Christian McArdle wrote:

Also, if the rafters are 100mm, you can just get away with building
regulations for loft conversions (but not new build) filling only between
rafters, although 25mm underneath is better to avoid cold bridging and
condensation, as well as improved energy efficiency.

With 4" rafters, if you are not stripping the whole roof, and, the
existing under tile felt is not breathable, then you can get away with :

Tiles
Felt
2" gap
2" PIR foam between rafters
25 or 30mm foam underneath
plasterboard

However you will need to arrange for ventilation at the soffits and
ridge of the roof.


--
Cheers,

John.

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