How to fix plasterboards to 100mm kingspan under ceiling

[Fred178]

I'm planning to insulate my loft conversion with kingspan; 35mm between the roof rafters and 100mm under, going across the rafters.
How do I then attach plasterboards to them? For the screws to go through the plasterboard, kingspan and into the rafters 25mm I'll need 137mm screws. I can only find 100mm plasterboard screws. Is there another way of doing this or do I need to keep looking for longer screws?
Fred178

 

[Alastairreid]

The 100mm goes between the rafters!

 

[cantaloup63]

The 100mm goes between the rafters!

Agreed - if you need to allow for airflow space between the kingspan and slates/tiles (used to be 50mm, but not sure what the current specs are), then screw appropriate thickness timbers onto the rafters first. Then you can put 135mm kingspan between them and screw the plasterboard straight onto the new timbers.

 

[Alastairreid]

100mm between 35mm across the rafters. (cold bridging).
50mm airgap soffit ridge vents.