Why does my loft have condensation?

People find water in a loft and look for a leak. In most cases the roof is sound and the water arrived as vapour from the kitchen, bathroom and bedrooms below, which is why the pattern is general dampness rather than a single drip.

Safety

Step by step

1. Tell condensation from a leak.A leak makes a defined stain in one place, worse after rain. Condensation appears as widespread dampness, droplets on the underside of the roofing felt, and is worst on cold still nights rather than wet ones.
2. Understand where the moisture comes from.Cooking, showering, drying washing and simply breathing put several litres of water into a house each day. That warm moist air rises and finds every gap into the loft, where it meets a surface at outdoor temperature.
3. Seal the loft hatch.Usually the largest single route. Fit draught-strip around the frame, insulate the back of the hatch, and add a catch that pulls it closed against the seal.
4. Deal with downlighters.Recessed ceiling lights are open holes into the loft, and there are often a dozen of them in a bathroom or kitchen ceiling. Fit purpose-made fire-rated hoods or replace them with sealed units.
5. Seal every pipe and cable penetration.Soil vent pipes, extractor ducts, cables and the top of stud walls. Expanding foam or sealant around each one. Individually small, collectively substantial.
6. Check the extractor fans actually vent outside.A bathroom fan ducted into the loft rather than through the roof or wall is a common and serious cause — it delivers the entire moisture output of the shower directly into the loft.
7. Check the eaves ventilation is clear.Lofts are ventilated at the eaves. Insulation pushed right into the eaves blocks the airflow completely. Pull it back, or fit eaves ventilation trays to hold the gap open.
8. Do not over-insulate the wrong places.Insulation goes between and over the joists at the loft floor, not against the roof slope in a cold loft. Insulating the slope without a ventilated arrangement makes condensation worse.
9. Check for missing or blocked roof vents.Some roofs are ventilated with tile vents or a ridge vent rather than at the eaves. If these have been covered by later work, the loft has no airflow at all.
10. Stop drying washing where the moisture reaches the loft.Drying clothes on a landing or in an upstairs room with the door open sends that moisture straight up. Dry in one room with the door shut and the window open.
11. Look for the water tank if there is one.An uncovered cold water tank evaporates continuously into the loft. It should have a lid.
12. Check again after the first cold spell.Go up on a cold morning after a few days of frost. That is when condensation is at its most visible, and it tells you whether the sealing worked.

Tips

• Do this work in autumn. Sealing gaps is far easier before the loft is cold and damp, and you want it done before the first frost.
• A loft that smells musty in October is telling you the same thing as visible droplets in January.
• Modern breathable roofing membrane tolerates this better than old bitumen felt. A loft with black felt and no eaves gap is the classic combination for this problem.

Common mistakes

• Stuffing insulation tight into the eaves — It blocks the ventilation path the loft depends on. Insulation should stop short, or be held clear with eaves trays.
• Treating it as a roof leak — Money gets spent on tiles and flashing while the actual source — moist air from the house — carries on. The pattern of dampness distinguishes them.

If it doesn't work

Questions people ask