Shutter Condensation Calculator

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Welcome to the Shutter Condensation Calculator

This small JavaScript application computes the risk of condensation behind an internal shutter on a window.

It uses the temperature and relative humidity inside the room as well the outside temperature, together with the U-Value of the window to work out the best loose-fitting shutter that can be used without causing condensation on the inside surface of the window.

In order to use a better shutter (i.e. one with a lower U-Value), it will be necessary to use one that seals closely to the window, either to the glass, the frame or the reveal.

Room Temperature Temperature scale
Relative Humidity ±Accuracy % ±% Kelvin
Outside Temperature Celsius
U-Value of window W/m2K Fahrenheit

Dew Point Temperature ±
Critical Heat Flux W/m2 BTU/h-ft2
Critical Shutter U-value W/m2K   R= ft2-F-h/BTU


The dew point temperature indicates how cold the inside of the window needs to get before condensation forms. Condensation will not form if the dewpoint is below the outside temperature, and the Critical Heat Flux and Critical Shutter U-value fields will display 'none'.

The Critical Heat Flux tells how much heat is lost per square metre when the inside of the window is kept just warm enough to avoid condensation.

The Critical Shutter U-value tells how thick a shutter you can add without provoking condensation, if the shutter is loose fitting. If you want to reduce heat loss below the critical value you need to add a shutter with a lower U-value than this but it will need to include a vapour seal to prevent condensation.

There's more detail on About the Calculator


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