Mold in Closets and Behind Furniture on Outside Walls

Mold behind a dresser, inside a bedroom closet or behind a headboard almost always grows on an outside wall, and it almost always shows up in winter. Nothing leaked. The wall got cold, warm indoor air could not reach it, and the moisture already in the air condensed on the surface. Fix the airflow and the humidity and it usually stops for good.

Why it happens on outside walls

In January the inside face of an outside wall is colder than the rest of the room, especially near corners, the floor and the ceiling. Furniture pushed flat against that wall, or a closet full of clothes with the door closed, blocks the warm room air from reaching it. The drywall surface gets colder still. When it drops below the dew point of the indoor air, moisture condenses on it. Drywall paper, dust and fabric give mold something to feed on, and black spotting appears within weeks.

It is more common in rooms with high moisture: bedrooms where people sleep with the door closed, rooms with a lot of plants or an aquarium, and homes where clothes are dried indoors. Older homes with thin or settled insulation, and corners where insulation is missing, show it first.

Where to look

• The back corner of closets on outside walls, especially at floor level.
• Behind dressers, bookcases and beds pushed tight to outside walls.
• Upper corners of bedrooms where the outside wall meets the ceiling.
• On the back of furniture, mattresses and boxes stored against those walls.
• Inside built-in cabinets or window seats on outside walls.

Is it a leak or condensation?

Condensation mold is usually spotty and spread across a cold area, worst in corners and low on the wall. It gets worse in the coldest weeks and fades or stops in summer. A leak tends to leave a defined stain, a tide line or soft drywall, and the moisture reading stays high in a single spot regardless of the weather. If you are not sure, a moisture meter reading and a thermal image of the wall answer it quickly. We do that as part of a moisture inspection, which runs $249 to $549.

How to stop it

1. Pull furniture 5 to 10 cm away from outside walls so air can move behind it.
2. Leave closet doors open, or install louvred doors, on closets that sit on outside walls.
3. Don't pack closets tight against the back wall. Leave a gap behind clothes and boxes.
4. Keep indoor humidity lower in cold weather. If windows are fogging every morning, it is too high for your house.
5. Run bathroom fans during and after showers, and use the kitchen range hood while cooking.
6. Don't dry laundry on racks indoors in winter if you are already seeing condensation.
7. Keep heat on in every room. Closing vents and doors on spare bedrooms makes their outside walls colder.

Cleaning it up

Small areas of surface growth on painted drywall, roughly the size of a sheet of paper, can usually be cleaned with a detergent solution and dried, with the room ventilated and a mask on. Do not dry-brush it and do not paint over it. Larger areas, growth that keeps coming back in the same spot, or soft drywall behind the growth point to something more than surface condensation, such as missing insulation or a hidden leak. That calls for an inspection before anything is cleaned or replaced.

Check the clothes, shoes and boxes that were against the wall too. Washable fabrics can usually be washed and dried completely. Cardboard boxes with visible growth are best emptied and thrown out.

If you keep finding mold in the same closet or behind the same piece of furniture every winter, send us photos of the wall and the corner. We will tell you whether it looks like condensation you can manage yourself or a wall that needs a closer look.

Common questions