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Last Updated: , Created: Wednesday, February 2nd, 2022

Overview - Condensation on the house side of thermal windows

Richard writes in:  

Hi, I could not find this second level question on your web site. I am familiar with the humidity levels required in a home to avoid condensation on the inside on a thermal pane window- we are still getting condensation with readings in the center of the room of 20 degree C /27% Relative humidity with outside temperature of -16.1 degree C. I believe this is within the acceptable levels to avoid condensation.  QUESTION -WHAT CAN BE THE CAUSE- although there is NO condensation between the panes could it be that these 14 year old windows have lost some of their efficiency- lost gas between the panes-so that they allow condensation inside the home even though the relative humidity is acceptable inside the home?

Hello Richard,

You are right that at those temperature and humidity levels you should not have any condensation on your windows. Let me give you an overview of all the things that can cause window condensation so you can hunt out a solution.

Since there is no condensation between the panes of glass, you probably have no gas leaks out of the thermal unit -- hence no reduction in the thermal resistance of the glass assembly itself.  I assume the condensation is on the bottom edge of the glass and on the frame. 


You need to understand two mechanisms that move towards the accumulation of condensation. 

First, the window is colder than the wall - even when it is a good window.  That means that warm room air hits the any window and begins to cool and flow down the face of the glass.  This air gets cooler the longer it is in contact with the glass and ends up much cooler at the bottom of the window.  Cooling any block of room air has a tendency to increase its relative humidity – actually compressing the air into a smaller block with the same amount of water in it.  If you sit your humidistat on something that holds it just at the bottom edge of the window you will see that the relative humidity is higher than the centre of the room. 

If there are curtains that restrict free air flow to the glass, this concentration effect is magnified.  So sometimes, just curtains can cause condensation – even indoor summer screens can cause wintertime condensation.  Those are simple to remove or open and see if the condensation goes away – nothing wrong with the window. Floor grill deflectors can also be a problem, preventing the heating system from warming up the window as it was designed to do.


The second mechanism is air flow from outside – cold air drafts.  If air has a path through the window or around the window, the constant flow of a tiny flow of cold air can over time reduce the temperature of the frame or even the glass enough to cause condensation or even frost on the inside.  Look up the keyword “Drafts” in my database and you will find a lot of information.  


The first thing to look for here is worn or missing weatherstripping, or warped windows that don’t close properly.  If you have fixed windows next to the opening windows and the fixed have no condensation problems, you probably have weather-stripping air gap problems with the openable windows as this is the only moving part of the window assembly.  You can use smoke pencils, now commonly available at the weatherstripping section of the renovation centres, to identify if and where air is coming through.  If you can’t get replacement weatherstripping from the manufacturer, or find the right stuff at the renovation centre, try glass stores.


If there is no problem with weatherstripping, then we look at the space around the window – the interface between the window and the wall.   If air is coming from behind the window trim, you could seal the trim itself to the wall.  Better yet, remove the trim, check the insulation behind it and seal that whole gap air tight before replacing the decorative trim.


All of that is to say, perhaps the windows themselves are not to blame – and all of this you can do yourself window by window for a lot less money than replacing all the windows in the house.  


I hope this helps,


Keywords: Condensation, Humidity, Weatherstripping, Glass, Walls, Frame, Relative Humidity, Renovation, Trim, Screen, Windows, Seals, Levels, Smoke, Temperature, Frost, Ice, Insulation, Drafts, Overview

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