💧 Condensation & Damp
Mould in the corner of a sloped ceiling, a musty smell that won’t clear, or staining that keeps coming back. Here is what is actually happening inside the roof, and how to tell it apart from other causes of damp.
Every home produces moisture — from breathing, cooking, showers, and drying laundry — and that moist air rises. In a dormer with cold-roof construction, insulation sits between the rafters or joists, leaving the roof deck above it cold. When warm moist air reaches that cold surface, it condenses into water, exactly the same way condensation forms on the outside of a cold drink on a warm day.
This is called interstitial condensation because it happens inside the structure, out of sight, rather than on a visible surface. By the time it shows up as mould or staining on your ceiling, the insulation and timber above it have usually been getting wet repeatedly for some time.
| Condensation | Roof leak | |
|---|---|---|
| Pattern | Diffuse, follows joist or rafter lines | Localised, irregular, often a clear point source |
| Timing | Gradual, worse in winter | Appears or worsens during/after rain |
| Location | Often in corners, near the eaves or ridge of the dormer | Anywhere water can track from a defect in the covering |
| Smell | Persistent musty smell even in dry weather | May not smell unless long-standing |
Both can occur on the same roof, and a covering defect can sometimes let in enough moisture to make a condensation problem worse. A proper inspection is the only reliable way to confirm which is happening, and we check for both when we survey a dormer.
The ventilation gap a cold roof depends on is easily blocked by insulation pushed too far, debris, or later building work that wasn't detailed correctly.
Adding loft insulation without addressing the dormer roof itself can increase the temperature difference driving condensation, without fixing the underlying cold surface.
Sealing gaps to reduce draughts is good for energy efficiency generally, but if it removes the roof's designed ventilation path, it can make condensation worse.
Drying laundry indoors, poor extractor fan use, or a lack of trickle vents all increase the moisture load the roof construction has to cope with.
Mould-resistant paint and bleach treatments deal with what's visible on the surface, not the moisture causing it. The mould returns, often within weeks, because the condensation cycle inside the roof structure continues untouched. In the meantime, the insulation stays wet and stops performing, and the structural timber is exposed to ongoing moisture. The only durable fix is removing the cold surface that the condensation forms against — which is what a warm roof upgrade does.
We move the insulation above the roof deck, fit a proper vapour control layer, and detail the junctions correctly so there's no cold surface left for moisture to condense against — signed off by an independent Building Control inspector, not us.
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