Because windows are usually the coolest surfaces in a house, they may collect condensation in winter, when outdoor temperatures are low and indoor air is warm. These conditions encourage mold and mildew and can cause window frames and sills to rot.

To reduce condensation, increase ventilation—just using bathroom fans and kitchen fans can make a big difference. Also keep showers short and run only full loads in washers and dishwashers to reduce indoor humidity levels.

If doing these things doesn’t help, contact an insulation contractor. (Also see the Dehumidifiers Buying Guide.)

If condensation forms between thermal window panes, there is unfortunately no cost-effective way to remove it; the glazing must be replaced.

Your best bet is to buy quality thermal windows that are backed by a strong warrantee (at least 10 years). One mark of quality is “IGCC approved,” a certification given out by the Insulating Glass Certification Council.