We have an 11 year old bungalow and this problem only occurs on the middle section of a bay window. We think that frost happens inside the insulation somewhere and when the temperature outside is mild the frost melts and water drips under the interior window trim and drips onto the slats of the blinds and pools on the window sill. We never have any water leakage when it rains.

We took out a large chunck of foam insulation in anticipation of mild weather and this morning the spot we had cleared was dripping. What do we do now?


Are you saying that you manually insulate the window on the inside with foam board to prevent heat loss? If so, this is completely expected. What's happening is that the insulation is doing its job by preventing the movement of heat, thereby keeping the glass colder. However, it sounds like the installation is not airtight. As a result, any moist air that goes through or around the foam board and touches the glass finds the perfect surface to condense on: something vertical, hard, and cold. So it does. If you want to manually insulate a window on the inside like this, the insulation must be absolutely airtight to prevent condensation. The more expensive but more correct solution to the problem would be to replace the window with a higher-performance one, likely a triple-pane. Low-E exterior storm windows are good too, and a lot cheaper, but that might be hard to do on a bay window.

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I would continue investigating upwards from there. Have seen similar situations where the roofing above the point had been set too flat: rainwater (or thawing frost) can run backwards under the overhang to the wall. If you have a beam or something coming out of the wall at that point, water can ingress through capillarity in the space around it.

Off topic, I have had this kind of problem on a Land Rover: absolutely no water coming in when running the vehicle -even through a summer storm-, but pronounced leaking around the windscreen and above the front doors after leaving it a night outside with light frost. These are points that are under a bit of overhang, so water drops falling down do not stike there. Capillarity at work, once more.


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These are Argon filled dual pane windows right? The chances of only one window condensating because of insulation problems isn't likely but the window is giving you your biggest clue. The seal between the glass is gone and the argone has leaked out leaving you a thermopane window that is no longer thermopane. Have a new glass panel made up and replace the old one, simple!

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