I plan on adding 60mm foil-faced insulation to our SIPs kit and had been intending using something like Kingpsan Thermowall throughout. We do though have vaulted ceilings in 80% of our first floor space which will make this approach bit more onerous and time consuming. I had ruled out insulated plasterboard on the grounds of cost but not sure if that's now mistaken having come across some potentially cheaper Celotex boards. These do though only have 12.5mm plasterboard, which seems the standard for insulated plasterboard, whereas everything I've read advises 15mm for ceilings. So my questions are: 

 

• How do costs compare?
• Does the insulation provide it with a bit more stability to make it suitable for ceiling use, less likely to sag etc?
• Where there's a foil face how does this affect the vapour control layer (which is on the wall side of the service cavity on the internal walls, but would be on the other side on the ceilings?

 

Cheers!

Following

Not using insulated pb on the slopes is a mistake Most of the jobs I price specify 70 or a 100 mil insulated 

We do the odd one in 37 mil

Insulated boards ar 12 mil to help keep the weight down 

Even a 37 mil insulated board is more ridged than a 15 mil plain 

and heavier ?

11 minutes ago, nod said:

Not using insulated pb on the slopes is a mistake Most of the jobs I price specify 70 or a 100 mil insulated 

We do the odd one in 37 mil

Insulated boards ar 12 mil to help keep the weight down 

Even a 37 mil insulated board is more ridged than a 15 mil plain 

and heavier ?

Thanks - so you think having 60mm insulated PB performs better/is a better option than applying insulation separately , battening a cavity out and boarding? I'm completely guessing at time but think for a couple of amateurs that even just getting the insulation on approximately 125m vaulted ceiling space will be a decent week of work so perhaps better making the labour saving on the walls and finding a few quid elsewhere.

You will be surprised how quickly 

I normally have one up on the scaffolding shouting the measurements down and the other cutting 

Good saving doing it yourself