Every year around this time, as temperatures drop, homeowners and apartment dwellers are plagued with the problem of what to do about sweating windows (sometimes even windows that have storm sash).

Although comdensation is usually much more serious on windows with metal frames, it is also quite common on those with wood frames In some cases the amount of sweating is minor, but in many homes the amount of moisture that accumulates is enough to cause paint to blister and peel from window frames and nearby wall and ceiling surfaces. It can cause mildew to form on walls and trim around the window. In extreme cases the dampness may cause wood sills or other structural members to rot, and plaster or wallboard around the window to decompose.

Condensation, the correct term for sweating, occurs when air that is relatively warm and laden with an average amount of moisture vapor comes in contact witha a colder surface. As the air is chilled by this contact, its dewpoint drops -- dewpoint being that remperature at which air becomes saturated with moisture vapor and can hold no more. Warm air can hold more water in suspension (in the form of moisture vapor) than can cold air, and when air is chilled the excess moisture condenses to form drops of water on the cold glass, much the way the outside of a pitcher of ice water sweats on a hot summer day.

This is the same phenomenon that causes eyeglasses to fog when one enters a warm room after being out in the cold -- as the warm air on the inside is chilled by the cold lenese, excess moisture condenses on the cold glass.

The same thing happens with windows when a house is hested on the inside, and the outside air is very cold. The clod air outside chills the glass to such a degree that when the warm air on the inside comes in contact with it moisture forms as the vapor condenses (if the glass is cold enough, the moisture will actually freeze and form a coating of ice on the inside).

This type of condensation can occur on the frames of metal windows because metal is an even better conductor of heat than glass is (which means it gets very cold on the iside when the outside is exposed to the cold). On metal window frames the condensation forms on the metal as well as on the glass, and adds to the problem. (This does not happen on some of the better quality new metal windows and doors because these new units have built-in thermal barriers consisting of plastic inserts between the outdoor and indoor portions of the metal frame. The plastic prevents conduction of heat from one side to the other and thus keeps heat from being lost on the inner side.)

In most areas condensation on windows can be eliminated by installing snug-fitting storm windows on the outside. The storm windows will act as insulation to keep the glass from getting so cold and the sindows will not be as likely to have condensation from on the inner face.

However, even if the storm window fits well, the problem may still crop up on very cold days. The extra layer of glass, coutled with the trapped pocket of dead air between the two windows, should keep the inner window warmer by cutting down on heat loss, but it doesn't stop the heat loss completely. On extremely cold days enough heat may escape to permit the glass on the inner window to become chiller -- then some condensation may form.

This is why in very cold climates more and more people are finding a third layer of glass advisable: either storm windows plus double-pane insulating glass in the main windows, or an extra plastic storm window on the inside with a storm sash on the outside. This creates a heat barrier consisting of three panes of glass and two pockets of dead air -- a combination quite effective in preventing condensation problems and heat loss.

When windows protected by storm sashes develop condensation on the inner face of the storm window, instead of on the inner face of the main window, the most likely immediate cause is air seepage around the main window due to the lack of adequate weatherstripping. If warm inside air seeps out around the edges of the window sash and gets into the space between the two windows, it will condense on the coldest surface -- the glass of the storm window (the glass on the main window will almost always be warmer because of the heat it absorbs from inside).

To correct this condition, more weatherstripping may be required on the main window to cut down on air leaks around the edges.Also, if the outside storm windows are permanently installed, make sure the weep holes or drain holes along the bottom, which allow trapped moisture to escape harmlessly, are not clogged.

If both these precautions do not stop condensation on the storm windows, the only other step you can take is to install an extra plastic storm window on the inside. Do-it-yourself kits are sold for this purpose in many hardware stores and lumberyards, as well as in home centers and department stores. Some contain flexible sheets of clear plastic that are taped or tacked up, while others use flexible or rigid sheets of clear plastic that are fastened in place with plastic moldings or trim strips (the latter are decorative and can often be left in place during the summer).

In addition to insulation and weatherstripping, there is another way to combat condensation: Try to lower the indoor relative humidity (RH) when it is very cold outside. Although an RH of 30 to 40 percent is ideal for most homes, it may be too high when outside temperatures drop down close to zero. In such a case an RH of no more than 20 percent will be required to prevent condensation on windows -- even if they are adequately protected with weatherstripping and storm windows. Indoor humidity can be lowered by using exhaust fans in kitchens, laundry rooms and bathrooms whenever moisture vapor is being generated. In houses with central humidifiers, lower the setting on these units when condensation becomes a problem.