On Feb 3, 7:59*pm, (Andrew Gabriel) wrote:
In article ,
* * * * David Robinson writes:

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 would not deliberately cut the felt. It's your second line of
defence against a cracked or slipped tile, although a hole at the
top would not be as bad as a hole at the bottom. There's usually
an overlap very near the top, on account of the top sheet going
over the ridge timber, and effectively being less than half width
left to go down each side.

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 wouldn't bother. The larger insects aren't a problem for the
house, and the smaller ones like woodworm beetles, you won't
stop anyway.

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

Providing there's an overlap between layers (as there should be)
so that any water running down won't run in, that sounds OK.
Normally the water runs down centrally between the rafters due
to the droop in the felt, and this helps keep the rafters dry.
Make sure you aren't disturbing the felt enough to redirect any
water onto the rafters. I would not use anything which might rot
as a spacer. Either use offcuts of pressure treated battens, or
some plastic pieces.

The flet in my roof has enough droop between layers to allow a
little air flow along all the overlaps without adding anything,
and overall it probably adds up to more than the 10mm all round
vent generally used nowadays.

Thanks for the feedback Andrew. Yes, the felt droops a little, but not
by much in most places. Will use plastic spaces though - good idea!
I'll see how much air I can get in at the top with that, without
cutting. I'm not sure there is a join within 1 foot or so of the top
though.

It amazes me how different the responses are to this question. It
seems that opinion hasn't quite settled to a single standard answer
when it comes to loft ventilation and felt.

Given that pros often leave a loft in a state of permanent
condensation, while Victorian properties with none of the modern
practices fair well enough, I suspect we still have something to
learn.

Cheers,
David.