On Feb 3, 5:09*pm, "Nightjar \"cpb\"@" "insertmysurnamehere wrote:
On 03/02/2011 13:01, David Robinson wrote:

Following on from previous threads...
http://groups.google.com/group/uk.d-...ad/262ef95c259...
...I'm putting 50mm celotex (equiv!) on the underside of the rafters.

As previously suggested, I'm going to cut some ventilation holes in
the (not breathable) felt near the ridge - a hole in between each pair
of rafters.

I thought that maybe I could buy some cheap fly screen (small meshed
plastic/fabric) and cut it to glue over these holes. This has the
obviously advantage of keeping insects out of the space between the
felt and celotex (unless the glue fails), but the likely disadvantage
of getting blocked up by dust/dirt eventually, so preventing the air
flow that's vital to stopping the timbers rotting.

I was also going to stick little 1cm wedges of wood into the overlap
between adjacent sheets of felt to get even more airflow.

There are no dedicated vents in the soffits into the felt/celotex
void, but the woodwork is quite loosely nailed together, so there's
plenty of breeze. If I ever have it replaced with plastic, I'll ensure
that suitable vents are included along the length. I _have_ put vents
in from soffit level directly into the loft space itself, which run
through, but don't vent into, the felt/celotex void.

What does the group think of the fly screen idea? And the wedges?

I think I would go for fitting something like vented ridge tiles.

http://www.marleyeternit.co.uk/Roofing/Ventilation.aspx

Thank you. Looks very good, but well beyond my budget (and climbing
ability!).

Cheers,
David.