Cavity wall insulation board in loft rafters?

mallen

Hi,
I am look at doing some DIY loft insulation and have found some Kingspan Kooltherm K8 Phenolic Cavity Insulation Board 1200mm x 450mm x 50mm (0.54m²/Sheet) which seems fairly priced.

My house has an old loft extension which is boarded out. There is an access hatch into the side of the roof and I was planning on getting some insulation board and retro fitting to the back of the plaster board between the rafters. There is currently a think rockwoll there.

Is it OK to use cavity wall Kingspan between the rafters as the one i have found above is considerably less than others online..

Thanks

stator

Sure you can do it.
I suppose if it burnt you might be concerned about where the fumes go

mallen

Fair point, I think I'll get the normal insulation boards.

Also the rafters depth to the plasterboard is 50mm. Should I leave a gap from the plasterboard to insulation board and use 30 or 40mm board leaving a 10mm gap?