Flat roof insulation

[GaryinAshby]

My flat roof (cold deck) on a new build extension has 9" x 2" joists, with firings tapering from 4" to 0.

I don't have my plans with me at the minute, but I want to sort out the roof insulation this morning. From memory I need to achieve a "U" value of 0.16 for a flat roof.

What product would be best to use between the joists to achieve this? It needs to be no thicker than 7" as I need to keep a 2" airspace over the top of the insulation. I also don't want to use the insulation-bonded to plaster board stuff thats available, just insulation between the joists & the 12mm plasterboard over the top, nailed to the joists.

Any suggestions?

Thanks

 

[^woody^]

I think that you will find that its something like 90mm board between the joists and 25mm continuous below the joist to prevent thermal bridging.

You can't just have insulation between joists nowadays

And you don't have the depth for any quilt insulation