Condensation
Dew point & condensation calculator
Why do the windows run wet on cold mornings? Enter the room's temperature and humidity, then a window, wall or a surface temperature you measured, to see the dew point and whether water or mould is likely.
Dew point and surface
Air 20 °C, 60% relative humidity; surface 12.7 °C (Older double glazing, 0 °C outside).
No water yet, but the air touching the surface is at 95% relative humidity, above the 80% that ISO 13788 uses as the limit for mould growth.
12 °C dew point
53.6 °F · for 20 °C air at 60% relative humidity
- Surface, estimated12.7 °COlder double glazing, 0 °C outside
- Humidity at the surface95%margin 0.7 °C
- Water in the air10.3 g/m³absolute humidity
To keep this surface dry, keep the room below 63% relative humidity at 20 °C; to keep it under the 80% mould limit, below 50%.
| Relative humidity | Dew point | Surface at 12.7 °C |
|---|---|---|
| 30% | 1.9 °C | Dry |
| 40% | 6 °C | Dry |
| 50% | 9.3 °C | Dry |
| 60% | 12 °C | Damp, mould risk |
| 70% | 14.4 °C | Condensation |
| 80% | 16.4 °C | Condensation |
| 90% | 18.3 °C | Condensation |
Td = b·γ ÷ (a − γ), γ = ln(RH/100) + a·T ÷ (b + T), a = 17.625, b = 243.04 °C
How to use the dew point calculator
- Enter the room's air temperature and relative humidity, ideally read from a hygrometer in the middle of the room.
- Choose the window or wall you are worried about and the outdoor temperature, or switch to a surface temperature you measured.
- Read the dew point, how damp the air right at that surface is, and the humidity that keeps it dry.
What the numbers mean
The dew point is the temperature at which the room's air would be saturated. Any surface colder than that collects water. Air touching a cold surface also gets more humid without reaching it; ISO 13788 uses 80% relative humidity at a surface as the limit above which mould can grow, so a surface can be at risk even while it looks dry.
The estimate treats the window or wall as a steady barrier: its inner face sits below room temperature by U × 0.13 × (indoor − outdoor), where 0.13 m²K/W is the standard inside surface resistance of ISO 6946. Frames, edges, corners and spots behind curtains or furniture run colder, so condensation usually starts there first.
Worked example
At 20 °C and 60% humidity the dew point is 12.0 °C. Older double glazing on a 0 °C night sits at about 12.7 °C, only 0.7 °C above it, with air at about 95% humidity against the glass: damp, and wet at the edges. Single glazing would be near 7.5 °C and stream with water. Bringing the room down to 50% gives a dew point of 9.3 °C.
Indoor humidity: what to aim for
The US Environmental Protection Agency advises keeping indoor relative humidity below 60%, ideally between 30% and 50%. In cold weather even 50% can be too much for single glazing or a cold corner, which is why the calculator gives the humidity limit for your own surface.
- Cut moisture at the source: lids on pans, extractor fans when cooking and showering, and clothes dried outdoors or in a vented dryer.
- Let it out: short bursts of window ventilation, trickle vents left open, and doors closed on steamy rooms.
- Warm the surface: better glazing, secondary glazing or insulation raise the inside surface temperature, and a steadier heating pattern stops walls from cooling right down.
Wipe condensation off sills before it soaks into timber or plaster. Mould that keeps coming back, or a damp patch that is not near a cold surface, points to a leak or rising damp, which needs a builder or surveyor.
Sources
- O. A. Alduchov and R. E. Eskridge, "Improved Magnus form approximation of saturation vapor pressure", Journal of Applied Meteorology 35 (1996), 601–609: the constants 6.1094 hPa, 17.625 and 243.04 °C, for −40 °C to 50 °C over water.
- ISO 13788:2012, Hygrothermal performance of building components and building elements: internal surface temperature to avoid critical surface humidity and interstitial condensation: the 80% surface relative humidity limit for mould growth.
- ISO 6946, Building components and building elements: thermal resistance and thermal transmittance: inside surface resistance 0.13 m²K/W for horizontal heat flow.
- BRE, SAP 10.2 Table 6e (window U-values) and RdSAP 10 Tables 6 and 13 (wall U-values).
- US EPA, A Brief Guide to Mold, Moisture, and Your Home: keep indoor humidity below 60%, ideally 30–50%.
Questions
What is the dew point?
It is the temperature to which air has to cool, with no change in its moisture, before it becomes saturated and water starts to condense. Air at 20 °C and 60% relative humidity has a dew point of about 12 °C, so any surface colder than that gets wet.
Why do my windows get condensation on the inside?
The glass is the coldest surface in the room. When its inside face drops below the dew point of the room air, moisture from breathing, cooking, showers and drying clothes condenses on it, usually at the bottom of the pane and along the frame, where it is coldest.
What humidity stops condensation on windows?
It depends on how cold the glass gets. With 20 °C indoors and 0 °C outside, older double glazing stays dry below about 63% relative humidity and single glazing only below about 44% at the centre of the pane, less at the edges. Enter your own window to get its limit.
Is condensation on windows a sign of mould?
Not by itself, but it shows the room is humid for its coldest surfaces. If it happens most days, the damp sill, frame and nearby wall corners are where mould tends to start. ISO 13788 treats air above 80% relative humidity at a surface as enough for mould growth.
How accurate is the Magnus formula?
With the Alduchov and Eskridge constants it tracks the saturation vapour pressure of water closely between −40 °C and 50 °C, far better than a hygrometer reads humidity. The surface temperature estimate is the rougher part, so measure the coldest spot if you can.