Lobster wrote:
Following on from earlier (related!) queries...

Plan is to insert 2" Kingspan insulation between the joists, and overlay
with vapour barrier and insulation-backed plasterboard. However, this
will leave a completely sealed cavity above the insulation and if there
is any ingress of damp in the future sounds like a recipe for dry rot,
no? The dormer is dry currently, but there's staining on the underside
of the now-exposed timbers so obviously it hasn't always been so.

On the underside of T&G floor boarding there are slots cut to prevent
forming a sealed box between the joists. You could cut similar shallow
slots in the beams/joists. When I built my Dormer I used 100m. thick
Rockwool Slab instead of Kingspan. It seems to absorb more sound.

Chris.