Insulating between rafters with awkward angle at apex/eaves

[MisterBoy]

I am insulating between rafters with PIR in a home project (my greenhouse conversion) and not sure what to do at the peak and in the eaves since the end of the board won't butt perpendicular to the ridge, etc. Do I try and mitre the ends of the PIR boards or accept it won't fit snugly and deliberately leave a gap to fill with rockwool/foam, or something else?

 

[liftfan]

Mitre the ends. Any gaps and your expensive PIR is essentially useless. The current regs require the joints to be properly sealed eg. with Gapotape.

Don't take my word for it though ... listen to the Master: Robin Clevett:

 

[MisterBoy]

He loves his gapo (watched a bunch of guys videos)...I love the concept but it costs half as much as the PIR! They need to invent squishy PIR

Thanks.

 

[MisterBoy]

So cutting PIR accurately is tricky enough at 90 degrees (I use the SkillBuilder tip) without trying to do at an angle.
I wonder if I can use my circular saw on an angle just to start it off - I am aware of the dust problem