QUESTION: We have French patio doors that constantly fog up between the two panes of glass, and also have visible drip lines. I'm worried that we're losing our heated and cooled air to the outdoors. Any ways to fix this?ANSWER: I checked with Ross McCluney, an energy researcher who specializes in windows, and he thinks your problem is that either the seals have failed on the insulated glazing units in your doors, which is allowing water vapor to seep in and condense, or the frames are not sufficiently insulated. If it's the seals, they need to be replaced. If it's low insulation, this should be fixed also, possibly by replacing the doors.The drip lines come from a lack of sufficient insulation in the windows so the interior surfaces are exposed to the more humid indoor air in winter that drops below the dew point temperature, causing water condensation. Water is either condensing on the interior surface of the glass and dripping down to the frames, or insufficient insulation in the opaque framing elements of the doors is leading to condensation and dripping.There's a great deal of information on windows at www.fsec.ucf.edu/bldg/active/fen/index.htm. The effect of windows on a home's energy use and comfort level is huge, and choosing the right windows is essential. Windows used to be considered the weak link in the building envelope, essentially creating holes in the insulated walls and causing all kinds of energy problems. It's a different world out there today, with high-quality windows being made by many manufacturers and proper installation being done by knowledgeable contractors. Today's windows can actually enhance a home's level of energy efficiency. Don't delay on fixing window problems.Ken Sheinkopf is a communications specialist with the American Solar Energy Society (www.ases.org). Send your energy questions to askken@ases.org.

By MCT News Service