Insulating 45 degree internal ceiling returns in loft space

[jc212]

Looking for advice again from you good folks if it's not too much trouble, this time it's in my loft. 

 

For whatever reason the architects thought it would be a good idea to have 45 degree internal plasterboard around the outside walls of the house. See attached photo for what I mean. Only issue with this - there is zero insulation on the back of the plasterboard and these points sit right in front of the soffit ventilation ports. 

 

We put vertical radiators in (not my decision) so literally radiating heat against a surface that's being cooled at high velocity in the loft with fresh air!

 

I am proposing cutting 100mm (or 75mm) PIR and placing this between the rafters in contact with as much of the plasterboard as possible. I would be sure to allow sufficient space 25-40mm in front of the pir so air can circulate freely through the soffit vents.

 

Questions are:

 

What's the best way to detail this

Should I friction fit, or use some kind of adhesive 

Should I foam around the edges in contact with the rafters and then also add foam in the 45 degree gap that will be formed between the PIR and vertical plasterboard

Any other thoughts or suggestions 

[Big Jimbo]

One of the reasons why i hate those slopes when i see them in any property. You are going to have to try and get some insulation in there, while maintaining airflow under the tiles if you have a cold roof.

[Mattg4321]

gapotape looks great. expensive though. Needs foaming in at very least imo. Airtight.

[jc212]

Not seen that, looks absolutely perfect. Id be struggling to reach down to foam in the gap between plasterboard coming up from the room, sounds as though applying this it would expand and fill it. But in principle this would make sense putting PIR here? 

[Iceverge]

72.5mm insulated plasterboard chamfered top and bottom and screwed through the existing plasterboard into the rafters. 

 

Foam either a good low expansion foam, tape and skim the joints to wall and ceiling. 

 

Don't bother taking down the existing plasterboard. No space gain in the room and will create an enormous mess.

Edited 7 hours ago by Iceverge

[jc212]

I was planning on leaving the inside finish as is (would prefer not to reskim etc). Preference would be to insulate in the loft space