Will a dehumidifier stop damp and mould?

Do not rely on a dehumidifier alone to stop damp and mould. The important question is why moisture is reaching a wall, window or floor in the first place. Government guidance says dealing with surface mould without addressing the underlying causes will not prevent it returning. A dehumidifier is therefore a possible part of managing a room, not a substitute for finding a leak, improving inadequate ventilation or dealing with a persistently cold surface.

Start with the kind of moisture problem

Condensation forms when moisture in indoor air settles on colder parts of a building. It often shows around windows, in corners and behind furniture against external walls. Everyday cooking, bathing and drying laundry put moisture into the air. If it cannot escape, condensation and mould may follow. The source of that moisture and the temperature of the surface both matter, so a room can keep developing a patch even while you try to manage the air in it.

A different problem may be feeding the same-looking patch. A roof defect, overflowing gutter, leaking pipe or failed seal can keep supplying water. In that situation, the priority is to repair the route by which water enters and allow affected materials to dry. More than one cause can exist at once: government guidance notes that penetrating damp can make condensation worse. A surface mark cannot tell you which part of the problem needs repair.

Where a dehumidifier fits

Think of a dehumidifier as something to discuss after you have checked the building and its ventilation. The supplied official guidance does not say that a dehumidifier repairs water entry or replaces an extractor fan. If you are using one, keep looking at the original damp patch and at what is happening elsewhere in the room. A change in the room's feel is not evidence that a damaged gutter or concealed leak has been fixed.

It also makes little sense to place all the responsibility on a household to operate a device when the building is letting in water or cannot be ventilated properly. Government guidance for rented homes says tenant measures for condensation and small areas of mould must not replace assessment and treatment of the underlying issue. A landlord should look at defects, heating and ventilation together, with the people living there involved in describing when the problem occurs.

Practical checks that matter more than a machine

First look for visible water routes. Check from a safe position whether gutters overflow or a downpipe leaks, and note any plumbing or roof defects. Then check whether kitchen and bathroom extractors work, whether vents are blocked and whether furniture is pressed against a cold external wall. Notice where condensation appears and whether a patch changes after rain or after indoor moisture is produced. These observations help distinguish an indoor air problem from water entering through the building.

Ventilation and warmth need to be considered together. Government guidance says insufficient heating and insulation make moisture and condensation on the inside of external walls and windows more likely, while inadequate ventilation lets moisture remain indoors. Improving airtightness without considering ventilation can create new problems. That does not mean leaving windows open continually; it means making sure the home's ventilation is suitable and actually works for the way rooms are used.

Yorkshire and Lancashire back rooms: check the wall as well as the air

A little-used back room or unheated spare bedroom in a stone house can have a cold external wall. If mould keeps appearing there, a dehumidifier in the room is only one observation in a wider investigation. Check for blocked air vents, furniture against the wall and moisture reaching the room from elsewhere in the house. Outside, look at rainwater goods and the condition of the wall. The cold surface may be part of a condensation problem, while a defect outside may be wetting the wall itself.

Keep notes about when the room is heated, when condensation appears and whether the wall changes after rain. That record helps when you report the issue or ask for advice. It is especially useful where a room is shut much of the time and a brief visit might miss the pattern. Do not assume a damp wall is simply the result of how often a spare bedroom is occupied.

When to seek a proper diagnosis

If damp returns, spreads or has no obvious source, ask an independent building surveyor with relevant damp and building pathology experience to investigate. RICS advises looking for all sources of moisture and making repairs proportionate to the building. If you rent, report the problem to the landlord or managing agent and describe both any measures you have tried and what remains wet. The useful outcome is an explanation of the moisture source and a plan to address it, rather than an assumption that a device can carry the whole job.

This page gives general information, not legal or medical advice. For your own situation, use the organisations listed under "Where this comes from" at the foot of the page.