Thinking ahead: roof ventilation/breathability

[Tim W]

year sometime, I'll be wanting to insulate the above wall and serveral others like it (dormer) with celotex.

I can arrange for facia vents on the front of bother dormer roofs, fiddly but quite possible.

Now, those timbers in that photo are 3x3". Notionally plenty of air space if I just cover over with say 75mm celotex.

The fly in the ointment are those noggins and similarly the windows.

Even if I cheat and batten out to allow only a 25mm airflow over the top of the timbers, that's taking 4 inches off the room, whereas if it weren't for all that, I could stick 25mm celotex between the wood and 50 over losing only 2 inches. As these extra 2 (maybe 3) inches apply to all walls, I'm losing space fast trying to insulate better. And I really out to be aiming for 100mm celotex, although 75mm will be twice as good as the miserable bit of glass wool that used to be there.

Other than reroofing with Tyvec or turning it into a cold roof (the roof is fine - this is not really a practical proposition expense wise), any ideas on how to maintain air flow?

Could I maybe drill some holes though the noggins (though that leaves the problem of the windows)?

Cheers

Tim

[geraldthehamster]

It's a bit hard to see what's going on at the bottom of those dormers

- I'd expect to the see the roofing felt turn vertically upwards where the cut-off rafters meet the dormer timber work, but that's not clear as the felt isn't well lit. Have you got soffit vents at the bottom? With soffit vents and new fascia vents, how much timber work is stopping the airflow from one to the other? I have a similar arrangment in my new dormers, but have a cold roof with a vented ridge, so less of a problem. I don't see any reason why you shouldn't drill the odd 10mm hole through any offending noggins.

As far as the volume of insulation goes, personally I'd batten out with more 3 x 2, and put 75mm in between, with a 50mm air gap at the back, and another 25mm over, taped for a vapour barrier. Not only would that be warmer but it would be what BC would expect (at least, in these parts), as you've removed a thermal element.

Cheers Richard