Q Our home inspector reported that three windows in our 9 year old house had fogging insulated panes, and he suggested that we contact the builder or manufacturer for warranty information. We contacted the manufacturer who sent a man out, but he said that only two of the windows were fogged. When we called the inspector, he said that insulated windows will fog only under certain conditions, and that we look at the windows at the same time of day that they were inspected. Is our inspector a bit foggy in the head?

A It has been my experience that insulated window panes, when the seal is broken, will fog only under certain conditions. The two pieces of glass in a double-pane window have an inert gas between them which is held in place by a seal. This thin space of gas is what allows the windows to slow down the transmission of heat or cold. When this seal is compromised, ordinary air is allowed to enter, and moisture may condense on the inside surfaces of the glass. These types of windows are most likely to fog on a winter morning a short time after the sun hits them. The outside of the window has been cold overnight, and the inside has been warm. When the sun hits the cold outer glass, moisture condenses and the foggy appearance occurs. A few hours later, as the temperatures stabilize, the fog may disappear altogether. In this case, the inspector was correct to suggest that the windows be inspected under the same conditions. In the case of your 9 year old house, your windows may still be under warranty. The earlier versions of insulated windows were somewhat prone to failure, but technology has steadily improved, and today’s windows are much more reliable. Warranties have gotten much better as well, so it you have foggy windows, check with the manufacturer to see if you can have them replaced under warranty.