Insulating Pitched Roof

16 May 2006
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United Kingdom
Hello all
I'm converting a pitched roofed garage and therefore have to fully insulate the roof between and below the rafters (which are 4" x 2").
The Building Inspector has asked for 50mm Kingspan (or similar) between the rafters, and 100mm below, then the plasterboard.

I can understand the need for airflow between the rafters - hence the 2" gap here - but how the hell do I fix the 100mm layer to the rafters, and then the plasterboard to that?! Am I missing something??

I guess I could fix another 4" x 2" timber onto each rafter then put all the insulation (150mm total) between, then fix the plasterboard direct to this timber.......would this be the simplest solution (seems like an *rse to me!) and would it be acceptable?

Any help or advice would be greatly appreciated.
Many thanks!
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Or you fit appropriate sized battens under the existing rafters and your 100mm ins goes between, then your 50 goes under. There was a poster on here who did 100 below (his choice btw) and just used super log screws twas a PITA of course I recall but proves it is possible.

This detail is a bit old hence only 25 below

Without wishing to appear to contradict what nose and Freddy have wrote, there is a lot of confusion with regard to insulation U values, especially with local building controls.
Whilst there is only a difference of 0.001Wm/K in the lambda value of certain Celotec and Kingspan products. we prefer to use Celotec
The U value of vented cold deck pitched roof with insulation either between the rafters or between and under rafters is 0.18 Wm2/K.
Technical note 10 states that to achieve this U value you either need 165mm GA between rafters, or 100mm GA between and 35mm below, or 50mm GA between 25x50mm counter batten to form cavity and then 70mmGA over counter battens.
All of these methods are grossly over the top. Take example 1. 165mm between joists. You already have a resistance value of 0.35m2W/K with inside and outside air resistance, roof covering, cavity, plasterboard and skim Add R value of 165mm GA = 7.50 less say 5% possible cold bridge, gives total R value of 7.45 which equals a U value of 0.14W/m2K.
The way we would go if rafters are 100mm deep. Fix either 50x50 or 2number 25x50 battens to underside of rafters. 100mm GA between rafters with 50mm gap above. Then below the rafters and across the rafters 37.5mm PL 4000 Celotec. This will giver you an R value of 6.03m2K/W and a U value of 16.58, call it 0.17Wm2.K.
37.5mm PL 4000 is 25mm celotec stuck to 12,5mm plasterboard, and going across the rafters stops any possible cold bridge.
Regards oldun
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You don't half waffle a lot oldun and really, when one sentence would do you make it an essay. There is no confusion with regards to U Values.
Any help or advice would be greatly appreciated.
Many thanks!

That was the question Fred, and we tried to answer in a comprehensive way and to explain the various options. Now if that is waffling on Fred, then yes, we are guilty..
Anyway Fred, post was not for your benefit, so why get your leotard in a twist and try to preach a sermon?
Regard U values Fred, it would appear that you are very confused, as you still do not know the correct insulation thickness to comply with AD L1B 2010.
Try Buddhist Meditation. Seek to understand.
Regards oldun :LOL: :LOL:
Oldun and Freddie - thanks very much for your replies.
I'm getting the feeling that the best solution is:

1) Fix 50x50 (2"x2") timbers to existing rafters
2) 100mm insulation between rafters, keeping a 50mm (2") airflow gap
3) 37.5mm PL 4000 - or 50mm insulation covered with 12.5mm plasterboard - across rafters.

Do you reckon this would be ok?
And do I fix (3) above with 100mm drywall screws?

Thanks again for your time :)
Ta for that oldun.......

....if I went with 50mm insulation covered with 12.5mm plasterboard - across the rafters (think the BI will insist on it somehow), what's the best way to fix the Kingspan/Celotec to the rafters? And then the plasterboard thro the lot?

Many thanks for not getting bored with me yet!
Ta for that oldun.......
....if I went with 50mm insulation covered with 12.5mm plasterboard -!

Not necessary, but your choice.
Use 62.5mm of PL Celotex with 100mm drywall screws. Screws jump from 75 to 100. No size in between.
PL Celotex is so, so much easier to use than separate celotex and plasterboard.
Regards oldun
Use 62.5mm of PL Celotex with 100mm drywall screws. Screws jump from 75 to 100. No size in between.

Drywall screws are sold in all kinds of sizes, we are currently using 90mm and have used 80mm in the past.

You need better glasses Old Codge! :p
Nose, many thanks for info Will have to look more closely next time. go to Tesco. Suppose you still drive them home with your favourite tool. Sledge hammer. Your mate must have nerves of steel, holding a screw and waiting for you to ring the bell.
Not the glasses nose, need new mince pies. Been trying to catch eagle for a while so that I could have eye transplant.
See yer oldun :LOL: :LOL:

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