Plastic Windows Expand And Contract
It is important to note that in all plastic windows or in all plastic use, one of the main reasons of failure is not considering the coefficient of thermal expansion. In a 20°C change in temperature, plastic can expand or contract about 0.004" per inch. This means that a 12" long window can change in length almost 1/16 of an inch from freezing to room temperature and that a 48" long window will expand 1/4" with a 20°C change.

Using Silicone As A Window Sealant
All gasket materials have limitations to movement and the better silicones have a 50% movement to thickness ratio. Therefore, if you seal a window with silicone in a 1/4" gap, the silicon could stretch or compress as much as 1/8" before it would tear and be damaged.

Silicone Caulking Failure
Silicone can be used successfully, but it must be applied correctly. Most people start off with the right idea, but end up with a poor result. Below is an account of what most of us do when caulking windows.

First: We make sure both sides of the mounting areas are clean and free from oils and other contaminants that may cause the silicone not to stick. We then apply a bead of silicone around the framing area where we want it to stick. Sometimes we are generous with the silicone since we have masked off the view part of the window so we can wipe the silicone off without damaging the viewing area. We then push the window in place and press it down firmly and sometimes put weights or cantilever something against the window to insure a good bond while the silicone cures. We wipe off the excess and even fill any gaps around the window. After 6-24 hours, we remove any of the fixtures and the window looks great. Please note that we pushed the window so much that in some areas there is less that 1/32 of an inch of silicone between the window and the window frame.

Now The Test: It looks like a huge rainfall is on its way and we wait in anticipation for our window test. The rain falls and our window appears to be leak free.

How The Failure Occurs: A few months later the weather has had some cool mornings and some blistering hot days. The 24" long windows have seen a temperature differential of over 30°C and the window has stretched and contracted as much as 1/4" of an inch. In areas where the silicone was less than 1/32 of an inch thick, the silicone has torn.

Finding The Leak: You notice a small droplet of water on one of the windows. You then proceed to look for possible areas the window might be leaking. When the window is dry you then take a tube of silicone and seal the outside and the inside of the window frame and this seems to stop the leak. After a few years, you notice that you have globs of silicone around most of your windows. What a sight!

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