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? The window in the room 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.
The temperature of a surface where water will condense is called the “dew point.” For any air temperature and relative humidity, you can find the dew point on the table below (Click To Enlarge). When the relative humidity is greater than 60% condensation will start to form on many surfaces in poorly insulated home.
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 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 — or increase insulation levels at the building corner, a much more complicated and expensive fix.
Controlling Indoor Humidity
The first step for lowering indoor humidity is to improve “spot ventilation,” that is, remove 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 store 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.
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 more easily addressed by strategically retrofitting fibrous or foam insulation. Some examples include cantilevered floors, attic kneewalls and floors, kitchen soffits, and band joists.
An experienced energy auditor or weatherization contractor can help you identify cold spots prone to condensation and recommend practical ways to beef up the insulation.
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.
If you suspect a leak, have a general contractor or roofing and siding contractor take a look. They may be able to identify the likely source of the leak and repair it with caulk or sealant. A repair with caulk 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 metal flashings in place.
Georgia Plevritis says
How To Eliminate Mold on Bedroom Wall Corners
I have a apartment that it collects mold at corners of bedroom. The wall is plaster .
We have re-insulated the attic, redone the corner flashing, and added a new roof. The mold keeps coming back in very cold days.
It is the outer walls of the bedrooms. Any advice??
buildingadvisor says
It sounds like you taken care of exterior water leaks, so the most likely cause is condensation from moisture in the indoor air.
Building corners tend to be cold spots due to a lack of insulation here and poor air circulation, as explained in the article above.
Water vapor in the air will condense on cold surfaces (think cold beer can on summer day) and can lead to this type of mold growth over time. Corners can be a problem due to poor air circulation there, especially if blocked by furniture. The moisture builds up, does not dry out, and mold grows.
Mold spores are everywhere, waiting for the right conditions to grow – wet and above 40°F. Mold will grow on just about any surface –I’ve even seen it on glass (using dust and dirt for food).
Condensation on windows is a sign that humidity levels are too high. You can also use a humidity meter to monitor the level, although the cheaper ones may not be very accurate. In general, indoor relative humidity should range from 30-50%.
Since it is difficult to add insulation to the corner of a room, you may need to lower the indoor humidity and/or increase the air circulation in the corner. Second-story rooms tend to be more humid in the winter due to warm, moist air rising upwards – the so-called “stack effect”. Forced-air heating systems tend to even things out, while water-based heating with radiators do not help with air circulation.
Many things can contribute to high indoor humidity levels – poor ventilation in kitchens and bathrooms, wet basement or crawlspace, lots of plants, and drying firewood to name a few of the most common. If you whole house is too humid, try to find and control the moisture at the source. If the moldy corner is blocked by furniture, try rearranging things to improve the air movement in the corner.
If the problem is just in the bedroom, you can just lower the humidity in that room with improved ventilation or a dehumidifier. To increase the ventilation, you may be able to adjust the HVAC system or add a quiet, low-volume exhaust fan. As a last resort, you can run a dehumidifier, although these are noisy, will increase your electric bill, and will raise the bedroom temperature.
With some trial-and-error, you should be able to reduce or eliminate the mold – after a good cleaning with diluted bleach – wearing protective gloves and goggles. An experienced home inspector could also help identify the cause and practical solutions.
John Gibson says
Condensation on Bedroom Windows
Well, for bedrooms you seem to be overlooking a big source of moisture, and that is one or two people sleeping in the room for an 8 hour stretch. Especially if you have baseboard heating with no air movement. Then you pull the honeycomb shades down for the night. Bound to have condensation galore on the windows. And to some extent on the walls too i would guess.
buildingadvisor says
You make a good point. People certainly generate moisture in a bedroom, which can condense on cold surfaces. The smaller and colder the bedroom, the more likely you are to see condensation on windows.
Remember, even a high-quality low-E window with argon gas has an R-value of less than 4, compared to R-19 for a basic wall built to code. So the glass surface is pretty cold. Add honeycomb and other insulating shades and the glass gets even colder as you have insulated it from the indoor air. Unless the shades have sealed edges, moist air will enter and condense. As you point out, homes without mechanical air circulation are prone to greater moisture levels in bedrooms and other spaces that generate moisture
Condensation and mold at wall corners, however, indicate some combination of:
• unusually high moisture levels in the room
• unusually bad thermal insulation at the corner
• poor air circulation in the corner
I live in New England where baseboard hydronic heating is the norm, and most houses do not have mold growing on drywall corners – although some do get minor condensation on windows