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PostPosted: Sun Aug 12, 2018 1:27 pm 
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I am doing some stuff in a late 70s/early 80s semi and extension in London, and would like to insulate the loft spaces. The whole roof is tile over tar paper.

I am concerned about ventilation under the tar paper, and the possibility for condensation and related damage. I tried to read up on the subject and I am wondering if insulation directly under the tar paper is okay if the roof plane is open to the rest of the house, and if the house is adequately ventilated.

The loft in the extension had been converted, with polystyrene blocks glued under the tar paper and wooden panelling nailed over this. When I took this out I didn't see any sign of a moisture problem. I've removed part of the floor, and that space also has a small window, so I plan to re-insulate with mineral wool under the tar paper. Does that sound okay?

The loft in the original house had the horizontal space above the eaves well filled with mineral wool, and somewhat above the plasterboard ceiling of the floor below. There is no insulation under the tar paper, so the roof is self ventilating. I would like to make more use of that loft for everyday storage, so I am considering insulating under the tar paper and making the ceiling permeable with a duct or vents. Does that sound okay? I mean include the loft space in the well-ventilated air mass of the house. Part of that would be to eventually actively ventilate the house, but for now I like having windows open whenever I can.

I don't see any vents under the eaves, so I don't think the roof plane was ever really ventilated bottom-to-top if that makes sense.

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PostPosted: Fri Aug 17, 2018 6:37 pm 
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All sounds ok to me, you said it yourself "I didn't see any sign of a moisture problem"

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PostPosted: Fri Aug 17, 2018 6:41 pm 
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Generally the ventilation will be from the eaves, so long as you don't block the ventilation from the eaves then there is no issue.

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