Condensation in the Loft

The "leak" with no hole — moist air meeting a cold roof.

You go up for the Christmas decorations and find the felt beaded with water, insulation damp, maybe mould freckling the rafters — but the roof has no hole, and it does it in dry cold weather. That's condensation: warm, moist air from the house rising into the loft and meeting a roof surface at outdoor temperature. The loft is doing what a cold beer glass does in summer.

It's a modern problem with modern causes. Better insulation makes the loft colder (the heat stays below); sealed double-glazed houses push more moist air up through the loft hatch and ceiling gaps because it has nowhere else to go; and non-breathable bitumen felt traps that vapour against its cold face. The classic trigger stories: "we topped up the insulation", "we had the soffits replaced and it started", "it began when we started drying washing indoors in winter". All three reduce ventilation or increase moisture — usually both.

It matters because chronic condensation soaks insulation (halving its effect), stains ceilings in ways that get misdiagnosed as leaks, and in bad cases rots battens and rafter feet as surely as any leak would.

Features

What's included

Leak ruled out first — pattern and inspection, not assumption
Ventilation paths assessed at eaves and ridge
Insulation checked for blocked airflow at the eaves
Moisture sources traced — bathrooms, hatches, flues
Fixed with airflow, not absorbent gimmicks

Repair options

Repair or replace

The cure is airflow and always has been: air must enter at the eaves and leave near the ridge, continuously. Practically that means eaves vents or over-fascia vents (and pulling insulation back where it's stuffed tight into the eaves, blocking the path), tile or ridge vents to give the air an exit — a ventilated dry ridge does this as a side effect — plus housekeeping below: bathroom fans vented outside (not into the loft, a depressingly common find), and a snug loft hatch. On roofs being re-covered anyway, a breathable membrane solves it at the source. What doesn't work: dehumidifier tubs, "anti-condensation" paint, and hoping.

Common questions

Frequently asked

How do I know it's condensation and not a leak? +

Pattern and distribution. Condensation: worst in cold snaps, present without rain, spread evenly as beads or frost across the cold surfaces (often worst on the north slope), mould rather than stain. Leak: follows rain, localised, leaves a defined track and stain. The five-minute torch inspection during a dry cold spell usually settles it.

It started right after we insulated the loft — did we make a mistake? +

Only a detail-level one, and a very common it: insulation pushed hard into the eaves blocks the airflow path that was keeping the loft dry, while the thicker layer makes the loft colder. The insulation itself is right; pulling it back from the eaves (or fitting vent trays) restores the airflow and keeps the benefit.

Is mould on the rafters serious? +

Surface mould is a symptom, not structural damage — it maps where moisture sits. Caught now, ventilation dries the loft and the mould dies with the moisture. Left for years, persistent wetting moves from mould to rot at battens and rafter feet, which is real money. The mould is the early warning worth heeding.