Mold in closets and on outside walls

The pattern

It appears in the heating season. You move a dresser, open a rarely used closet or pull out a bed, and find grey or black speckling on the wall, on the back of the furniture, or on shoes and clothes. The room itself seems dry.

This is one of the most common mold problems in Ontario homes and it rarely involves a leak.

Why it happens

Air can hold a certain amount of moisture, and warm air holds more than cold. When warm indoor air touches a surface that is cold enough, moisture condenses onto it, or the humidity right at that surface rises high enough for mold to grow.

Three things combine:

1. A cold surface. Outside walls, and especially outside corners, are the coldest surfaces in a room. Areas with thin or missing insulation are colder still.
2. Still air. A closet with the door shut, or the gap behind a wardrobe, gets no warm air circulation. The wall there is several degrees colder than the open room.
3. Indoor humidity. Cooking, showers, laundry, plants, aquariums and people all add moisture. Newer or newly sealed houses hold more of it.

Typical locations

• Closets on exterior walls, especially at the floor and in corners
• Behind beds, dressers and bookcases pushed against outside walls
• Outside corners of bedrooms, low down or at the ceiling
• Behind curtains that hang to the floor
• North-facing rooms
• Rooms over a garage or a cantilever

How to clean it

For a small area on a painted wall, wear gloves, eye protection and a well-fitting N95 respirator, wash with water and dish detergent, and dry thoroughly.

• Clothing and bedding: launder in hot water and dry fully.
• Leather shoes and bags: wipe with a damp cloth and mild soap, then dry in moving air. Heavy growth may not come out.
• Furniture: wash hard surfaces. The unfinished back of a wardrobe may need sealing afterwards.
• Cardboard boxes: discard.

If the drywall is soft or the stain comes straight back, check for a leak or get a moisture reading.

Stopping it returning

Let air reach the wall

• Pull furniture a hand’s width away from outside walls
• Leave closet doors ajar, or fit louvred doors
• Do not pack closets tightly against the back wall
• Keep stored items off the floor

Warm the surface

• Keep the room heated, including spare rooms
• Do not block heat registers
• Have missing insulation corrected

Lower the humidity

• Run kitchen and bathroom fans that vent outdoors
• Vent the dryer outdoors
• Do not dry laundry on racks indoors in winter
• Turn down or switch off a furnace humidifier that is set too high
• Watch the windows. Regular heavy condensation on the glass means the house air is too humid for the weather.

When it is not condensation

Suspect a leak or building problem if:

• It appears in summer
• There is a water stain or the wall is damp to the touch
• It is below a window, a roof junction or a bathroom
• The baseboard is swollen
• It returns quickly despite the changes above

Then the wall needs investigating with a moisture meter.

Renters

Tell your landlord in writing, with photos. Even where condensation is the cause, insulation and ventilation are part of the building. Keeping furniture off the wall and using the fans is the part within your control.

Measuring instead of guessing

A humidity gauge, called a hygrometer, costs little and tells you whether the air in the house is the problem. Health Canada recommends keeping relative humidity in a home between 30 and 50 percent. In an Ontario winter, aim for the low end, and in practice for whatever keeps condensation off the windows. Put the gauge in the affected room, not beside the furnace.

If the reading is high through the winter, the fix is ventilation and moisture control for the whole house. If the reading is normal and one spot is still growing mold, that spot is unusually cold, and the fix is insulation or air sealing there.

What is damaged and what is not

• Paint and drywall surface. Light growth cleans off. If the paper face of the drywall is soft, crumbling or stained right through, that section is replaced.
• Clothing and shoes. Most washable fabrics come clean in a hot wash and a thorough dry. Leather can be wiped and conditioned. Anything with a musty smell that survives washing has growth inside the fibres.
• Cardboard boxes. Throw them out. Cardboard feeds mold and holds moisture against the wall. Use plastic bins with lids.
• Furniture backs. The unfinished hardboard on the back of a dresser or wardrobe often grows mold first. It can be cleaned or replaced.
• Mattresses and upholstery pushed against a cold wall can grow mold on the hidden side. Check it.

Why it is worse in some houses

• Houses built before modern insulation standards, with little in the walls
• Corner rooms and rooms above an unheated garage
• Homes with electric baseboard or radiant heat, where there is no fan moving air
• Apartments and condos with concrete walls or slab edges exposed to the outside
• Homes with more people, more cooking and more laundry than the ventilation was designed for
• Houses where new windows and air sealing have cut the natural air leakage

A longer-term fix

Where the same wall grows mold every winter, the lasting answer is to raise the surface temperature. That can mean adding insulation, sealing air leaks at the top and bottom of the wall, or correcting a gap in the attic insulation above. An energy adviser or a thermal imaging survey on a cold day shows exactly where the heat is being lost.

More: mold removal and remediation.