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NotThamesWater
Forum opinion on this, we get these patches in our bedroom. My suspicion is condensation, there is nothing on the exterior to suggest penetrating damp, the render looks okay.
The only thing is that it is always these patches and they NEVER dry, even after a heatwave - guessing stained? Why would condensation pick these random spots?
Id be 100% sure it was condensation but the staining is making me question it, however they dont get notably bigger after it pisses it down and if it was penetrating damp id expect the staining to be worse?
Had a surveyor round a year or so ago who was equally baffled but said likely condensation.