Roll or board - Pitched roof

[Moggaman]

Hi

I have alot of pitched roof insulation to put in as I have a room in roof design.

I have 135mm rafters. The detail i have been given is 180mm knauf omni fit roll with insulated plasterboard fronting the rafters. Clearly that cannot work. I propose to use a 110mm PIR board (kingspan or similar) leaving a 25mm airgap behind and insulated plasterboard fronting the rafter also. I will have insulation detailed above the flat part of the ceiling also which will be rolls.

Do you think my rafter plan is ok? .Shouldnt affect  U value too much?

Thanks

 

Edited September 10, 2021 by Moggaman
inserted drg

[Moggaman]

Apologies my rafters are 175mm (7 x 2)

[Iceverge]

Similar topic here a couple of weeks ago. 

 

[ETC]

If this is a new build check your SAP for the thicknesses of roof insulation to be used. Your SAP assessor will have used certain thicknesses of insulation to calculate the TER and DER.

 

However for 175mm deep joists I would normally expect to see 125mm Kingspan between the rafters giving a 50mm air gap behind the insulation and anything from 32.2mm to 82.5mm Kingspan insulated plasterboard across the face of the rafters giving U-Values ranging from 0.16Wm2K to 0.11Wm2K. 0.18W/m2K is required by TBF1 for extensions.

 

Don’t forget that you can always batten down the rafters to achieve a larger rafter depth if your design calls for insulation that is thicker than the rafters.