Do Dehumidifiers Stop Damp and Mould?

Dehumidifiers get bought by the thousand every winter, and I understand why. You plug it in, it fills a tank with water, and it feels like it’s doing something. And it is. But it’s worth knowing exactly what it can and can’t do.

Where they help

• Condensation and mould where the problem is too much moisture in the air.
• Drying washing indoors in a closed room.
• Drying a building out after a leak, a flood or building work, including new plaster.
• Cellars and storerooms used for storage, alongside ventilation.

Where they don’t

• Rising damp. The moisture is coming up from the ground through the wall. A dehumidifier might dry the surface a bit, but it won’t stop the source.
• Penetrating damp. If a gutter is pouring water down the wall, you need to fix the gutter.
• Leaks. Ditto.
• Salt-contaminated plaster that keeps attracting moisture from the air. You’ll lower the humidity a bit, but the plaster is still the problem.

Choosing and using one

• Compressor (refrigerant) dehumidifiers tend to be more efficient in warmer rooms. Desiccant types tend to cope better in cold spaces like garages and unheated rooms. Check the manufacturer’s information for the conditions it’s designed for.
• Get one sized for the space. A tiny one in a big house won’t keep up.
• Use it with the doors and windows of that room shut, otherwise you’re trying to dry the whole street.
• Empty the tank, or use a continuous drain hose, and clean the filter.

The bigger point is this: a dehumidifier recirculates the air that’s already in the room. It doesn’t bring fresh air in. If you’re relying on it permanently to keep mould at bay, that’s a sign the house needs better ventilation, such as working extractor fans, trickle vents or a PIV unit, and possibly warmer surfaces.

Used for the right job, a dehumidifier is a useful tool. Just don’t let it hide a problem that needs fixing.