Tub insulation

[Dodgyal]

I lost my green credentials at work because I got a hot tub, it was free and I couldn't say no, anyway I've worked out it consumes more that my house in electric and so I've take a few steps to try and insulate and and turn the temp down when not in use also been an avid (lurker) on Frotter 'Donut o Delite' solar heating array post

A question on the insulation though, under the tub their is a plastic base (no insulation straight onto a cement base) and the fibreglass tub has a small amount of spray foam 10-15mm on it. I wanted to stuff it full of plastic insulation from B&Q which I have done a little of already (steering clear from the pump and stuff) but now I not so sure this is the best way, should I leave a gap between the base and tub for heat to build? does this improve things?

[Contadino]

I have a friend who builds custom bathroom furniture (the sort of stuff you'd see in a Moroccan hammams.)  He's doing our bathroom next summer and I asked him about insulation of the tub.  He uses a kind of powdered/crumbled cork (from the builders yard, dirt cheap and environmentally responsible), mixed in with a lime render, and puts a couple of coats (like 30cm in total) under the final waterproof layer.

Maybe you could do something similar on top of the existing insulation? I'm not sure how you'd get it to stick, and you'd have to be sure that there was no flexing of the tub itself, but it could be a solution.