The windows in my house is 1973 vintage. Aluminum frame with an air gap of about 3 inches. The house is well insulated: R20 walls R40 attic. Humidity set and confirmed by a hygrometer at 40% RH. No condensation on the windows. However, between the windows I get condensation on the inside of the exterior glass and some mould on the frame and wood between the glass. What can I do if this is not normal? Would replacing the windows, an expensive proposition, be the solution or is there something I can do to reduce the moisture between the glass. I have weather stripped the frames, the channels. Is the problem the exterior glass is just too much cooler than the interior glass.
Answer:
The secret to keeping frost off windows of this type is to have the exterior window leaking a bit more than the interior. That gives you a good air exchange in the cavity and it doesn't frost up. Make sure you have a bit of air leakage (less than 1/8") along both the top and bottom of the outside window and that should do it.
