Condensation in your basement can pose a significant threat to items you have stored down there. Books, photos and other paper goods absorb the moisture, causing irreversible damage. Metal objects start rusting. Furniture, carpeting, and clothing show signs of mold and mildew.
Condensation occurs when moisture-laden warm air strikes a cold, dry surface, such as a basement wall or window. A similar thing happens with cold water passing through copper pipes, as the metal pipe is usually much colder than the surrounding air.
The solution for most basement condensation problems is to reduce the humidity, which can come from a variety of sources.
Wrap the Pipes
You may already have foam insulation covering the hot-water pipes in your basement, to conserve energy. Do the same thing on the cold-water pipes and you will take a big step toward controlling condensation. Buy foam sleeves sized to fit your pipe’s diameter (1/2-inch is typical). Slip the sleeves over the pipes. Cut the foam with scissors or a utility knife, making miter cuts at the corners. Foam pipe insulation usually has one adhesive edge covered with a plastic strip. After fitting the insulation tube around the pipe, peel off the strip and stick the adhesive edge to the mating edge to seal the insulation. For maximum protection, also wrap all joints and gaps with a compatible tape.
Seal the Ducts
The ducts in a forced-air heating and cooling system frequently leak conditioned air, which both wastes your money and creates the opportunity for moisture to escape.
A humidifier on a forced-air heating system can be particularly troublesome in this regard. To prevent leakage of moist air into the basement, seal all joints and any holes in the metal ductwork, using duct mastic or metal duct tape. Do not use standard plastic duct tape, which quickly dries up and peels away from ductwork.
Stop Incoming Moisture
Moisture seeping through basement walls and foundation joints can be a significant source of humidity in basement air. If you have seasonal or ongoing moisture problems, correcting them may go a long way to reducing condensation. Simple solutions like directing gutter downspouts away from the house and improving outdoor drainage can solve many common seepage problems. More extensive issues may require adding a foundation drain and sump pit to deal with significant water infiltration.
You can also reduce condensation by making cold surfaces warmer. Air-sealing prevents cold air from getting in and keeps the basement interior warmer. Insulating basement foundation walls creates a thermal barrier between the cold wall and relatively warm interior air. The most important place to air-seal is along the mudsills and rim joists—the wood framing members that sit atop the masonry foundation walls of the basement. After air-sealing, insulate the joist cavities above the mudsill with rigid foam insulation board or standard batt insulation (such as fiberglass). Insulate foundation walls with insulation board, or install a framed and insulated wall in front of the foundation walls, with an air space in between.
Note: Do not insulate basement walls if moisture is seeping through them to the inside. Covering damp walls with insulation can lead to mold problems.
Seal Dryer Ducts
If you have a clothes dryer in the basement, make sure the dryer's vent duct is thoroughly sealed so that moist air is not leaking into the basement. To maximize dryer efficiency and minimize humidity in the area, use rigid metal duct for the vent, and make the duct run as short as possible leading to the outdoors. Never vent a dryer in a basement or other indoor space.
For occasional condensation problems, the easiest solution may be to open a window or door from time to time to encourage air circulation. However, this makes sense only when the outdoor air is dryer than the basement air. During humid weather, ventilation only increases the humidity in the basement.
Use a Dehumidifier
Dehumidifiers help reduce condensation by removing moisture from the air. This can be an effective solution during periods of high humidity, but it's not practical to run a dehumidifier all the time. For one thing, they use a lot of electricity. And if you need continuous dehumidification, you probably have bigger moisture problems that require a more permanent solution.