Home improvement/Attic

anneobrien1975 · 21-01-2026 09:35AM #1

Over the lasf 12 months ive noticed a film on attic felt which looks like mold. Over the last few months it has got worse there are roof vents and plenty ventilation in attic its baltic up there. I inspected it yesterday and feels like sand on the felt any knowledge or advice . The rooms underneath seem to be cold at one side even though radiator is roasting.

10-10-20 · 21-01-2026 10:26PM

Hi Anne,

That sand is just sand - it's applied to the bitumen felt when it's being made so that the external surface is protected against the elements and the whole roll doesn't all stick together when it's being stored in the heat. You can safely ignore it.

If you're seeing a light white haze on the underside of the felt then this is mold. It's common in damp and poorly ventilated attics. The normal remediation is to check the vents around the eves and to relay the insulation in the gap between the roof and the wall so that there is a layer of insulation covering the top of the wall, but that there is still a 50mm gap for air to flow from the eves to the attic space.

Hope it helps.

Space_Echo · 09-02-2026 12:35PM

Adding to the above advice.

One simple thing to check is if your kitchen and bathroom extractor fans are working OK. If these have broken down or gotten blocked this will leave more moisture in the air inside the house, which will also make its way up into the attic.

Another thing to check is if there's a lot of air leakage from the upstairs rooms into the attic. In this case the warm air from the house carries moisture upwards, which then condenses on your roof felt. This could also be why the upstairs rooms are getting cold.

Air leakage can happen around the attic hatch, which needs to be fitted with draught strips and closed tight with a latch. But also through any small gaps in the ceiling, at the edge of plasterboard, or around ceiling lights and other wiring. There are special airtight tapes available for sealing these. (If you have downlighters, these are trickier to seal, and they need to have a gap between them and any insulation, to stop them from overheating).

Home improvement/Attic

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