Why does condensation appear after loft insulation?
Insulation keeps heat in the rooms below, so the loft runs colder. Warm, moist air that leaks up from the house now meets colder surfaces and condenses, usually on the underside of the felt. Blocked eaves ventilation makes it worse.
What are the usual causes?
- Insulation pushed into the eaves, blocking airflow
- A bathroom or kitchen extractor ducted into the loft
- An unsealed hatch, downlights or pipe holes letting warm air through
- Drying washing indoors with windows shut
- Old bitumen felt with no ventilation
How do you fix loft condensation?
Seal the air leaks from the house first, then make sure the loft can ventilate: clear the eaves, fit ventilation trays and add vents if needed. Ducting extractors to outside is often the single biggest fix.
When should I worry?
Light condensation on the coldest nights that dries out is common. Dripping water, wet insulation, mould or soft timber needs investigating.
Related services
Sources: Energy Saving Trust: roof and loft insulation; Which?: loft insulation costs and savings; LABC: building regulation requirements for loft insulation; GOV.UK: extending the ECO4 end date, government response; GOV.UK: VAT on energy-saving materials (notice 708/6); Property Care Association: spray foam guidance.
