David M wrote:

the other area that you must take adequate care over is ensuring a 50mm
air gap between the insulation and roof, and also adequate roof vents
and at the eaves. Insulation manufacturers normally have decent
application notes covering this.

The usual trick with lofts is some insulation between the rafters, and
then some under them before the plasterboard goes on. If you use foil
tape over the joins in the final layer then you have a good seal without
any additional barrier.

--
Cheers,

John.

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