Taking the plunge...

[goodnoisefella]

I put all the poly packaging I used to get in the void beneath the bath....kept it loads warmer for longer soaks.
In the new place, i use multilayer foil & loft insulation under the bath....it's even better (and I couldn't wait for all the packaging to arrive)
It's still good for loose flll round the DHW tank, though.

/TP

[heatherw]

Yes, I've always wondered why baths don't come insulated underneath.  That spray-on polyurethane foam surely wouldn't do any harm.

[Bob]

Polystyrene would be good but fiberglass works quite well.