Breathable membrane

[leafsta]

Currently building an extension on my house as a permitted development but i'm a bit out of touch with roofing membranes and the amount of airgap required when insulating between and below the rafters.. from my understanding it's a minimum of 50mm above the insulation so i'm guessing that means on a 175x50 roof, I can use no more than 125 PIR between the rafters... can anyone confirm if thats correct?

 

Thanks in advance!

[Temp]

Yes that's correct. The membrane should drape/sag about 25mm between rafters leaving 25mm air gap. Should be ventilated at ridge and eaves.

 

The drape is to stop water pooling above battens causing them to rot.

[Thorfun]

You can batten and counter batten above as well to give the air gap above when using breathable roofing membranes. Before doing so check with the manufacturer and building control.