Condensation in the loft: causes and fixes

How to tell it is condensation, not a leak

Condensation is usually spread across a wide area of the underside of the roof, often worst on the north-facing slope, and appears after cold nights rather than after rain. A roof leak tends to show as a localised wet patch or drip line that appears during or after rain, often near a chimney, valley or damaged tile.

Check in the morning after a cold night and again after heavy rain. Black mould spotting on rafters and the felt is a sign that condensation has been happening for some time.

Where the moisture comes from

Warm air rises and carries moisture with it. Any gap in the ceiling lets that air into the loft: the loft hatch, holes around pipes and cables, recessed downlights and the tops of internal walls. Bathroom and kitchen extractor fans that vent into the loft rather than outside are a common and serious source, because they push steam straight into the roof space.

The more moisture the house produces, the worse the loft gets. Drying washing indoors without a dehumidifier, long hot showers and cooking without lids all add to it. Our explainer on how much water a wash load puts into your home shows the scale.

Fix 1: stop warm air getting up

Fit draught-proofing to the loft hatch so it closes tightly, and seal gaps around pipes and cables with suitable sealant or foam. Recessed downlights in the top floor ceiling can leak a lot of air; an electrician can advise on fire-rated covers suitable for the fittings. Do not cover light fittings or cables with insulation unless the product is rated for it.

Make sure every extractor fan ducts all the way to the outside through a roof or wall terminal, with the duct joints sealed and the duct sloping so condensation inside it drains away rather than back into the fan.

Note:Take care in the loft: walk only on joists or boarding, use a light, and get an electrician for anything involving wiring or downlights.

Fix 2: let the roof breathe

Many lofts rely on ventilation at the eaves to carry moisture away. When extra loft insulation is laid, it can be pushed into the eaves and block that airflow. Pull it back and fit eaves trays or vents so air can pass from the soffit over the insulation. Tile or ridge vents add more ventilation if needed.

Some modern roofs use a breathable membrane designed to let vapour through, and the ventilation needs are different. A roofer can tell you what your roof has and whether more vents are needed.

Fix 3: reduce moisture downstairs

Bringing humidity down in the house reduces what can reach the loft. Use fans and lids while cooking, keep the bathroom door shut while showering, and dry washing with a dehumidifier in a closed room rather than on radiators. A dehumidifier on the landing of the top floor helps catch rising moisture in damp homes.

See dehumidifiers for condensation →

When to call a professional

Call a roofer if condensation continues after you have sealed gaps and ducted fans, if timbers show widespread mould or softness, or if you are unsure about ventilation. If you also have damp patches on bedroom ceilings, the cause could be either, so our guide on damp walls in a bedroom may help narrow it down.

Frequently asked questions

Is some condensation in the loft normal? A little on the underside of the felt on very cold nights can occur, but regular dripping or wet timbers means too much moisture is getting in or too little is getting out.

Can a bathroom fan vent into the loft? It should not. Extractor fans need ducting to the outside, or the steam condenses in the roof space.

Will more insulation cause loft condensation? It can make the loft colder and block eaves vents if laid badly. Keep the eaves clear and seal air leaks from below.