Attic Insulation -overkill or underspec???

gooseygander

All opinions welcomed, attic insulation on new build which is storey and a half with 7 dormer windows is planned to consist of 400mm of isover g3 softtouch to all flat areas of attic and 150mm of metac to sloped area that rises from walls, my rafters are only 7inch so I am confined to 150mm in the slopes and that leaves 25mm air gap for roof to breath 62.5mm insulated slab will then be applied to sloped area and same then plastered. Is a normal plasterboard sufficient for the rest or should I also look at the option of insulated board on inside of normal flat ceiling also.Sorry if this is all a bit confusing but I am prepared to push out the boat for best insulation. All informed opinions and criticism welcomed.

BryanF

Ceiling and slopes areas: install a Vapour barrier taped & sealed between insulation and plasterboard or of you intend on using spot lights etc install a service cavity inside the air-tightness/vapour barrier

7 dormer windows: what air-tightness target have you set? How many visits for testing? Have you seen previous results from air-tight installer?
With 400mm insulation on the flat you don't need the insulation below the joists, but you do on the slopes

sydthebeat

You need 50mm gap to comply, not 25

gooseygander

BryanF wrote: »

Ceiling and slopes areas: install a Vapour barrier taped & sealed between insulation and plasterboard or of you intend on using spot lights etc install a service cavity inside the air-tightness/vapour barrier

7 dormer windows: what air-tightness target have you set? How many visits for testing? Have you seen previous results from air-tight installer?
With 400mm insulation on the flat you don't need the insulation below the joists, but you do on the slopes

Thanks brianf, my air tight contractor is a passive house builder who I trust at the minute and have checked out his credentials before he came on board, slopes is definite for the insulation boards but I was going to do the flats as well if budget allows, I figure I will only get one chance to do it, I know I am 100mm over the regs of 300mm for the flat of the attic by going with 400mm of the softouch, do I still need the vapour barrier if I go with insulated slab or does the foilback on same suffice and am I mad to put insulation below the joists, many thanks

BryanF

gooseygander wrote: »

I was going to do the flats....going with 400mm of the softouch, do I still need the vapour barrier if I go with insulated slab or does the foilback on same suffice and am I mad to put insulation below the joists, many thanks

i prefer to have a vapour barrier, which can double as an air-tightness layer

as regards the flat ceiling areas: instead of insulated slabs put 500mm off insulation over top - give the floor to ceiling in those rooms that extra few mm

why is your arch not advising you of this stuff?

gooseygander

BryanF wrote: »

i prefer to have a vapour barrier, which can double as an air-tightness layer

as regards the flat ceiling areas: instead of insulated slabs put 500mm off insulation over top - give the floor to ceiling in those rooms that extra few mm

why is your arch not advising you of this stuff?

Arc has just specified enough to meet regs and told me if budget allows go big on insulation, builder suggested the insulated board. I will run it by him tomorrow, thanks for your input.