# How do I calculate the interior temperature of a window at a given outside temperature with a fixed interior temperature?

| Posted in GBA Pro Help on

I am trying to predict at what outside temperature will I get condensation on my windows. If it is 70 degrees inside with 40% relative humidity and my window has a u-value of .3 at what outside temperature will the window (frame and center of glass) have condensation. How do I calculate this?

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### Replies

1. GBA Editor
| | #1

Chaz,
The answer depends on your indoor relative humidity level. Another complicating factor: your window is not at a uniform temperature. The frame, the center of the glass, and the edge of the glass all have different temperatures.

Here is a link to an article with more information on this issue: Rating Windows For Condensation Resistance.

2. Expert Member
| | #2

The dew point of 40% RH/70F air is about 46F if you run it through a psychrometric calculator or look at a chart. At 45F window surface temperature you'd get a light fogging, but it has to be to be cooler to have copious condensation.

There are no simple calculations, but there are multiple reference documents with curves showing when copious condensation begins that may be useful. Different windows with a calculated U-factor of 0.3 may have different center-glass U-factors, but a typical U0.30 window uses a single low-E coating with an argon gas fill. The loss through the glazing isn't perfectly linear with temperature, but if you take a peek at this graphic you'll see that typical U0.3 glazing will have condensation at an outdoor temp of about -45F or colder:

A different representation would agree assuming it's a typical low-E argon filled glazing:

At 40% interior RH a typical non-argon clear-glass double pane starts to get visible condensation at about 0F outdoor temp.

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