I've often wondered the same thing. I'm afraid the glib answer is: do the experiment. It won't be too hard. You'll need to ensure that conditions are the same for two bathfuls of water. Try to do both baths on the same day when the ambient temperature and air pressure are the same. Fill to the same level and start at the same water temperature. If someone is to wash in the bath, then it should be the same person each time. Moving around a lot could affect the results, so best to lie still. Take temperature readings every minute or so, for about half an hour - that will give you some data to show differences in heat transfer.

My guess is that the bubbles would make a minuscule difference in a plastic bath. If you have an old cast iron bath, then I bet the any difference would not be measurable without lab conditions.

I used to use bubble bath because it prevented the formation of "bath rings", and I certainly did not try Pete's controlled experiments, but I did occasionally wonder why I noticed no dramatic effects of heat conservation.

I concluded that although the bubbles would limit radiation and conduction, the concentration of heat at the bottom of the foam layer must cause the bubbles to rotate and convey warm water upwards and cold water from the top downwards.

Personally I would like to see some experiments with infra-red cameras, but my camera does not seem to work in the relevant part of the spectrum. <siiigh!>

I first thought that "bubble" bath meant some air bubbled through the

water from below, as it is done in so called "jacuzzis" (right?)

In this case, the bubbles would carry away a lot of heat, because

the air has to be saturated with water vapour.

Now, Petes and Jons answers

deal about some more or less stable foam layer on top 

of the water. In this case, the evaporation of water at the surface is

blocked, the heat loss from evaporation (main loss in a bath tub!)

does not occur.

Plastic or steel or cast iron tub is secondary, when the tub is

"built in". But maybe that is something specific for Germany?

For a free standing tub, on the outside the heat transfer is

ruled by air speed, the tub material is not so important in most cases.

Georg

Georg,

What you describe is altogether what one might expect, but it does not seem to tally ell enough with the facts. That is why I (and I think Pete) would be unwilling to commit to any theory in the absence of controlled experiment.

Cheers,

Jon