I just want a long hot bath

[sirspiffy]

My mind was wandering today.  I was looking at bathtubs b/c I'm too tall for mine and one of the problems of tubs is heat waste and conductivity.  This explains why I can take a bath for only about 5 minutes before the water gets tepid, then chilly.  After a brief ride on the information supa' highway I found that some people say a layer of bubbles will actually insulate the water from the air.  Interesting but I think the tub is the bigger problem.  A few internet solutions were to foam insulate everything, the pipes, the water heater, and the tub, as well as preheating the tub and refilling. Others say to keep a small trickle of hot water going to prevent large dips in temperature. Seems pretty inefficient to me. 

Most bathtubs are made of metal (iron, steel), some are ceramic (acrylic), some are stone (marble), and others (fiberglass, polymers, resins).  Metal has high conductivity, ceramic and stone stay cold and the others have varying properties. 

Maybe the tub needs to be taken out of the equation, and the basin could be built out of solid forms.  Maybe a water re-heater is necessary.

What does everybody think?  How can we design Maximal water heat retention tub?  Materials? Techniques?  Anyone have thermal ratings for tubs and materials?

[velocistar237]

This link is about a shower rather than a bathtub, but maybe it will help to think about it.
http://www.sunfrost.com/efficient_shower.html

You want materials with low thermal mass, so it doesn't take heat from the water, and high insulation, so the water stays warm. A plastic-type tub with foam insulation sprayed underneath might do the trick. Enclosing the area above the tub will reduce air flow and prevent cooling from evaporation.

Also, builders often install the tub early in the construction process, before the sheathing goes on, and as a result, there's a huge air leak if the tub is on the outside wall. See here for details. This is true of my tub, but it looks like too big of a hassle to fix.

[Parizade]

The original tub in my last house was metal and didn't hold the heat all. When I upgraded the bath I put in a fiberglass one piece shower tub, but it didn't hold heat either.

My current tub retains heat very well, but I'm not sure why. It seems to be made of fiberglass and I assume there is some insulation in there too. It's still not long enough for someone over 6 ft, but it is deep so it works out anyway. It is separate from the shower and was called a "soaking" tub in the brochure if that helps.

Good luck, a hot bath is a wonderful thing.

[slugsworth]

I wanted to second the notice that your could be suffering from poor air sealing.  It seems that it is pretty common for plumbing penetrations to not be well air sealed. If your tub happens to be on the 1st floor I would guess that there is cold air from outside cooling the bottom of your tub. You would probably notice a bigger difference in the winter than the summer if this is the cause of your problem. In any case I would make sure to air-seal the penetrations if you replace your tub.

[HeidiO]

   My first house had a beautiful clawfoot tub.  Freezing cold.  I don't know what the best kind is, but it certainly isn't an antique one!  I like high shower curtains - ones that are hung just an inch or 2 (or 3) from the ceiling.  You can buy them or make them extra long (a regular length one hung really high looks kind of silly.)  They look classy, and there is less of the thermal chimney effect (as described in the sunfrost link.)  I also don't understand turning on the vent while taking a shower.  Run it when you're all done!  Of course, I live where there is an avg of < 10" of rain a year, so mold is not a big issue here.
   While on the subject, I really like cloth shower curtains.  I grew up with plactic liners, but as an adult discovered the polyester ones that you don't need a liner for.  They can be more expensive (i.e. $10 instead of $2 when I bought one at Big Lots) but they wash really well in the washing machine, and can last forever.   http://www.target.com/p/Inter-White-Waterproof-Shower-Curtain-Liner/-/A-13100913  is an example.  96" long white fabric, for $17.39 .  I'm sure there is a more frugal option, but this was the easiest link I could fine for this post.
Heidi

[Mirwen]

I just installed this tub for the same reason.  It's been fantastic.

http://www.amazon.com/Swanstone-BT-3060L-010-Veritek-60-Inch-Finish/dp/B0014GDN62/ref=sr_1_1?ie=UTF8&qid=1332779007&sr=8-1

[catalana]

Two simple solutions:

A small bathroom
Run the hot water on its own first (particularly useful if you have a metal bath)

[Welmoed]

I love long, hot soaks, and solved the problem by installing an inexpensive hot tub in the back yard. It's always hot, yet doesn't add much to our electric bill (maybe a few bucks a month). I also think they're more environmentally responsible because you're not constantly filling and emptying it for each soak; we drain it twice a year and the system it has requires minimal chemicals (it's an ionizing type). Best investment we ever made. Check Craigslist; I've seen hot tubs show up there all the time.
--Welmoed