Insulating flat garage roof

[Tom]

I need to insulate my flat garage roof, [cold deck] to a u value of .25, the joists are 9 inches deep and I have to leave a two inch gap above the insulation to provide room for cross ventilation. I have priced celotec at nearly £700.00, is there a cheaper way, eg pad out the bottom of the joists so that I can use 300mm of fiberglass, If so is there a minimum clearance between flor and ceiling? Is there a better way, does rockwool have a better u value than fibreglass? If relavant the garage is constructed with a brick outer skin and a celcon block inner leaf and is integeral to the house on one long side, the garage is 15ft by 9ft and is to be used as a study/music room. Any sugestions gladly accepted, Regards all, Tom

[The Natural Philosopher]

Shop around for a better price on the celotex. I would have expected a lot less than that. IIRC an 8x4 sheet of 50mm is about 30 quid.

Nothing is as good.

You COULD use double the thickness in rockwool, but if it gets damp, it rots timber faster than anything. I don;' think it would come much cheaper, either. AND its a bastard to isntal.

Fiberglass and rockwool are almost the same thing.

15x9 135 sq. foot. I make that about 120 quid at sensible prices.

[Christian McArdle]

No minimum height requirement.

Get quotes both for ecotherm and for seconds Kingspan from

quid sounds very steep for 12m2. You might even get away with 4 boards, if the joists are thick and going the right way. Worst case is 6 boards, which is 17.3m2. Did the quote include the Celotex equivalent to K17 for underboarding? Perhaps it included wall insulation?

Christian.

[Lobster]

price was 107 GBP including delivery.

David

[Lobster]

(sorry, should've said: their delivery is 20 GBP + VAT - standard charge AFAIK - if you want to do the sums for your own quantities)