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This seems really odd. Are you 100% sure the membrane is impermeable?
Could there be another cause all the condensation that's building up? What humidity level is it showing at the moment, and is that reading constant across the space? - wondering if it could be ingress from a gutter or steam coming from your boiler/similar.
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Where is the insulation? Make sure nothing is blocking the eavese / soffits.
If it's a cold roof and a sealed off loft, where is the moisture coming from?
hugo7
NotThamesWater
@tyeness
Any tips on the best mechanical ventilation approach to combat condensation in a cold roof?
Our roof was fitted (by a previous owner) with a completely impermeable membrane, and no ventilation at all. The seller fitted some vent tiles, and I've since fitted some lap vents but some leadwork has recently failed and the considerable moisture that's built up has nowhere to go.
We can't afford to replace the roof right now, but I want/need to do something relatively quickly to protect the timbers in the roof. The best (read: relatively affordable and easy) idea I can come up with is to fit some mechanical ventilation in the loft space.
Should I go for extraction or PIV in these circumstances?
The loft is well insulated/sealed from the livable spaces, if that makes a difference