Roof insulation design

[Linto]

So after giving up hope of finding a builder to start this year and knowing prices have increased substantially I thought I'd take another look at what we were planning on doing with our roofs insulation build up to see if I can make any savings.

 

Currently we are planning a roof detail as per the image aiming for about a u value of .14 or below. But as the roof has supporting dwarf walls and areas that we aren't planning on plastering (we would just use equivalent thicknesses of PIR in these areas) I thought about putting the PIR insulation over the rafters instead. In my mind this will be easier to achieve with less complications and simpler for any builders. It seems like a straight forward swap from under to over and there are a number of details available from manufacturers but I fear that I'm missing a key detail. Ie where to put a VCL if needed?

 

So the new proposed build up would be:

Tiles

Battons

Membrane

PIR insulation

Mineral wool 175mm or 200mm (rafters will be 195mm thick so I'm unsure what to go for, compress the 200mm (probably will be 2x 100mm rolls) or 175mm and thicker PIR)

Plaster

 

What are your thoughts, any other ideas?