What is an attic ventilation smoke test, and how do you do one?
Short answer. An attic ventilation smoke test uses a small, harmless stream of smoke to see which way air is moving at the vents: smoke drawn into the soffit openings and out at the ridge means the system is flowing the right way. It is a quick, visual way to spot blocked intake, short circuiting exhaust, or air leaking up from the house.
The simple version
- Pick a day with light wind, ideally a warm afternoon when the attic is hotter than the outdoors.
- Use a smoke pencil, a smoke puffer bottle, or an incense stick. Keep any burning source well away from insulation and wood.
- Outside, hold the smoke just below a soffit vent. If it is drawn up into the vent, intake is working there. If it drifts away or blows out, that section may be blocked or acting as exhaust.
- Repeat at several points along each eave, especially near corners, porches, and additions.
Inside the attic
- Go up early in the day, step only on joists, and hold smoke near the inside of the eave. It should flow up the rafter bay, not hang.
- Release smoke near the ridge. It should rise and leave through the ridge vent or upper exhaust.
- Check near gable vents or box vents. If smoke is pulled in through one exhaust vent and out another, the system is short circuiting.
- Around light fixtures, top plates, and the hatch, smoke moving into the attic from below shows air leaks that carry moisture up.
The professional version
Energy auditors pair a blower door, which depressurizes the house, with smoke or infrared imaging to find ceiling leaks. That test is about air sealing, but it often explains why a ventilated attic still has moisture problems.
What to do with the results
Dead soffits usually mean insulation blocking the eave or painted over vents. Short circuiting usually means mixed exhaust that should be reduced to one type. Read roof ventilation basics and ridge vent vs box vents.