Sealing Joist Ends: The Hidden Airtightness Weak Point
Where timber joists meet an external wall is one of the most overlooked air-leakage paths in a building — hidden in the structure, hard to reach once the floor is on, and easy to skip. Multiplied across a floor, joist ends can sink an otherwise good air test.
Why joist ends leak
Joist ends built into masonry, or sitting on a wall plate, leave a gap around every single timber where the airtight line is broken. Each one is small, but a typical floor has dozens — together they add up to a significant, invisible source of air leakage and draughts that’s very hard to fix after the floor and ceiling are closed up.
The solution: a joist-end sealing bundle
The Joist End Bundle pairs our Passivhaus Airtight Sealant with a foil applicator gun so you can seal around every joist end quickly and consistently — restoring the airtight line through the floor zone as part of the wider system.
How it is applied
- Seal each joist end before the floor and ceiling are closed up
- Gun flexible airtight sealant around the timber-to-structure gap
- Tie the seal into the wall’s airtight layer (membrane or tape)
Benefits
- Targets a common hidden leak path
- Fast, consistent application
- Flexible seal that moves with the timber
Frequently asked questions
Why not just rely on the wall membrane? The membrane covers the wall face; the joist penetrations through it still need sealing individually.
When should this be done? Before the floor build-up and ceiling close it off — access afterwards is very difficult.
See the whole approach in How to Make a Building Airtight.