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Window & Wall Condensation Risk Calculator

Will condensation form on a window or cold wall? Inside-surface temperature from the U-factor and indoor/outdoor temperatures, compared with the dew point of the room air — the safe indoor humidity, the outdoor temperature at which dripping starts, and mould-risk thresholds.

Surface U-factor

Custom U-factor

Indoor air temperature

Indoor relative humidity

Outdoor temperature

Heavy curtains / blinds closed or furniture against the wall?

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How it works

T_surface = T_in − U × R_si × (T_in − T_out), R_si ≈ 0.13 m²K/W (0.26 behind curtains); dew point from Magnus formula; condensation when T_surface ≤ T_dew; surface RH = e_s(T_dew) ÷ e_s(T_surface)

Condensation appears wherever a surface is colder than the dew point of the air touching it. The inside face of a window or wall sits between room and outdoor temperature in proportion to how much of the assembly’s total resistance the inside air film represents: a single pane (U 5.8) drops most of the way to the outdoor temperature, a triple-glazed unit stays within a couple of degrees of the room. Meanwhile the room’s dew point rises with both temperature and humidity — 21 °C at 50% RH has a dew point of 10 °C. Curtains and furniture make the surface colder still by shielding it from room air. Mould needs no liquid water, only about 80% relative humidity at the surface for a few weeks, so the safe indoor humidity in winter is lower than the condensation limit suggests, which is why 40–45% is the usual cold-weather target.

Worked example

A low-e double-glazed window (U 1.6) with the room at 21 °C and 50% RH and −5 °C outside: the glass surface is 15.6 °C against a 10.2 °C dew point — dry, though surface humidity is a marginal 70%. Indoor humidity could rise to 71% before dripping, but should stay under 57% to keep the surface below 80% RH; the same room with single glazing (surface 1.4 °C) would be streaming.

Frequently asked questions

Why is the frame edge wet when the glass is dry?

Spacers and frames have higher U-factors than the centre of glass — aluminium frames especially — so their surface temperature is several degrees lower. Condensation at the edges is the first sign.

Between the panes?

That is a failed seal, not a humidity problem; the unit needs replacing.

Related calculators

Dew Point Calculator

Find the dew point — the temperature at which air becomes saturated and dew, fog or condensation forms — from temperature and humidity.

Relative Humidity Calculator

Compute relative humidity from the air temperature and the dew point using saturation vapour pressure — the physical definition of humidity.

Absolute Humidity, Vapor Pressure & Mixing Ratio Calculator

Psychrometric properties from air temperature (°C, °F or K) and relative humidity, dew point, wet-bulb temperature or absolute humidity: saturation and actual vapour pressure, vapour pressure deficit, absolute humidity, mixing ratio, specific humidity, enthalpy, air density, heat index and the water to add or remove in a room — with a choice of Magnus, Arden Buck, Tetens or Wagner–Pruss formulas.

Dehumidifier Size Calculator

Estimate the pints-per-day capacity your dehumidifier needs from the room’s square footage and how damp it feels, then match it to a standard unit size.

Building-physics estimate using steady-state formulas and typical material values; real assemblies, workmanship, air leakage and climate data can move results significantly — confirm with a qualified designer for code compliance.