How to Find Air Leaks: Smoke Testing a Building
You can’t seal a leak you can’t see. Before and after airtightness work, smoke testing makes airflow visible — showing exactly where air is moving through the building fabric so you can fix it before the formal air-pressure test.
Why find leaks before the test?
Approved Document L now requires every new dwelling to be pressure-tested, and a failed test means delay and rework. Smoke testing lets you find and fix the leaks on your own terms — cheaper and far less stressful than discovering them on test day.
How smoke testing works
A smoke source releases a fine, visible smoke that drifts toward draughts and leakage paths. Around junctions, penetrations, frames and joist ends, the smoke shows you where air is escaping (or, under pressure, where it’s being drawn in) so you can target your sealing.
The tool: a smoke stick
The Wöhler Smoke Stick produces a controllable plume of visible smoke, ideal for pinpointing leaks at specific details. Once you’ve found them, seal with airtight sealant, tape or brush membrane as appropriate.
How to use it
- Work methodically around junctions, penetrations and openings
- Watch where the smoke is pulled through or pushed out
- Mark and seal each leak path
- Re-check before the formal air test
Benefits
- Makes invisible air leaks visible
- Pinpoints exactly where to seal
- Cuts the risk of a failed air test
Frequently asked questions
Does this replace the formal air test? No — it’s a diagnostic tool to prepare for it; the certified pressure test is still required.
When should I smoke test? Before closing up, while leaks are still accessible.
See the whole approach in How to Make a Building Airtight.