Condensation Ventilation Calculator — Dew Point & Extract Rate

The dew point is the temperature at which air becomes saturated and water vapour condenses into liquid water. If any surface in your home (windows, cold walls, behind furniture) is at or below the dew point temperature, condensation will form. At 20°C and 70% RH, the dew point is about 14.4°C.

Indoor RH should be maintained between 40-60% for comfort and to prevent condensation. Below 40% can cause dry skin and respiratory irritation. Above 65% increases condensation risk, and above 70% promotes mould growth. The ideal target is 55-60% RH.

A typical UK household puts about 12 litres of moisture into the air per day, rising to over 20 litres where clothes are dried indoors (BRE / industry figures). The main sources are breathing and perspiration (~1.5L per person), cooking (~3L), bathing and showering (~1.5L) and indoor laundry drying (~5L). This calculator sums those sources for your home. The moisture must be removed by ventilation or it accumulates as humidity and causes condensation.

A PIV (Positive Input Ventilation) unit is often the most effective whole-house solution. It introduces filtered, slightly warmed air from the loft, diluting moist air and maintaining positive pressure. Extract fans are essential at source in wet rooms: Approved Document F 2021 Table 1.1 sets the intermittent minimums at 30 L/s for a kitchen where a cooker hood over the hob is ducted to the outside, or 60 L/s where the extract is elsewhere, there is no hood, or the hood only recirculates, 15 L/s for a bathroom or shower room, 30 L/s for a utility room and 6 L/s for a WC.

Heating alone does not solve condensation — it raises air temperature which can hold more moisture, but without ventilation the moisture stays. The solution is adequate ventilation PLUS heating. Warm, well-ventilated rooms rarely suffer condensation. Cold spots (thermal bridges) should be insulated.