Q: I have a damp spot in the corner of the bedroom. It is and outside wall corner. It is a bungalow so no bathroom over it. The damp spot seems to dry at times but there are tiny mold spots on it. Could it be that there is not enough circulation in the room? Window becomes full of condensation when blind is down in winter so thought that may be part of problem. Could it be that there is not enough attic insulation in that corner? Thanks — T.M.
A: The majority of building problems involve water and it often takes some detective work to discover the source of the water. Sometimes the cause is not at all obvious and requires some trial and error to diagnose and fix the problem.
The first question is the source of the water: Is it a leak from outside the house, a plumbing leak, or condensation from the air inside the house?
Condensation at Wall & Ceiling Cold Spots
From what you describe, my first guess would be that condensation is forming on the surface of the wall in the corner. Mold can start growing in the corner if the area stays wet for more than 24 hours and the air temperature in the corner is at least 40°F – conditions easily met in an older home in a cold climate. The warmer the air, the faster the mold will grow.
The condensation on your windows (especially if you have double-glass or storm windows) indicates that your indoor humidity levels are pretty high. Humid air will condense into water droplets wherever it meets a cold surface, such as window glass. It’s the same reason that droplets form on a cold beer can on a hot, humid day.
Wall corners are often very cold because there is mostly wood at the corner and not much insulation — called thermal bridging or a thermal short-circuit. Older homes are have a number of such cold spots that weatherization contractors can quickly identify with an infrared camera and can sometimes fix on the spot.
Additionally, there is not a lot of air movement in corners, especially if there is tall furniture in place and no air vent nearby. This can make the area even colder and limit the drying potential once it gets wet. Mold growth in cold corners or inside cold closets is a common problem.
Ceiling corners can have similar problems, usually due to reduced insulation near the attic eaves. Better insulation near the eaves usually eliminates the problem.
How To Test for Condensation
To test this theory, I would thoroughly dry out the area with a hair dryer, clean the mildew with diluted household bleach (wearing rubber gloves and eye protection). Then reduce household moisture levels until you stop seeing condensation on the windows. Use bathroom and kitchen exhaust fans when the rooms are in use, and run one bath fan all day (or intermittently with a timer) if needed to get the humidity level down. Running a dehumidifier is another more expensive option.
If there’s no condensation on the window and no mold in the corner, you’ve got your marching orders. You need to lower indoor humidity levels in cold weather.
Controlling Indoor Humidity
The first step for lowering indoor humidity is to improve “spot ventilation,” that is, removed the moisture at the source. That means effective ventilation in kitchens and baths, with short duct runs that are not broken, blocked, or otherwise not working. (Ventless kitchen hoods that blow the vented air back in your face don’t count.)
You may have other sources of moisture such as a wet basement, uncovered crawl space, drying firewood in the house, etc. Cover the crawlspace with a heavy plastic ground cover, improve basement drainage, and put the firewood outside. If necessary, you can run dehumidifier on occasion.
Try to keep indoor humanity levels between 30% and 50%. Too high and you’ll have condensation problems, too low and you may experience some minor health problems and discomfort like dry skin. The right level for you will depend upon the climate, house tightness, and your personal preferences. In a cold climate, you will probably be fine near the lower end and of the range and in a warmer climate, the higher end of the range.
Warm Up the Corner
You can also add insulation to the wall corner if you are remodeling, but otherwise this is usually not practical. Other cold spots on walls and ceilings can be addressed by strategically placing fibrous or foam insulation. Some examples include cantilevered floors, attic kneewalls and floor, kitchen soffits, and band joists.
Also remove any furniture from the corner and other obstructions that restrict airflow at the corner, at least until the problem is solved. The added airflow will keep the corner warmer and drier.
If the Source Is A Water Leak
If condensation is NOT causing the problem, then you probably have an exterior leak, either in the roof, corner boards, or flashing at the junctures of these components. Most roof leaks are around valleys, penetrations for skylights, chimneys and pipes, and intersections with walls and other building components.
If there is a bathroom or kitchen above or below, you may also want to have a plumber take a look. A slow leak in a toilet tank, drain line, or other plumbing fixture could be at fault. Pin holes can also form in older copper supply lines in areas with acidic water.
Have a general contractor or roofing and siding contractor take a look. They may be able to identify the likely source of the leak or caulk and seal any place that could possibly leak. A repair with sealant could be short-lived or last for years if a high-quality sealant is used.
A more permanent repair may involve pulling apart the components that are leaking and rebuilding them with the proper flashings in place.