Replies

1. 
   Mike_Willms | Nov 06, 2001 05:14am | #1

   *
   Sand and gravel concrete - R 0.08 per inch. In otherwords, it's not an insulator

   Other materials R-values, for those interested:

   Stucco R 0.20 per inch
   Building paper R 0.06
   1/2" plywood R 0.62
   Softwood R 1.25 per inch
   1/2" drywall R 0.45
   Carpet and foam pad R 1.23
   Asphalt roof shingles R 0.44
   Shakes R 0.94

1. 
   Mike_Smith | Nov 06, 2001 05:31am | #2

   *another way of looking at it ...wood is 1 to the inch..concrete is 1 to the foot..

1. 
   Cloud_Hidden | Nov 06, 2001 07:07am | #3

   *However, that might not be the best question to ask. Think of it this way.... If you put concrete on the warm side of insulation, the combination of insulation and mass perform far, far better than the sum of the parts. Where that's leading is that we built one of the type you describe, but we insulated it. Dug a hole. Put layers of plastic. Sprayed 2"+ of urethane foam. Placed jets and hoses. Added 4" of shotcrete. Plastered. Filled with water.No big heat loss to earth be/c of insulation. So concrete with properly placed insulation does great from an energy perspective. Have pictures of the process. If you want I can scan and post them. Also should have a shot or two of it on tv in a few weeks or so.As long as you can insulate it, your plan sounds like a neat idea! Bonus: the insulation will also protect the pipes.

1. 
   teo_ | Nov 06, 2001 08:18am | #4

   *How's that plaster on your butt? I've spent some time in concrete tubs and doubted I'd want to have that red butt all year long. Can you burnish the plaster?

1. 
   Cloud_Hidden | Nov 06, 2001 05:40pm | #5

   *Smooth as you'd ever want. No scratching at all. Our 4 yo daughter is in there and never scratches. Like the sidewall of a good swimming pool--smooth, but not slippery.

2. 
   Mad_Dog | Nov 06, 2001 09:23pm | #6

   *I build houses with insulated concrete forms as well as more traditional means. An advantage to concrete surrounded by R-25 foam is more in terms of thermal mass than as an insulating member. It takes longer to heat it and cool it, but does a good job of keeping heat/cold transfer to manageable levels.Don't know how it would relate to your tub situation though.

3. 
   David_Thomas | Nov 06, 2001 10:05pm | #7

   *Teo: How about steel-troweled shop floors? Really smooth. Probably smoother than you want with water around. The finish is certainly controllable. -DAvid

4. 
   Cloud_Hidden | Nov 06, 2001 10:53pm | #8

   *For a hot tub, the advantage to me was that it would last longer and not degrade in UV, as fiberglass will. Longer life means cheaper in the long run. Once the decision to use concrete is made, then insulating it properly seemed to make sense. Didn't pick concrete/foam for energy values (my house, yes; hot tub, no). For hot tub, picked concrete for strength and longevity, foam for energy savings.teo: the plaster was also good at filling in any curing cracks in the concrete

2. 
   Ken_C_Van | Nov 06, 2001 10:53pm | #9

   *
   I'm working with a friend who is a concrete precaste specialist and we are combining talents to build a life time hot tub, but we are concerned about the energy costs, so we are hoping that someone out there can give us an idea of the R value of concrete. We are on Southern Vancouver Island so its not to cold here in the Winter but we want to make this tub as kind to the enviroment as possible.
   Thanks for your help
   Ken from Winter Creek