Part of Ward Property Services UK LtdLoft condensation is frequently mistaken for a roof leak. Water drips onto stored boxes, insulation is found soaking wet, and black mould appears on the underside of the sarking felt and the rafters — yet the tiles are sound. The moisture is coming from inside the house: warm, humid air escaping through the loft hatch, downlighters, cable penetrations and gaps around soil vent pipes, then condensing on the cold underside of the roof.
The problem has become more common as homes are insulated more heavily. Thicker loft insulation makes the roof void colder, and bituminous felt is not breathable, so any moisture reaching it condenses and runs back down. The fix is a combination of stopping the vapour getting up there and ventilating what does.
Common signs
- ✓ Water droplets on the underside of the roofing felt
- ✓ Black mould on rafters, trusses and the top of stored items
- ✓ Wet, flattened loft insulation with no rain damage above
- ✓ Damp patches on upstairs ceilings in cold weather
- ✓ Rusting fixings, nail plates and staining on timber
Likely causes
- • Uninsulated, unsealed loft hatch
- • Bathroom extract ducting discharging into the loft void
- • Recessed downlighters allowing warm air to escape
- • Blocked or absent eaves and ridge ventilation
- • Non-breathable roofing felt with no ventilation path