Risolant

15:16 Jun 17, 2013 

@Chris, of course you're right. However, I did spend quite a while looking for an equivalent in a similar "low-energy" context. I now have a published example of the variable I can show my client if he should ask.
However, your point is valid.
Your link was very instructive.

15:10 Jun 17, 2013 

that there is no "correct" or "precise" translation since these are all variable names, ie mathematical shorthand. It could just as well be "Rinsulat" or "Rinsul", etc.

15:02 Jun 17, 2013 

I found the precise answer here: A handbook on Low-Energy buildings by L.D. Danny Harvey (cf. Google books).
It is in fact R_ins
I am aware that this should be written in subscript, but I wasn't sure that KudoZ would take HTML in the actual word.
Thank you for all your interesting links.

16:06 Jun 14, 2013 

to understand the distinction between U-values and R-values, the one being (kind of) the inverse of the other. See http://www.architecture.com/SustainabilityHub/Designstrategi... for enlightenment.
Analogously with electrical resistance, thermal resistance increases with thickness, while thermal conductance decreases with thickness. The total resistance is inversely proportional to both area and thermal conductivity, so conductivity is sometimes a more natural parameter to use. Values expressed are usually "specific" values, meaning that they are given in terms of units per square metre.