Attic Top-Plate Air Sealing
Sealing the seams where drywall meets the wall top plates — and the wiring holes through them — while the attic insulation is out.
Air-Seal a Beam Penetration Through the Middle, Not Only the Sides
A built-up beam passing through the air barrier leaks between its sandwiched plies even after you caulk all four sides — a small hole drilled through the beam at the barrier line lets you inject sealant into that hidden middle gap.
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The Problem
When a built-up beam — two 2x members sandwiched together — penetrates a sheathed wall that's serving as the air barrier, sealing it looks straightforward: caulk the gap on all four sides of the beam pocket. But a doubled beam has a fifth gap you can't see: the seam between the two plies, running straight through the wall. In a tight house, that hidden channel is exactly the kind of small leak the pressure difference finds and pulls air through continuously.
The Technique
This builder's fix on a screen-porch beam:
- Drill a small hole — his was a half inch — through the beam at the plane of the air barrier, passing through both plies.
- Inject sealant through the hole so it fills the seam between the plies where they cross the barrier line.
- Caulk the perimeter — all four sides of the beam where it meets the sheathing.
- Tool the beads smooth. The beam pocket is now sealed on all four sides and through the middle.
Pro tip: If you're earlier in the sequence, you can skip the drilling entirely — run a bead of sealant between the two 2x plies before they're sandwiched, or hang the beam from the wall face so it never penetrates the air barrier at all. The drilled-hole version is the retrofit for a beam that's already built and placed.
Safety note: A beam is structure — don't drill holes in one without knowing what it's carrying. This creator judged a single half-inch hole acceptable on a lightly loaded porch beam; a heavily loaded beam, or multiple holes, is an engineer question, not a caulk question.
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