Mold on drywall: clean it or replace it?

Why drywall is such a good host

Drywall is a gypsum core wrapped in paper. The paper is cellulose, which mold feeds on, and the core soaks up water and holds it. Once the board has been wet for more than a day or two, growth can start on the front, on the back and inside the paper.

The back of the board, facing the wall cavity, is dark, still and often next to damp insulation. It commonly has far more growth than the side you can see.

Clean or replace: how to decide

Usually cleanable

• Light surface growth on intact paint
• Caused by condensation or humidity, not a leak
• The board is hard and flat
• The patch is small

Usually has to be replaced

• The board is soft, swollen, crumbling or stained through
• There was a leak, flood or long-term moisture
• The growth returns soon after cleaning
• The paint is bubbling or peeling
• The mold is on bare, unpainted paper

When in doubt, a moisture meter reading and a small inspection hole settle the question.

Cleaning surface mold

Wear gloves, eye protection and a well-fitting N95 respirator. Dampen the area lightly, wash with water and dish detergent, rinse with a clean cloth and dry completely. Do not soak the board.

How replacement is done properly

1. Contain the area with plastic sheeting, and for larger jobs keep it under negative pressure with HEPA filtration, so that dust and spores stay put.
2. Cut well beyond the visible mold. Growth and moisture extend further than the stain. A margin of about 30 centimetres past the affected area is common, cut to a line that makes the repair simple.
3. Bag the pieces inside the containment before carrying them out.
4. Remove wet or moldy insulation behind.
5. Clean the framing by HEPA vacuuming and damp wiping.
6. Dry the cavity until moisture readings are normal.
7. Fix the cause.
8. Close up with new insulation and board.

Cutting out moldy drywall without containment is the usual way a small problem is spread to the rest of the house.

Common causes

• A plumbing leak inside the wall
• A leaking shower or tub surround
• A roof or window leak
• A basement flood where the lower wall was left in place
• Condensation on a cold exterior wall behind furniture
• High indoor humidity

After the repair

Use a primer suitable for new drywall and a paint with a washable finish. In bathrooms and basements, moisture-resistant board helps, but it is not waterproof and does not replace fixing the moisture source.

Special cases

Bathrooms. Mold on a bathroom ceiling is usually surface growth from steam. It cleans off, and a working exhaust fan vented outdoors stops it returning. Mold on the wall beside or below a shower is more often a leak behind the tile, and that wall needs opening.

Basements. The bottom of a basement wall is the classic location after a flood or seepage. If the lower drywall was never cut out after a past flood, the back of it is the first place to check.

Ceilings below a bathroom or roof. A stain with mold on a ceiling means the drywall was wet from above. The insulation and the space above need checking as well as the board.

Behind furniture and in closets on outside walls. Cold wall, still air, and condensation. Usually surface growth that cleans off. Move the furniture out a little and improve air circulation.

Around windows. Condensation running off the glass soaks the sill and the drywall below it. Clean it, repair the damaged section if it is soft, and reduce indoor humidity in winter.

What the repair costs depends on

The size of the section, whether it is a wall or a ceiling, whether tile or cabinets have to come off to reach it, how much containment is needed and what caused it. A patch in an open basement wall is a small job. The wall behind a tiled shower is not, because the tile and the plumbing are part of the repair.

Do not forget the other side

If the wall backs onto another room or a closet, check that side too. Water that soaked one face of a wall cavity has usually reached the other.

More: mold removal and remediation.