U Value for a pitch roof

[Phillip Stephens]

Hello All, I'm trying to work out the U value requirement of a pitched roof. Its an extension roof and Building Regs require a .16 U value at the ceiling level.
This is the roof makeup thus far
Clay Tiles 20mm
Breather membrane (0.1 mm)
Air layer/battens 30mm
Rafters 200 mm

To get to a .16 the U calculator at Rockwool for example has the following after the tile/membrane
HARDROCK® Multi-Fix (DD) (50 mm)
FLEXI® (200 mm)
Unventilated air layer (10 mm)
Polyethylene/polythene, low density 0.15 mm (0.15 mm)
Standard Wallboard Plasterboard (12.5 mm)

I'm trying to understand what the below rafter height would be based on the above. Would it be 12.5 mm plasterboard + a further 60.15 mm to accommodate the air layer/Hardrock multi-fix+Polyethylene? So a total of 72.65 mm below the rafters as the insulation (200mm) would fit within the rafter gap. Thanks

 

[^woody^]

300mm of Rockwool or Dritherm on the ceiling.

 

[Phillip Stephens]

Thanks Woody. I knew I could rely on you for an answer.
Could I get away with using Kanuf Earthwool Rafter Roll 2x75+100mm / Rafter (250mm)? The calculator comes up with .16 U. That would mean the plasterboards would only lower the ceiling height by little over 60mm as the rafters are 200mm. Thanks

 

[^woody^]

Yes if the calculator gives you that. Quilts tend to give the relevant u-value between 250 and 300mm thick depending on the actual product, so 300mm is just an easy general guide.

You mention the rafter depth, so must be doing a whole roof calculation? It does not tend to matter though as generally the insulation will perform based just on its thickness at ceiling level.