Excess Celotex board - can I use it between rafters/studs?

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I am extending my house and I have 8 sheets of 90mm Celotex board (which was orignally going to be laid on the floor slab) spare.

I have no floors or ceilings at the moment, just the open floor joists and exposed roof rafters (the roof is tiled and weatherproof) and I got to thinking that it might be a good use of it to slice it up and fill the gaps between the roof rafters at ceiling level as its starting to get a bit nippy.

I also have more 100mm and 170mm rockwool insulation than you can shake a stick at. So I was thinking fill the gaps between the roof rafters/trusses with Celotex, then Rockwool over the top in the roof void, and hold the whole lot up with the plasterboard - is there any reason not to use Celotex between the rafters? or any downsides?

Equally, could I use it within the the stud wall cavities? or the 1st floor voids? or is there a better product for the job?
 
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you can lie it on top of the upstairs ceiling, and put chipboard or ply in top to walk on if you like a boarded loft. This is a good way to get the modern extra-thick loft insulation.

this is usually done after you have put loft insulation between the timbers, to top up thr thickness. I can see this will be difficult if you have not got ceilings yet. I suppose you could lift it up after the ceilings are done. Or you could just stack it up in the loft and fit it later.
 
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Thanks for your reply, but I can't lie it on top of the rafters as there isn't enough room... a picture should explain:-
You can see that the roof 'void' is tiny - hardly big enough to crawl through. I want to know if I can use the Celotex between the lowest rafters, the ones in the bottom of the pictures and then use the Roackwool on top to fill up to the level of the middle rafters. Whatever happens I can't get pieces of Celotex into the roof void any bigger than the strips between the rafters as there is no opening big enough, so I either use it between the lowest rafters or I don't use it in the roof. Is there any reason not to use it between those rafters? like sweating of timber, building regs or something?
 
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no, I was just thinking that it is usually dificult to get a good tight fit between rafter with a rigid board. unlike the quilt which will squash to fit. Your roof looks nicely made so if the timbers are straight and parallel, with even spacing, you could probably cut it to a tight fit. You need to avoid gaps that will allow leakage.

the void has to be ventilated to the outside air, as warm humid air will rise up and cause condensation otherwise, even though I think I can see a breathable membrane, this is not always enough (though very much better than felt) and you will find it difficult to get up there and look.
 
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This may be becoming an issue now.... does anybody know whether Rockwool could be laid on top of Celotex board in a loft situation? For example might the Rockwool make the Celotex sweat? Anybody have any idea what the thermal insulation values are for 100mm Celotex and 270mm Rockwool respectively?
 
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Which bit ?

By the way, do you update your non-smoking interval manually or is it automatic ?
 
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Thank you for the replies, thats good to hear. It was the builder who suggested it may sweat.... I have no idea why (unless he didn't fancy the idea of crawling around in the roof space laying the stuff).
Anyway, with that question answered and approved by building regs. we are going to stick 170mm down on top of 100mm Celotex. I reckon this should give R8.25? which sounds toasty to me :D
 
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he might have thought it would sweat because warm moist air can rise up through rockwall or similar, and condense on a cold impervious surface. It can do this on felt or flooring. that's why a vapour barrier is required on the warm side of insulation.

As the celotex will not be very cold, it might not suffer this way. I don't know. But it will certainly prevent any condensation getting away. A ventilation gap might be wise.
 
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John

if there is a vapour barrier, why are you suggesting a gap which would be :

a) significantly more work to put in place
b) serve no function

This kind of suggestion just serves to confuse people.
 
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