Damp felt, wet nail heads, musty smell — loft condensation is warm moist house air meeting a cold roof. Fixable, and usually cheaply — but don’t ignore it.
Cooking, showers and breathing push ~10 litres of moisture into house air daily. It rises through gaps (downlights, the hatch, pipe runs), hits the cold loft and condenses. Modern airtight living + blocked eaves ventilation = the classic combination.
1. Seal the leaks: hatch strip, downlight covers, pipe collars. 2. Restore airflow: clear insulation from eaves (use vent trays), add tile/lap vents if soffits are sealed. 3. Check the insulation itself — compressed or damp quilt loses performance; 270mm evenly laid keeps the ceiling warm side warm. Extractor fans vented OUTSIDE, never into the loft.
Black mould on timbers, persistent pooling, or condensation under spray-foamed roofs needs professional eyes — timber decay is the risk. Otherwise it’s a £50–£200 DIY weekend.
AC-coupled (battery has its own inverter) is the retrofit default: no rewiring of your array, works with any brand or age of system, £3,500–£6,000 installed for 5–10kWh usable. DC-coupled only makes sense if your inverter is due replacement anyway — a hybrid inverter + battery swap can be cheaper combined.
Feed-in Tariff payments are unaffected by adding storage — generation payments continue and deemed export isn’t metered down. On SEG, a battery lets you shift export into peak-price windows instead.
Match usable capacity to your evening consumption and summer surplus — the sizing guide has the tables. Most 4kWp homes land at 5–8kWh usable.
Occasional winter misting isn't; persistent wetness, mould or dripping is — it risks timber decay and needs ventilation fixed promptly.
Only when it blocks eaves airflow. Fit vent trays at the eaves and insulation reduces the problem, not causes it.
Never — always through the roof or wall to outside. Loft-vented fans are a top cause of this problem.
Facts checked 21 August 2026. We link to official sources and show when facts were last verified. If a scheme changes, we update the page.
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