Ceiling & additional wall Insulation

[anorakdiyer71]

New single storey side extension (wc/ utility & office) with tiled pitched roof (inc breathable membrane/felt) with rafters & ceiling joists

So small void /storage area potentially all be it small, and insulation is priority

Double skin (Con. blocks, cavity insulation , Fenlite Con blocks,)
& floor to current insulation requirements etc etc

CEILING ??
Specified is :
300mm of insulation roll in two layers;
One between ceiling joists, then next layer over joists.

Is this best solution ?
what about celotex , foam backed p/board , or foil quilt products

INTERNAL WALLS? (Even though double skin & insulated)
Whats best method of adding insulation internally (just to improve u values etc)
Foam backed plasterboard ?
25mm celotex , then p/board w/ instastik or mechanical fix ?
DONT WANT TO batten out - defeats the object

 

[cdbe]

You could stick PIR plasterboard on with foam, but if the extension is to current regs I think the returns would be marginal. I would focus on the eaves as there won't be 300mm there - PIR cut to match the roof pitch while maintaining the air gap. You could also underboard the ceiling with PIR.