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, around the weatherstripping, or around the rough opening. This might be cooling the inside surface of the window below dew point temperature.

Another issue to consider is that post-and-beam houses are built with timbers that often have a high moisture content. Typically, the timbers are installed green, because it's expensive to kiln-dry such large pieces of lumber. This may raise moisture levels inside the house for a year or two. Because the window units are typically installed within a timber framework involving headers, sills, and side members that are wet, each window has a local moisture source. Get a moisture meter and check the moisture content of the timbers. Eventually, they will stabilize at a moisture content of 8%. Even if the inside surface of timbers (the side facing the living space) is dry, the surfaces within the wall cavity are most likely wet. If wet timbers are the problem, you can raise the indoor temperature, circulate warm air toward the windows, and use a dehumidifier for the short term.