Insulation

[RASPUTINS-DOG]

we have a bath shower combination....the bath is situated against the wall with the window...I have been told that by putting insulation underneath the tub and filling any air pockets will improve the heating of the water in the tub.....?

[gingejbee]

Do you mean that the hot water in the bath will stay hotter longer? If so, yes it will...but don't expect miracles!

[Old_Geezer]

Surely the water is heated before you put it in the bath ?

That aside, for sure, answer above is correct. Improve insulation between bath and cold outer wall will ensure less heat loss per unit of time to the outside world.

[Old_Geezer]

Unsure what the reference to air pockets was about, though. Air is a good insulator already as long as you avoid convection currents.

[buildersmate]

Doing this would slow down the rate at which the water in a bit cooled, but only very slightly. The whole of the top surface of the water is still transferring heat into the air of the room.
There can't be too many people who lie in a bath for more than 20 minutes, and it is immaterial when showering.
This is going to make such a trivial amount of difference as to not be worthwhile the effort.

[The Builder]

I've always stuffed fibreglass roll under baths as a matter of course. It's not that important though. Far better to have a really warm bathroom.

Use an extractor fan over the shower. You can keep the window closed then.