Mould on Walls: Causes, Cleanup and When to Call a Pro

Mould needs moisture. EPA: "The key to mold control is moisture control" — clean the mould and fix the water problem, or it will most likely come back^[1].

Typical causes on walls

• Condensation on cold surfaces — outside-wall corners, behind furniture, around windows (EPA lists the surface of walls behind furniture as a common hidden-mould location)^[1].
• Leaks — plumbing, roof, gutters^[1].
• Thermal bridges that keep a strip of wall cold — see cold walls and thermal bridges.

Can I clean it myself?

EPA: if the mouldy area is less than about 10 square feet (roughly 3 ft × 3 ft), in most cases you can handle it yourself. For more than 10 sq ft, a lot of water damage, contaminated (sewage) water, suspected HVAC contamination or health concerns, use EPA's professional guidance or an experienced contractor^[1].

EPA cleanup basics

• Scrub mould off hard surfaces with detergent and water, and dry completely^[1].
• Porous materials (ceiling tiles, carpet) may need to be thrown away^[1].
• Do not paint or caulk over mould — clean and dry first; paint over mould is likely to peel^[1].
• Wear an N-95 respirator, gloves and unvented goggles^[1].
• Routine bleach/biocide use is not recommended; never mix bleach with ammonia products^[1].

Prevent the return

Keep RH below 60% (ideally 30–50%), dry leaks within 24–48 hours, and warm or ventilate the cold spots^[1]. Use the calculator to see whether a wall is at risk: mould risk starts well before visible condensation.

Sources

1. US EPA – A Brief Guide to Mold, Moisture and Your Home