Insulating the rafters

[marsaday]

I am planning on using tri iso for my loft conversion as the bld inspector will pass it.

however, can i add simple rock wool insulation (100mm) into the 100mm gap between the slate and the underside of the rafter?

My roof has no membrane on it, just old fashioned back pointed slates in great nick.

or maybe i could use 50mm kingspan. because the tri iso needs to have a 25mm air gap between it and the kingspan. if i used rock wool, this gap wouldn't be there unless i built out the rafters by 25mm

 

[masona]

Best to have an air gap with the top ridge vents as well for air circulation otherwise condensation problem