Spray Foam Insulation

Hello

Looking for some opinions I am Building an extension at the moment and I was considering filling the space between Roof rafters with spray foam insulation before slabbing ceilings with insulated slab

I think the spray foam will fill roof cavity completely avoiding ventilation Problems etc

I cant fit insulation on outside of roof ie warm roof as I am tight for slope on roof

any advise welcome please

thx

glengar

Make sure the foam is vapour open or your timber will rot, While spray foam can be usefull for new build, it does not have sufficient U-Value for new build. You'd need about 300mm thick to get a 0.25 U -Value. PIR board is good but you need expandable foam to seal it. You will need to line the rafters also to eliminate the cold bridging through the joists. It probably too late to think about energy and comfort and sure lob in a bit of insulation and it'll be grand. Firstly, Energy prices are going up, people are investing in oil instead of gold to hedge against currency drops coming next year. Secondly condensation through cold bridges will destroy the fabric, and you'll be re-roofing in 10 years. I'd recommend softboard and about 250 of cellulose to give a good stable insulant. Insulation isn't just cheap it makes you money. It'll pay for itself after 3 years and there after its putting cash in your wallet. Anyway I'm wasting my time, irish builders have no building skills and no qualifications needs whatsoever, to the point where amateur selfbuilders can do a far superior job. Soft touch regulation how are ya.

Hopefully can help me here... Firstly I am not sure of what the items are properly called but here goes...

Dormer Bungalow with 4' eve (usual wasted space out in eve instead of in bedrooms). A 12" block on the flat built up for about 2' up above joist to the wall plate. We think that the gap between the joist and the blocks was never filled in thus allowing wind/air/cold into the house between the celing and the first floor (loads of recessed lights not helping either).

We were wondering if we were to have this 2' wall sprayed with closed cell spray foam would this A) fill the gaps around the joist and B) insulate that section of the wall.

A guy looked at it and said that we are right but to also use open cell spray foam up around the wall plate to seal at that point - I do not want under any circumstances want to block ventilation to our roof timbers, our roof has traditional felt not breathable membrane as it is 11 years old.

Sorry for hyjacking this thread, but I did'nt see the need to start another one.

We are planing on having the gutters and facia removed & to prop the soffit (if needed be) to allow access to this wall.

All advice much appreciated as I am sure it is going to be an expensive project and naturally we don't want to waste money.