Pitched roof insulation

[Tony1919]

I have a question about insulating my pitched roof, I intended to insulate between and under the rafters but my rafters are only 50mm deep.
Rather than increase the depth of the rafters to allow insulation and 50mm air gap between insulation and felt, could i simply insulate under the rafters with maybe 100mm celotex and then plasterboard.
The rafters are 400mm centres supported by huge purlins approx 1200mm apart.
I was thinking of using full sheets of celotex, trimmed/foamed in as required, screwed to the rafters, taped joints, then plasterboard over the lot and leaving what remains of the purlins exposed.

The house is 250 years old.

Any tips/advice would be very much appreciated.

 

[datarebal]

50 mm that is skinny rafters.
might be easier to use one of the multi foil insulations.. much easier in fact

 

[Tony1919]

BCO won't accept multifoil insulation, a question mark still hangs over the validity of the tests they use and u values claimed.

 

[datarebal]

So your converting the loft into living space? Have you spoken to BC ? what insulation is there now?

 

[freddiemercurystwin]

BCO won't accept multifoil insulation, a question mark still hangs over the validity of the tests they use and u values claimed.

Yes they will but it requires an air gap above and below, effectively eradicating any advantage in using it.

 

[Tony1919]

In detail, we are removing a ceiling and creating a vaulted ceiling so the loft now becomes part of the bedroom. The insulation will have to be good enough to satisfy BCO but more importantly, the insulation needs to work.
An ex-neighbour did a loft conversion and said at the time BC would not accept the multi foil option, maybe things have changed, personally I'm not convinced it would be up to the job, especially with the 2 air gaps mentioned above.
Headroom is not an issue so 100mm+ of celotex or similar looks like how I will do it.
I suppose the question is with the rafters only being 50mm deep, can all the insulation be fitted below rafter, rather than between and below ? And how could this be fixed prior to plasterboards going up ?

Many thanks

 

[Tony1919]

Oh and will also be fitting dry ridge system and over fascia vents.

 

[^woody^]

Don't you have a designer or is it on a building notice?

 

[Tony1919]

Not got either, still at the planning/pricing up stage, will be doing the work myself and giving BC the required notice etc.

 

[datarebal]

Multi foils do indeed work. There was a period where bc decided not to agree with it. but in fact its come a long way since then. Its been in use in France for over 20 years. (their regs generally exceed uk, take a look at their plumbing you might not think so)
Advantages are not just space saving, but ease of fitting. try fitting 100 sheets of 100mm foil board in 30c!

 

[freddiemercurystwin]

Multi foils do indeed work. There was a period where bc decided not to agree with it. but in fact its come a long way since then. Its been in use in France for over 20 years. (their regs generally exceed uk, take a look at their plumbing you might not think so)
Advantages are not just space saving, but ease of fitting. try fitting 100 sheets of 100mm foil board in 30c!

France led the way with multi foils until it was proved upon their final admitance that their testing methods and theoretical results were flawed and fraudulent, which was the reason UK Building Control resisted them and hence the need now for an air gap either side making their use obsolete.

 

[^woody^]

I've yet to be convinced about the performance of foil insulation, because the science does not stack up.

Foil is good for radiant heat, but there is not much of that within a structure. And the minimal amount of fibre within this foil insulation, is not enough to deal with the types of heat that transfers through material.

 

[datarebal]

its still in use in France.
When the first multi foil insulation started to be used in the UK battening and counter battening was standard. (if you followed the rules) so nothing new.
As I said it has moved on. Infact there is at least one that states should space be an issue a counter batten can be omitted, with slight reduction in thermal insulation..
certainly not obsolete.
of course now we also have the hybrids..

deviating from the original post i think

 

[Tony1919]

It was the conflicting opinions and uncertainty that made me dismiss the multi foil option, that and headroom not being an issue.
Does anyone see an issue with fitting all the insulation below rafter ?

 

[freddiemercurystwin]

Nope.