Why does condensation show in the corners first?

Because the corner of the glass and the frame are colder than the centre. The calculator only looks at the centre of the glass.

What do I do if the result is "risk"?

Lower the humidity (air the rooms, use the extractor, do not dry clothes indoors without airing), raise the temperature a little or choose glass with a lower Ug. See the guide on condensation.

What humidity is normal at home?

A hygrometer shows the value in your home. The calculator gives the limit for your glass and your temperature: below that value there is no condensation at the centre of the glass.

Is the Ug of the glass the same as the Uw of the window?

No. The Ug is for the glass only. The Uw is for the whole window and includes the frame and the spacer. For the centre of the glass the Ug is used. See the glossary.

Window condensation calculator: will the glass mist up?

Your values

Temperature inside (°C)

Between 5 and 35 °C.

Relative humidity inside (%)

A hygrometer shows it. For example, 55.

Temperature outside (°C)

Must be lower than the one inside.

Ug of the glass, W/(m²·K)

It is on the glass data sheet or in the quote. The lower, the better it insulates.

Example values for the Ug

Plain double (2.7)

Double Low-E with air (1.4)

Double Low-E with argon (1.1)

Catalogue values from one maker, only as an example. How to read the glass

Result

No condensation at the centre of the glass with these values. The corners and the frame are colder, so water may still show there.

Glass temperature (centre, inside): 14.7 °C
Dew point of the room air: 10.7 °C
Margin (glass minus dew point): 4.0 °C
Maximum humidity before it condenses on the glass: 72 %

The calculation is for the centre of the glass, with steady temperatures. The corners of the glass, the frame, shutters, curtains and furniture next to the window are colder or block the air, so condensation shows there first.

Read the guide on window condensation

How it works

The air in your home holds water vapour. When it touches a surface colder than its dew point, the vapour turns into drops.

The calculator estimates the temperature of the inner glass surface from the Ug and the temperatures, and compares it with the dew point of the room air.

The maths

Dew point: the Magnus formula, with the constants b = 17.625 and c = 243.04 °C (Lawrence, 2005), chosen for the range −40 to +50 °C.
Glass temperature inside: T = Ti − 0.13 × Ug × (Ti − Te). The 0.13 m²·K/W is the reference interior surface resistance of EN ISO 6946 for horizontal heat flow.
Maximum humidity: the relative humidity at which the room air reaches its dew point at the temperature of the glass.

Limits

It is an estimate for the centre of the glass, with constant temperatures. It is not a design calculation.
It does not count the thermal bridges of the frame, the spacer or the finishing: at those spots the surface is colder and condensation starts sooner.
The outside temperature changes through the night. Use the lowest one it usually reaches in the morning.

Sources

Wikipedia: Dew point. The Magnus formula and Lawrence's (2005) constants, with the range they cover.
The surface resistance of 0.13 m²·K/W is the EN ISO 6946 reference value for horizontal flow, confirmed in secondary sources. For a design calculation, check the standard.

Frequently asked questions

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