Adding insulation without ventilation is one of the most common and expensive roofing mistakes. What happens, and how it's put right.
Loft insulation is one of the best-value upgrades a homeowner can make. But done wrong — and it's done wrong constantly — it creates a problem that costs far more to fix than the insulation saved.
What Goes Wrong
Warm, moist air rises from the house into the loft. Traditionally it escaped through the roof. Seal the roof up with insulation and block the vents, and that moisture has nowhere to go. It condenses on the cold underside of the roof covering, and you get a wet loft, damp timbers and eventually rot.
The Signs
- Black staining or mould on the underside of the roof covering or on rafters
- Water droplets or frost on the roof timbers in cold weather
- Damp, musty smell upstairs that's worse in winter
- Rusting nail heads and fixings in the loft
- Sagging or softening timber over time
Why It Matters More In Older Homes
Period properties in Rochester, Chatham and Strood were built to breathe. The old plaster, lime mortar and unsealed materials all moved moisture. Modern insulation, modern sealants and modern windows close those routes down. On a period roof you have to add ventilation deliberately rather than assume it's there.
How It's Put Right
- Keeping insulation clear of the eaves so air can still move up under the covering
- Fitting tile vents, ridge vents or slate vents where ventilation is missing
- Making sure the loft hatch and any penetrations are sealed so moist air isn't being fed in from below
- Where a roof is being re-covered anyway, fitting a breathable membrane — which manages moisture rather than trapping it
The Time To Get This Right
If you're re-roofing, it's the cheapest it will ever be — the roof is open and vents can be fitted while it's exposed. If you're upgrading insulation in an existing loft, it's worth a survey first to check the ventilation is adequate for what you're adding. Getting both right together costs a fraction of fixing condensation later.
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Costs and timings in this article are guides for Medway and Kent properties. Every roof is different — a survey is the only way to get an accurate figure.