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March, 2007

Fix Frost Problems in Basement Walls

By Steve Easley

Q:  A Maryland builder asks, “During a recent cold spell, we had an issue with frost forming on the inside face of the OSB sheathing located on the exterior walls of the framed portion of our walkout basement. The basement was finished with fiberglass-insulated walls and gypsum board, and heated by a forced air furnace and ducts. “What is really puzzling us is that the walls on the first floor, directly over the affected wall in the basement, are bone dry. “Why would so much ice exist only 18 inches away from a wall that shows no moisture? And by the way, the homeowner does have a humidifier on the furnace.”


A: I get this question five or six times a year. I even see this problem in mixed climates. To answer, it’s worthwhile to review some basic physics to help with my explanation. First, heat moves from warm areas to cold areas. During the cold winter, heat inside your home is always trying to move outdoors. Second, moisture always moves from wetter areas to drier areas. That means moisture in the more-humid warm indoor air will seek a drier environment outdoors. Third, air pressure always moves from areas of higher pressure to lower-pressure areas.Most homes with finished basements have supply ducts that carry warm, humidified air to heat the basement, as is the case with your example. In most homes, the return air duct is located upstairs. That means that when the basement door is shut, it creates a condition where the basement is under a positive pressure. Every time the furnace runs, it pushes hot, humid air into the basement. With the basement door closed and no return air duct in the basement, there is no way for the air to return to the furnace; that’s what creates the pressure imbalance. This warm humid air eventually works its way through all the cracks and gaps in the drywall and insulation of the framed walls of the walkout portion of the basement. When this air reaches its dew point and condenses, it freezes when it comes into contact with the cold OSB sheathing. If this problem isn’t remedied, eventually mold will start to grow and the sheathing will deteriorate.

Fortunately, you have a few simple solutions from which to choose to fix this problem. If the insulation in the basement area is exposed, it must pass code for fire and smoke developed, or have an approved ignition barrier over the foam such as gypsum board. You could rip sheets of Thermax or polyiso foam insulation board to fill the cavity. Be sure the fit is tight and caulk it in place. You could also use closed-cell spray foam to insulate the cavity. This material comes in cans, and there are many spray foam applicators that do this. You could also purchase contractor kits that have around 600 board ft. of spray foam in a do-it-yourself container. Another solution is to balance the HVAC system. You’ll need to talk with your heating contractor to find the correct ways to balance the HVAC system to prevent these pressure imbalances. (For instance, if you were to place return air ducts close to combustion appliances such as a water heater, it could cause them to back draft. That’s why I always recommend direct vent water heaters and furnaces.)You should also consider using a paperless, mold-resistant gypsum board product like Georgia Pacific’s Dens Armor for the finished wall of a basement.  Finally, it’s very important to watch humidity levels in a home. Oftentimes, homeowners do not adjust their humidifier’s output, and some humidifiers can put out as much as a gallon of water per hour. This means that during cold weather, a furnace that runs for 20 hours could be pumping 20 gallons of moisture into the home. Condensation that forms on windows is the first indication that the humidifier output needs to be monitored.

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