Warm Roof - Celotex XR4000 Insulation Board

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2 Sep 2015
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Country
United Kingdom
New Roof
Vaulted Ceiling
8"x2" (203mmx50.8mm) Joists
I plan to leave a 50 mm airgap and then use 110mm Celotex XR4000 Insulation Board.
Do I also need to leave an airgap between plasterboard and Celotex? I assume not?

I may consider using 150mm Celotex XR4000 Insulation Board if no airgap is required on the plasterboard side.

Thanks
 
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Don't you need insulation below the rafters?

You definitely don't have an air gap between the board and insulation
 
As woody says 40mm PIR below, then plasterboard. Or PIR backed plasterboard. Don't forget to mark the position of the joists (rafters!) on the wall so you can aim the screws!

I would aluminium foil tape the life out of it too.
 
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As woody says 40mm PIR below, then plasterboard. Or PIR backed plasterboard. Don't forget to mark the position of the joists (rafters!) on the wall so you can aim the screws!

I would aluminium foil tape the life out of it too.
Our latest vaulted...

The first image shows 100mm Celotex between the rafters. Note the use of foam too. Also note the 50mm x 25mm batten framing the Velux - this is so that when the 50mm Celotex is laid across the rafters, the plasterboard outer edge has summat to fix to as does the plaster bead. You may also see a timber centrally fixed (now covered in foil) in the second image for ceiling light fixture. Oh and the spray paint wall markings for the rafter positions is essential.
 
Some pics from mine - My first DIY extension...
 

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