[TimSmall]

Radiant barriers aren't that useful on upward facing horizontal surfaces, as over time they get dirty and stop working. Also their contribution is pretty minimal anyway.

In A/C's proposed sketch, I think I would omit the EPS, as it's relatively high position will keep the timbers cooler, plus it's expensive + labour intensive to install. Just use batt (what ever is cheapest - so long as it has the PIR beneath it, it'll be fine.

Incidentally, for fixing the PIR upwards, I would use plastic discs and stainless screws. Don't put any timber down there, it'll soak. I think the timber support columns will have similar problems and will probably need replacing with something which won't mind the relatively high humidity environment...