insulating a roof structure

[murphs1]

Hi everybody. Can someone advise me please. I have a vaulted roofspace that we intend to use for living accommodation in the future. I know that building regs require a sellotex style insulation (rigid foam) with siver foil on the faces between the rafters and again over the face before the plasterboard goes on. Not only will it reduce the space up there but the materials alone are £5,500. Now i'm for good insulation and the enviorement as much as the next man but I can't help feeling that just maybe this is a bit of overkill and the lunatics really are running the asylum now!
We have a breathable membrane under the battening. Is it ok to insulate using a loft style fiberglass or possible polystyrene and if so does an air gap need to be left between the insulation and the membrane. The whole thing will be plasterboarded. If anyone can advise I would be grateful.
Thanks,
Peter

 

[hotrod]

5.5k - what thickness celotex are you using and what's the m2?

 

[murphs1]

Yes £5,500. The thickness they require is 100mm between the rafters and 40mm across the face. The total are is 110square meters. But that means 110 meters of 100mm and 110 meters of 40mm. It is a lot! Do you think that I can use wadding style insulation or polystyrene. Will it need an air gap with a breathable membrane as I do not want any condensation.

Pete

 

[hotrod]

Well by my reckoning you would need:-

40 n.o. Celotex GA3100 2400 x 1200 x 100mm = £1200+vat
40 n.o. Celotex GA3050 2400 x 1200 x 50mm = £640+vat

Can't find a price for 40mm. So I would shop around for some prices as you may get some further discount off those prices. Not 100% sure you need 140mm to achieve required U values either but am sure someone else / celotex can confirm - you need to achieve 0.20 W/m2k.

You need 50mm ventilation above insulation.

 

[Balenza]

hotrod wrote

You need 50mm ventilation above insulation

Why would you need that for ?.
Its not mentioned in this link...................

http://www.quinn-group.com/downloads/quinntherm_technical_manual.pdf

 

[AtoZ]

would the reflective rolls solve your problem. (actis) that type of thing.
its expensive but im sure it will be better than celotex.