[Exeric]

Hi AC,
The point I was trying to address was that efficiency is not always the be all and end all. This is especially true in more temperate climates. If one is using a renewable resource such as a solar collector or collector(s) to heat the water then it may be less of an issue. Your mileage may vary.

For instance, if you go for maximum efficiency with a heat source going almost directly into an example hardwood floor covering, then the pex will be seeing an R~0.9 . If the person decides to change to carpet later it will go up R~2.5 minimum, probably more. I don't see how that system will be able to handle that 180% increase. In other words, the change in a less efficient system compared to this more efficient heat transfer represents a 60% vs 180% increase in heat requirements with a change to carpet from hardwood. It is just good engineering to have a robust but simple simple that allows one to change to carpet at a later date without reengineering the system. It will come at the expense of maximum efficiency. This may be impossible in a very cold climate but in other circumstances it may not, especially using a renewable resource to heat the water. It also avoids the hot spots and installation problems of pex next to the floor coverings.

Also, if you look closely in my previous comment the R = 4.3 included the carpet on top of two layers of wood.

Best,
Exeric