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There is always going to be warm, humid air leaking into your loft space at this time of year the membrane will be cold, especially on a north facing elevation do when the warm air comes into contact with it condensation will form and is nothing to worry about. This is part of the reason why modern roofing membranes such as tyvek are breathable, but as any cyclist can tell you, breathable fabric doesn't stop moisture build up inside it, it just slows the build up somewhat.
Bobbo
I was up in my loft yesterday (about lunchtime maybe) and noticed that a lot of the membrane on the north facing part was damp with drops of water on it. There's been no rain recently so I assume it's condensation. Standard Victorian terrace.
Is this a bad thing or to be expected? There's a few vents and air bricks up there but not sure how many there should be. Loft insulation was done about 10 years ago and about 200mm down so far as I can see.