Stop Attic Air Leaking Into the House by Sealing the Wall Top Plates

The Problem

Every interior wall is capped by a 2x4 top plate, and the drywall on each side of the wall meets that plate at a seam. From the attic, those seams are rarely sealed — which connects the attic directly to the wall cavity below. The wall cavity, in turn, has holes cut into it in every room: switches, outlets, smoke alarms, security keypads. Attic air moves through the whole chain, carrying heat, humidity, fiberglass particles, and dust into the living space. In humid climates that moisture-laden air can also condense when it reaches cooler surfaces.

The Technique

The time to fix this is when the attic insulation is being replaced, because the plates are only reachable with the insulation out of the way.

1. Remove the old insulation.
2. Find the top plate of every interior wall and seal the seam where the drywall meets the plate — both sides of every wall — with caulk or canned spray foam.
3. Seal the penetrations through the plates too: every hole drilled for wiring or pipe is another leak into the wall cavity.
4. Only then put the new insulation down.

Insulation slows heat, but it does not stop air. Replacing insulation without air sealing underneath leaves the leak paths running exactly as before, now hidden under fresh material.