thinnest plasterboard or other plaster substrate?

[The Natural Philosopher]

you could expanded metal mesh, or lath...

[harry]

Or, I have removed the foil and used plaster and PVA to put a thin skim of plaster direct onto the foam. No problems so far, five years ago.

[Andrew Gabriel]

Yes, you can even PVA glass and plaster onto that.

It is also manufactured without foil on one or both sides, although finding a reseller might be a challenge.