Mold in the basement: causes and fixes

Why basements are prone to it

A basement is the coolest part of the house and is surrounded by damp soil. Three sources of moisture account for almost all basement mold.

Humidity and condensation. In an Ontario summer, warm humid air enters the basement and meets cool walls, floors and pipes. The moisture condenses, most of all in still corners, behind furniture and inside closets.

Water from outside. Seepage through cracks and the wall and floor joint, poor grading, short downspouts and blocked window wells all keep the foundation wall damp.

Water from inside. A past flood, a slow leak from a pipe or water heater, a damp laundry area, or a bathroom without a working fan.

Where to look

• The bottom 30 centimetres of finished walls
• Behind furniture, shelving and stored boxes against exterior walls
• Under carpet, especially along the edges
• Around the laundry, water heater and furnace
• In the cold room
• Under the stairs
• On cardboard boxes, paper and fabric stored on the floor

A musty smell that is stronger when you first come downstairs is often the first sign.

Finished basements hide it

In a finished basement, moisture gets trapped. Framing sits on the slab, insulation is pressed against the concrete, and carpet covers the floor. A wall can be growing mold on the back of the drywall for months before a stain shows on the room side.

If a finished basement has flooded in the past and the lower drywall was never removed, assume there is something behind it.

How it is removed

Small surface patches on concrete, painted walls or hard items can be washed with water and dish detergent. Health Canada treats three or fewer patches with a total area under one square metre, as a small amount a homeowner can generally handle.

Larger or hidden growth is removed professionally: the area is contained, affected drywall, insulation and carpet are taken out, framing and concrete are cleaned, and the space is dried before it is rebuilt.

Stored items. Hard goods can be cleaned. Cardboard, paper and fabric with growth are usually discarded.

Keeping it away

• Run a dehumidifier through the humid months, set to keep the basement comfortably dry, and empty it or drain it to the floor drain.
• Fix the outside. Extend downspouts, clean eavestroughs, and regrade soil that slopes toward the house.
• Seal active leaks and repair plumbing drips promptly.
• Keep air moving. Leave closet doors ajar and furniture a little off exterior walls.
• Store off the floor in plastic bins, not cardboard.
• Vent the dryer and bathroom fan outdoors.
• Do not carpet a basement that has ever been damp.

Building it back

If you are refinishing after mold, choose materials that tolerate moisture: tile or vinyl plank flooring, closed-cell foam board instead of batts against concrete, and wall assemblies that keep paper-faced drywall off the slab.

How humid is too humid?

Mold grows readily when indoor relative humidity stays high for long periods. Health Canada’s guidance is to keep the home dry and well ventilated and to control humidity, and it points to dehumidifiers for damp basements. A small humidity gauge costs very little and tells you whether your basement is staying damp through the summer. If readings stay high even with a dehumidifier running, there is a source of water to find.

Signs the problem is coming from outside

• Dampness is worst after rain or during the spring thaw
• There is a white powdery deposit, called efflorescence, on the concrete, left by water passing through it
• The damp is concentrated on one wall or one corner
• A downspout empties near that corner
• The soil outside slopes toward the house

Fix those before spending money inside. Sealing the inside of a wall that is being fed water from outside rarely works for long.

Cold rooms

The cold room under the front steps is the most common spot for basement mold. It is unheated, poorly ventilated and built against the outside. Keep cardboard and fabric out of it, make sure its vents are open, and keep the door to the basement closed.

More: mold removal and remediation.