can of foam, go around the basement perimeter and look at the rim joist pockets. Where can you see daylight, feel air, or see cobwebs waving? Seal those first; it’s an afternoon and a few cans of foam. Then measure the rim pockets for rigid foam: most will be the same size, so you can cut them in batches.