Following on from earlier (related!) queries...

I'm insulating an existing dormer window, having pulled down the lath
and plaster; this has left exposed the 4" horizonal joists which form
the roof of the dormer (roof is about 5'x3'). Dormer roof is composed
of timber boards with lead(?) covering.

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.

So what to do? I could drill ventilation holes in the joists to connect
the separated cavities, not sure if that's worthwhile though? Moe
significantly I was wondering about drilling a row of upward slanting
0.5" holes in the front of the dormer above the window and into the
cavity - would that be sensible? (would need to stop insect ingress I
suppose...) And as the room will be completely vapour-sealed from
below, any future leaks to the dormer roof would probably go completely
undetected until really serious damage had been done.

Difficulty seems to be the merging of 1912 and 2006 building practices!

Thanks for any pointers
David