On 24 Jul, 20:27, JP Coetzee wrote:
Thanks for your advice so far. A little more background:

* The shed is already installed. It backs up against a rough stone
wall i.e. I can't get behind it.

No problem. Either foil, yellow foam or even (with a struggle)
rockwool can retrofit from the inside alone.

* The internal wall battens are about 44mm thick, the roof battens are
about 66mm. I won't build out thicker than that or the shed will
become too claustrophobic. What insulation can I use in those depths?

If you're that tight for space, then by all means go with the foil.

I'd suggest attaching a "sub-batten" alongside the wall battens and
stapling the foil down to those. You can get away without the full
airgap, but you do need _some_ reliable airgap. If you have contact on
the sides of the foil, then its insulation ability drops radically.

BTW the Frame Foil expert at Sheffield Insulation said it would be
fine to put plasterboard directly on top of the foil i.e. no air gap
on the internal side. Would you disagree?

Yes. Of course you _can_ do this, but if you put pressure on the foil
then it presses flat and turns into a cheap vapour barrier and no
more. So don't do that.