Ventilating timbers on a dormer window

[Lobster]

Following on from earlier (related!) queries...

I'm insulating an existing dormer window, having pulled down the lath and plaster; this has left exposed the 4" horizonal joists which form the roof of the dormer (roof is about 5'x3'). Dormer roof is composed of timber boards with lead(?) covering.

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.

So what to do? I could drill ventilation holes in the joists to connect the separated cavities, not sure if that's worthwhile though? Moe significantly I was wondering about drilling a row of upward slanting

0.5" holes in the front of the dormer above the window and into the cavity - would that be sensible? (would need to stop insect ingress I suppose...) And as the room will be completely vapour-sealed from below, any future leaks to the dormer roof would probably go completely undetected until really serious damage had been done.

Difficulty seems to be the merging of 1912 and 2006 building practices!

Thanks for any pointers David

[mcbrien410]

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. When I built my Dormer I used 100m. thick Rockwool Slab instead of Kingspan. It seems to absorb more sound.

Chris.

[Lobster]

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

[Pete C]

Hi,

Try asking on the forums on

cheers, Pete.

[Lobster]

Hmm, they're a bit scary on there: if you use phrases like "pulling down lath and plaster" and "inserting Kingspan" they think you're a troll...

David

[Lobster]

Further to the above enquiry... tomorrow is the point-of-no-return day so I've decided I'd better go for it and drill the holes as above.

These will be through the front of a decorative moulding, so difficult to fit any form of grille over the front - so, anyone got any smart ideas for a reliable and totally maintenance-free solution for preventing insects entering a hole of that size... what could I shove up which would stay put and prevent bugs while still permitting air ingress?

David

[The Natural Philosopher]

You should have some kind of venting between the joist and either the rest of the loft space or the outside ideally.

Holes is no bad thing, but really there is probably some leakage anyway. The staining is probably due to failure of the lead at some time past.

[Lobster]

Unfortunately there's no way through to the rest of the loft space - complicated to explain, but trust me on that! So yes, it's vent holes through to the outside, but through this moulding not a flat face, so the followup question was how to prevent bugs from getting in...

David

[marvelus]

I'd be more worried by rain being blown in than insects.

When I did what you are planning the dormer ceiling void was open to the attic. After I'd completed it, it wasnt nice to see all the dew drops forming on the t&g over the dormer when it turned cold. I put it down to damp from some new plastering and I've put an air brick in the attic and I'll see what happens this winter.

[Lobster]

Point taken - I'd hoped that angling the holes downwards would help on that front but let's add that problem to the criteria then!

David

[The Natural Philosopher]

Oh. ask at the builders merchant. There are either grilles or mesh available for that. Or staple perf. zinc etc. over the holes.

[Lobster]

Thanks, I've been and done just that at TP. Doesn't seem to be anything to suit my purpose really but I bought a length of soffit vent grille stuff, and with some judicious carving with a Stanley knife have produced a length of L-section uPVC grille which doesn't look too awful from ground level! And will hopefully stave off my rot worries.

(Got the undivided attention of a Very Nice Man at the hated TP for 20 mins, who sorted me out for 3 GBP - so that's a first!)

David