Reverse draught at a vent
Initial assessment: reverse draught at a vent. Measurements, risks and action order.
- Do not permanently cover the symptom.
- Provide safe air supply.
- Call a professional where fuel-burning appliances are present.
What to check first
Start with a simple review. Do not permanently cover the symptom. Then provide safe air supply. Finally, call a professional where fuel-burning appliances are present. This order helps separate a short humidity spike after showering or cooking from a problem that persists for days.
How to take measurements
Measure temperature and humidity in the same place and at a similar time. If the problem concerns a corner, window or wall behind furniture, add the surface temperature. For air-quality assessment, record CO₂ as well. A short series of readings is far more useful than one isolated number.
How to interpret the result
Treat the result as a screening indicator. Dew point shows the condensation temperature, while surface relative humidity helps assess whether a cold part of the building may remain damp for long enough to create risk. The calculations cannot detect leaks, insulation defects or installation faults.
Technical basis: moisture control is central to mould prevention; EPA also advises keeping indoor relative humidity below 60%, ideally 30–50% where possible.
When a calculator is not enough
Call a professional if there is reverse draught, combustion fumes, a persistent mould smell, major water damage, wet structural materials or a recurring issue despite reducing moisture sources and providing reasonable ventilation. Health symptoms require medical advice, not an online calculator.