The Natural Philosopher wrote:
Lobster wrote:
Lobster wrote:

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...)

Further to the above enquiry... tomorrow is the point-of-no-return day
so I've decided I'd better go for it and drill the holes as above.

These will be through the front of a decorative moulding, so difficult
to fit any form of grille over the front - so, anyone got any smart
ideas for a reliable and totally maintenance-free solution for
preventing insects entering a hole of that size... what could I shove
up which would stay put and prevent bugs while still permitting air
ingress?


You should have some kind of venting between the joist and either the
rest of the loft space or the outside ideally.

Unfortunately there's no way through to the rest of the loft space -
complicated to explain, but trust me on that! So yes, it's vent holes
through to the outside, but through this moulding not a flat face, so
the followup question was how to prevent bugs from getting in...

David





Holes is no bad thing, but really there is probably some leakage anyway.
The staining is probably due to failure of the lead at some time past.