marvelus wrote:
On Mon, 14 Aug 2006 23:50:17 GMT, 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?

I'd be more worried by rain being blown in than insects.

Point taken - I'd hoped that angling the holes downwards would help on
that front but let's add that problem to the criteria then!

David