Metric and Imperial
In the interests of making the debate on R-values, and U-values more accessible to European readers, or indeed, anyone who lives in a part of the world which uses the metric system (does that include Canada btw?), would it be possible for GBA to compile together a conversion table? Or if there is any available already, to simple link to the same.
I can see from a definitions section in a state of florida building code the following:
Rated R-value of insulation, are in units of h.ft2.degree fahrenheit per BTU, at a mean temp of 75 degrees fahrenheit (24 degrees Celcius).
U-factor, thermal transmittance, units are BTU per h.ft2.degree Fahrenheit.
I have no idea what the ‘h’ in ‘h.ft2’ stands for even.
In the metric parts of the world, U-value is watts per meter squared kelvin.
U-value is the reciprocal of the ‘resistance’ (one divided by the resistance).
The units for resistances are meters squared kelvin per watt, m2k/w.
BTW, a kelvin degree and Celcius degree are the same for all intents and purposes.
The higher the resistance the better, the lower the U-value the better. But no one ever talks about resistance values in Europe. We only talk about U-values.
Sometimes for thermal bridging detail properties, and also for different grades of insulation, with use a measure called resisivity.
It’s units are watts per meter kelvin, w/mk.
When you divide the thickness of your material expressed in meters by the resistivity expressed in watts per meter kelvin, you obtain the thermal resistance of that part of the construction.
Which is expressed in units of meters squared kelvin per watt.
You get a reciprocal of all total resistances, and that gives you the U-value in watts per meter squared kelvin.
But to be honest, when I listen to any of the Green Building Advisor podcasts or read articles, I have no idea what is implied by the R-values or U-values quoted.
I notice that in terms of windows, the U-value is used, rather than the R-value.
In Europe, we would never mix and match our U’s and our R’s.