In message , John
Rumm writes
On 03/09/2019 11:58, The Natural Philosopher wrote:
On 03/09/2019 11:55, Tim Lamb wrote:
I expect current building regs. will require more than 25mm foam.
The tragedy is that they will. Thus preventing many people from
improving insulation.

It can be worth talking with your local BCO though since they will
often accept a less than ideal improvement if its the best that can be
done in the circumstances.

On a wood frame workshop, I used 75mm Celotex sheet. Battens secured
with distance screws and topped with feather edge.

For a domestic job I suppose you need to consider a vapour barrier.

--
Tim Lamb