Dormer roof- what's "unventilated air gap"?

[Chrischecked]

Hi - please can anyone explain what all this means for a dormer roof 3mx3m (especially the "unventilated air gap" and "TLX Rafter Fix")?
Would the roof be good, durable, energy-efficient if built to specs below?

"3 layers felt on 80 Celotex on 12 ply decking on firrings at 1:60
unventilated air gap ????
TLX Rafter Fix ????
150 rafters at 400 crs
unventilated air gap
12.5 plasterboard and skim

Cheers as always![/b]

 

[jeds]

TLX rafter fix is a multi foil insulation. That type of insulation works because the surfaces of the foil are ultra low emissivity, which reduces surface radiation. (must spell that correctly in case Robin is watching) The unventilated gaps are crucial to the emissivity - if the surface of the foil touches anything emissivity drops to zero.

 

[freddiemercurystwin]

i would be asking your designer why he specified the TLX, specifically what advantage is to be gained.

 

[jeds]

i would be asking your designer why he specified the TLX, specifically what advantage is to be gained.

I agree with that. More PIR foam insulation would be more beneficial and easier to fit. The necessary separation gaps are not easy to achieve and in reality every time I've seen it used the builders haven't bothered that much and the stuff is touching all over. I've even seen it boarded directly over, which makes the stuff pretty well useless thermally. But they did save the client 25mm of space - which I imagine they were very grateful for.

 

[Chrischecked]

Thanks John. What is the best way to fit the insulation to get an air gap?