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Lobster

wrote:
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.

Makes sense, except that this is an existing dormer which I ain't
dismantling!

Avoiding the 'sealed box' between joists sounds good; but that just
generates a larger 'sealed box' comprising the space between several
joists?? Hence the inquiry about venting that to the outside...

David