Guide

Mold Behind Drywall: Signs, Testing and Repair

Why cavities grow mold so reliably

A wall cavity is a dark, unventilated box containing two excellent growth media. Paper-faced drywall is cellulose, which is food. Fibreglass batt insulation is not food itself, but it holds water against the framing and the drywall paper and keeps it there. Add any moisture and the cavity has no mechanism to dry, because the whole assembly is designed to stop air moving through it.

That is why a leak that would evaporate harmlessly off a tile floor in an afternoon can run a cavity for years.

The signs, roughly in order of reliability

• A musty smell with nothing visible. The most common presentation. See musty smell but no visible mold
• A moisture-meter spike in one area of an otherwise normal wall
• Bubbling, peeling or crazed paint, usually low on the wall
• Drywall that feels soft or gives slightly under thumb pressure
• Staining that comes back after being painted over
• A visible bulge or a seam that has opened
• Baseboard separating from the wall, or cupping
• Rusty nail or screw heads showing through paint
• Insects that favour damp, such as silverfish or springtails, concentrated at one wall

Where to suspect first

Cavity growth is not random. It follows water, and water follows plumbing and the building envelope.

1. Any wall containing supply or drain lines, especially behind a shower, a tub or a toilet
2. The wall below a bathroom on an upper floor
3. Exterior walls below a window, where flashing or caulk has failed
4. Any wall in a finished basement, particularly the lower two feet
5. The wall behind a washing machine or dishwasher
6. Walls adjoining a chimney chase
7. Anywhere a roof leak could be tracking down inside the assembly, which may be a long way from the actual hole

How to check without wrecking the wall

Moisture meter first

A pinless meter reads through the surface. Sweep the wall in a grid and compare readings against a known-dry section of the same wall. You are looking for a spike, not an absolute value, because different materials read differently.

Then a small inspection hole

If the readings are high in one area, a hole a couple of inches across, low on the wall in an inconspicuous place, tells you in thirty seconds what an air test cannot tell you at all. Use a torch and a phone camera, or a cheap inspection camera. Cut low: water runs down, and cavities wet from the bottom.

What an air test will and will not do

An air sample measures spore concentration in the room, not location. It can support a suspicion but it cannot point at a wall, and a cavity that is sealed may not be releasing much into the room even while growth is extensive. Inspecting is cheaper and more decisive. See testing vs remediation.

What the repair involves

1. Fix the water source. If it is plumbing, that is a plumber, and it happens before the drywall work, not after
2. Set containment. Cutting drywall with growth on the back of it releases a lot of material, and running the HVAC during that distributes it through the house
3. Cut back well beyond the visible edge, guided by moisture readings
4. Remove all wet or contaminated insulation in the affected bays
5. Clean and HEPA vacuum the framing. Solid wood can usually be cleaned and kept
6. Dry the cavity to a verified moisture content, with the wall open, before closing it
7. Re-insulate and re-board only once the readings hold

Step six is the one that gets skipped under time pressure, and it is the one that decides whether you do this again. Closing a wall over damp framing restarts the entire problem in a sealed box where nobody will see it.

When it is worse than a drywall job

If the framing itself is soft rather than merely stained, or if a sill plate or bottom plate has been wet long enough to rot, the job now involves a carpenter and possibly structural repair. That is uncommon in a single leak and common in a leak nobody found for several years. See mold on wood framing and what it costs.

Common questions

How far back do you have to cut?

Standard practice is to remove drywall a foot or more beyond the visible edge of the growth or staining, because on a porous surface the growth is already past what you can see. The exact distance is a judgement based on moisture readings, not a fixed number.

Can I just cut out the mouldy bit and patch it?

Only if the moisture source is fixed and the cut goes far enough. Patching a hole in a cavity that is still getting wet buys a few months. The wall is not the problem, the water is.

Do I need to replace the insulation too?

If it is wet or has growth on it, yes. Fibreglass batts hold water and cannot be cleaned. Discarding a section of batt is inexpensive compared with re-opening the wall in a year.