A.Paul Fisette responds: The problem is that the inside surface of the window is below the dew point temperature of the air that comes in contact with it. This means either that you have too much moisture in the house or that the window is not insulative enough, so that its inside surface is too cold. You might have both conditions. The indoor relative humidity (RH) should be around 35%, but it should not go above 50%. Monitor the RH for a week with a device that records high and low readings. You can purchase these at stores like Radio Shack for about $20. If you learn that the humidity in the home is too high, figure out why and lower it.

A good place to start is by investigating exhaust fans in bathrooms, kitchens, and other high-moisture areas. Be sure that the fans are working correctly and are being used. If you find that the RH is okay, I would suspect that either the window is not as energy efficient as you think or it leaks air. Air may be leaking through the window seals,...

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