Workshop roof insulation

[Dijon]

Hi,

I'm putting up insulation in a workshop with a single pitch corrugated steel roof on perpendicular 8" tall (200mm) rafters.

I'm using 2" celotex and 2" loft insulation between the rafters with plasterboard underneath.

Should I leave a 2" gap at the top although I'm not sure how that works if the flow down the underneath of the corrugate hits every rafter as it goes.

Do I need to put the loft insulation over the celotex? Or the other way around?
I was thinking put the celotex up high enough for the 50mm gap above and seal it in with foam then have a 50mm air gap then the loft insulation underneath flush with the bottom of the rafter, on the plasterboard.

200mm of;
50mm air flow, 50mm sealed celotex, 50mm air gap, 50mm loft insulation.
(A richer and more intelligent person would have used 100mm loft insulation.)

Are there any possibilities of condensation below the the celotex if sealed above? And in the loft insulation unable to escape?

There aren't any condensation problems, but it's currently been a single brick thermalite building with a tin roof, so who would?

The better thermal option might not be the best option, though it's mainly just sawdust in there, no wet towels or anything...

Thanks for any help.

 

[big-all]

dont use plasterboard use ply 6mm if nothing is to hang off it [assuming 600 centers] or 12mm if you want strength

the strange thing is you need a 1" air gap but in a shed the air gap in the walls or roof are full off static air as there is no vent to atmosphere as its a box section construction ??

in my shed i had no moisture ingress over around 3 years so i gave a further 2 coats off waxy finish filled the 3" void with 4" loft insulation and 3 years further on it is still perfect
now this relies on 98% protection as the ply lined untreated walls on the inside can only breathe a little
any way the point i am making is if you can gtee no leaks you need no breather or air gaps but this relies on constant outer treatment to a high degree

 

[Dijon]

Well it had plasterboard on it so that's what I'm replacing it with.

Presumably the 1" gap needs to be around the walls. That should be pretty simple.

Thanks.

 

[Dijon]

Is there actually anything wrong with plasterboard, or is it unnecessary and more expensive.
Thanks.

 

[big-all]

plasterboard is great and cheap
its just ply will cope with leaks impact and timber movement better without problems

 

[Dijon]

Thanks. It's certainly something I may bear in mind for the future.
Now for some sleep. As I'm sure there's a lot of work to e done tomorrow