1. The Critical Target: The Rim Joist and Sill Plate

The “rim joist” (or band joist) is the area where your house’s wooden frame meets the concrete foundation. It is notoriously leaky because wood and concrete expand and contract at different rates, creating gaps.

How to Seal It on a Budget:

• Silicone Caulk ($8–$12 per tube): Use this for small cracks (less than 1/4 inch) between the wooden sill plate and the concrete foundation.
• Expanding Spray Foam ($6–$10 per can): For larger gaps where the joists meet the rim board, use a “gap and cracks” insulating foam.
• Rigid Foam Board ($25–$35 per sheet): Instead of expensive professional spray foam, buy a sheet of 2-inch XPS (extruded polystyrene). Cut small squares to fit into the “bays” between joists, then “seal the edges” of the squares with spray foam. This creates a high R-value barrier for a fraction of the cost.

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