loft ventilation - fly screen or open holes in felt?

[David Robinson]

Following on from previous threads...

'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?

Cheers, David.

[Nightjar]

...I'm putting 50mm celotex (equiv!) on the underside of the rafters.

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

Bignell

[Andrew Gabriel]

...I'm putting 50mm celotex (equiv!) on the underside of the 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 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.

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.

[David Robinson]

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.

[David Robinson]

ability!).

Cheers, David.

[Andrew Gabriel]

Yes, there's this incorrect belief that loads of insulation on the ceiling and plenty of ventilation will somehow stop condensation, whereas it actually causes it. You have to have some heat leaking into the loft space to stop condensation, because on a clear night the loft will otherwise be colder that the outside air, so the circulation of outside air into the colder loft will cause condensation.

There's also a potential problem with highly insulating some walls, in that the brickwork originally used some leaking heat from the house to dry out, and without that it will be constantly holding more water, hence spall and fall to bits faster (for similar reasons a garden wall built from the same bricks as your house will only last a fraction of the time).

I have a feeling that some of the super insulation initiatives we're seeing now will result in some serious building damage in 20 or so years time.