High insulation thin wall material for uPVC conservatory
 

I intend to build a uPVC conservatory onto the back of the rear of our
semi. It needs planning permission because it is 4m deep.

One side wall of the conservatory will be adjacant to the 7' high
party-wall wall that runs the length of the garden, so it won't get
any sun. If I use a standard uPVC panel it will lose a lot of heat.
I could build a cavity brick/block wall instead which would solve the
insulation problem, but it would make the conservatory considerably
narrower.

Is there a thin walling material, maybe 2" or 3" thick, that has high
insulation properties, and that might satisfy planning, that I could
use instead?


Thanks

Tony