Another insulation/plasterboard fixing question

[SimonH2]

In our house, the last 2ft of ceiling is sloping with the plasterboard fixed to the rafters. The rafters are 2x4, and there's only about 2-3" of glass fibre insulation. It's a cold spot and suffers from damp (and mould).

I already have a load of nominally 100mm (it's actually mostly about 90mm) tissue faced rigid foam insulation, and I was planning :

• Pull off the existing boarding and remove the glass fibre.
  
• Fix one layer of insulation between the rafters (all foamed in)
• Fix another layer of insulation under the rafters
• A sheet of plastic as a vapour barrier.
• And standard plasterboard under that.

The roof was redone a few years ago, and it's got a breathable membrane, so I believe it don't need an air space between insulation and membrane.

Any problems with that plan ?

That just leaves, how to fix the board.
"drylining" screws seem to stop at 100mm, while I reckon I'll need at least 125mm (90+13+enough to go into the rafters). The only other thing I've found that seems suitable are these "nail" :
https://www.twistfix.co.uk/drywall-insulation-fixing-stainless?cat=31
though I would have preferred a screw rather than a nail - though that's perhaps because I like things that give you a second chance when you don't quite get it right first time
Anything else I should be considering ?

 

[noseall]

The space has two purposes. One is airflow and the other is to allow for draping without the membrane coming into contact with the insulate material.

Your other bits sound ok. We would foil tape all the joints also.

 

[SimonH2]

We would foil tape all the joints also.

Is that because you are normally using foil-faced insulation ?

 

[noseall]

We would foil tape all the joints also.

Is that because you are normally using foil-faced insulation ?

Yeah, belt and braces. It completes the VCL when using foil sandwich insulation.

 

[SimonH2]

It's been "niggling away" in the back of my mind ...

... to allow for draping without the membrane coming into contact with the insulate material.

Does that actually matter ? The membrane is in intimate contact with the rafters and presumably that's OK.

 

[SimonH2]

And one more thing ...
Any suggestions on the best way to hold up the second layer of insulation ? Once the plasterboard is on it'll be sandwiched tight, but there's that short gap between putting up the insulation panels, and offering up the plasterboard.

I suppose I could put a dob of foam on the back and prop it in place while I foam around it. Then when it's all set it's not going anywhere

And keep the dog away from the foam

 

[theprinceofdarkness]

You could try these screws :- http://www.screwfix.com/p/turbo-ultra-woodscrews-double-self-countersunk-6-x-120mm-pk50/23654. The holes will need to be piloted and the board counter sunk.
Frank