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Can i install a breathable membrane under the rafters?


I have a 1930s semi-detached house with little insulation in the loft. The loft tends to get quite dirty because the torching between the slates keep falling off. Since it was built in the 1930s there is no roof sarking. I'm planning to add more insulation in the loft and raise the boards of the loft.

I was wondering if a breathable membrane could be added under the rafters in order to keep most of the loft clean. I've read on other forums it has been done but I'm not sure if it's good idea. The previous owner fitted a couple of plastic sheets, one above the loft trap and another one in one of the corners.

Thank you for your help.

3 Answers from MyBuilder Roofers

Best Answer

breathable membrane to work successfully really can only be fitted from the out side by removing the tiles, anything you do from the inside will direct any water ingress down into your lower roof @ wall plate which may not be discovered for a while by which time it has done thousands of pounds of damage.
good luck Alex.


Answered 23rd Oct 2017

Your best bet will be to insulate inbetween joists and don't waste your money putting breather on the underside of the rafters plastic sheeting in lofts is not a good idea as it traps moisture wait for a reroof and put the felt the right way round


Answered 25th Oct 2017

I would insulate it with 100mm kingspan insulation cut down to fit between rafters


Answered 2nd Nov 2017

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