celotex + PB or insulated PB ?

[sm_jamieson]

My ceiling spec includes 25mm celotex under the ceiling joists to minimize cold bridging, and then plasterboard. I can't to a warm roof for various reasons. Is it cheaper to get the celotex (secondsandco etc) and PB, or get insulated PB ? I've never used celotex / PB sandwich onto joists before - seems to me the celotex would squish slightly and fail to give the PB and solid enough fixing to avoid the skim cracking in the future. Is this something I should consider ? Thanks, Simon.

[Dave Plowman (News)]

That's what I have here and it seems to be fine. There are some cracks between plasterboard sheets - but no different from anywhere else here. Flexible caulk fills them ok.

[Andrew Gabriel]

No it won't squish.

[John Rumm]

Probably cheaper separately...

No, its quite tough in fact - it may compress very slightly under the tension of the screws - but it won't move any further afterwards.