I am looking at putting in some celotex insulation material between the rafters in my house on the 2nd floor that provide the pitched ceilings for 2 bedrooms (and a flat roofed landing). The pitched roofs has 4inch rafters, and that flat roof 6 inch.
What depth of celotex should I be looking at, and more importantly what air gap do I need to leave between the top side of the celotex and then roofing felt?
There seem to be many different types of celotex, so how do I know what is best suited for me?
I hope I am wrong but I have the impression that any work on a roof is subject to building regulations that will require the roof to be to be brought up to the current standard, which would require 65mm of PIR insulation.
But having a 2" air gap is meaningless unless it can vent out at the highest point and vent in from the lowest point. Failure to do this may cause rot - moreso than wool as the latter can breath sideways.
Lower vents are usually a vent strip or repeated round vents in the soffit board.
The upper vents are more of a pain - tile vents or ridge vents are a common solution.
Both can be avoided with a breathable roofing "felt" like Tyvec, but unless you have it already, that implies a re-tile job.
This is an issue that's causing me some headaches as I have a dormer conversion in a full hipped roof, so the airways are not clear from base to top - I'm having to find creative solutions...
Back to your point about depth - you could put another 25 or 50mm under the rafters too if you can lose the space - that will up the insulation and reduce cold bridging via the wood.
I got mine off the mate of a supplier local to a another uk.d-i-y poster in Essex (I'm in East Sussex) - the mate in question happens to do runs past my village on a regular basis so is happy to drop off 20 odd sheets.
So - either local knowledge needed - these people can be hard to find, but worth the looking.
Or, sometimes, the internet can turn up the goods at a fairly reasonable price.
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don't know what they're like - I just noted them down when I stumbled across them...
Anyway, the magic word is "seconds" or "2nd grade". It means dinted, imperfect, maybe a few mm off correct thickness, recovered surplus materials from mega building jobs etc. The BCO might reject it for a new build if he's being arsey, but it's fine for general work.
In practice, most people who have 2nd grade are happy with it. You can chop damaged edges off for house work anyway - and PIR foam is PIR foam. It doesn't really matter who makes it or if it's a bit bent - it still does the job.
Roger Chapman wibbled on Monday 02 November 2009 22:20
Let's not go there Roger ;-O
I do believe that you're right - but I think most BCOs would be happy if people did something rather than nothing. I don't think anyone will ever get prosecuted for failure to notify in this case. It's bloody stupid anyway...
I told my BCO flat out that I couldn't bring it up to book standard, but I thought I could make it twice as effective as before (it is currently 3" glass wool - very crap). I hope to replace with 3" celotex and maybe 4" in some segments...
He was perfectly happy with this and was more concerned with helping me with useful suggestions about ventilation over it.
Both are highly flammable, although PIR slightly less so. I thought it was all treated with fire retardant, but I bought a sheet from a builders merchant that I found wasn't when I tested it - it sustained and spread flame, albeit not very quickly.
Andrew Gabriel wibbled on Tuesday 03 November 2009 00:12
Hmm. I'll definately try setting fire to mine - I'll be very upset if it does burn, because, unless it was specifically a type made to go under concrete floors, it would be unfit for purpose in any other use...
Thanks for the warning.
The problem is of course, that despite printing loads of crap like logos and phone numbers on the foil face, neither Celotex nor Ballytherm seem to print the part number of the particular board which makes identification hard.
Ballytherm say this generically about their boards:
"Fire Ballytherm Insulation remains stable at temperatures up to 400ºC; at higher temperatures a protective char forms on the surface, slowing the spread of flame. Ballytherm insulation may be used safely behind non-flammable materials such as plasterboard. The products have been fully tested in accordance with B.S. 476: Part 7: 1997 Surface of Flame and have achieved a Class 1 Rating."
you register you can download the complete solution. I ended up with a 50mm air gap, 70mm of celotex and then thinsulex
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battened on underneath. Building control were happy with it, in fact here is what they actually approved:
Detail of works:
Remove lath and plaster ceiling (D)
Re-roof and add plasterboard ceiling: (re-slate, impermeable membrane, 50mm air gap, rafters (currently 85mm deep at 360mm centres), extend down by 35mm, 70mm Celotex between rafters, Thinsulex, battens, plasterboard).
As i remember it, you could not get the required U value simply by putting insulation bhetween the rafters becuase the rafters them selves conduct too much heat,
I think you can also do away with the air gap if yo uuse a breathable membrane, but the pitch of my roof was very low and I didnt; want to do that.
The whole roof does not have to be brought up to modern regs, but if you change it then the heat insulation does have to meet new regs (according to my BCO).
It's Ok if you have a virgin roof, but not so easy if you have this:
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noggings are a problem as the block the vertical airflow - as do the windows. So effectively I can't have a continuous inch between the wood and
2" over. The best I can do without butchering the wood is 1" between and leaving a gap round the noggings, 1" over, leaving an air gap again, then another inch or two over the lot. Now I'm not sure how effective that arrangement will be as there will be colder spots amongst the well insulated bits. Perhaps it will work well enough - need to think about it more...
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