Insulation in sloping ceiling in Timberframe

[Podooser]

I am building a timberframe chalet style bungalow. I want to do the insulating and tacking in the bedrooms in the roof void myself. I have been supplied all the insulating materials and was told to use the 90mm celotex sheets and 80mm glasswool insulation rolls together in the sloping ceiling. I have 2 questions.

Do I have to insulate with the celotex first then glasswool or can I use the celotex to hold the glasswool up?

And secondly, do I stop the insulation of the sloping ceiling where it meets the flat ceiling (above which is the roof void)?

 

[Deluks]

Not sure if there's a right way, if using the wool inside, use string or thin wire in a zig-zag to hold the insulation in, you can also get insulation clips (big discs that feed through)

By flat ceiling, do you mean crossmembers that triangulate the roof at the apex? If you are boarding out then you can just insulate between these. If you want these crossmembers open then go up into the peak.

 

[noseall]

90mm celotex in the ceiling rake, 270mm quilt between and over joists.

i don't know why you need quilt in the rake too? sound insulation maybe?

you will stop the cellotex once you meet the horizontal joists, whereby you will go with the 270mm quilt insulation. 100mm between the joists and 170 over the joists.

we just use 150mm everywhere, both between and over.

 

[Podooser]

Thanks for the replies.

I really don't want to have to insulate the rafters as this will lower the sloping elements and reduce the usable living space. So I spoke to celotex and they said that if I doubled up the layers of celotex ie 2*90mm=180mm between the rafters (I was supplied a huge pile of 90mm celotex) then I will not need to insulate the rafters themselves as the overall U value will be 0.17. The rafters are 220mm deep, so will easily accomodate the celotex.

The problem is the dormers (5 of them).They have a small sloping element that will not take that amount of celotex. Can you suggest a way od insulating usig both one layer of celotex and the 80mm glass wool that was supplied?