R-values come in different units. On Canadian websites, where they acknowledge that imperial and metric measurements are both used, I have seen them listed as R for imperial and RSI for metric. American websites tend only to use imperial values. Some other values seem to be a mixture of metric and imperial!
The UK regulations use U-values, which is fine for working out heat loss but doesn't really acknowledge that R-values are more useful for working with insulation to conform to the regulations. R-values of individual layers are summed to get the overall R-value, which is the reciprocal of the overall U-value. There is no point working out U-values for individual layers but you can average the U-value for a wall with windows.
The K-value of a material does not depend on thickness. Rule-of-thumb K-values (SI units) include 0.115 for timber, 0.063 for Vermiculite, 0.040 for mineral wool and 0.024 for PIR. To calculate R-value, you divide the thickness (metres) by the K-value. Thus 250mm of mineral wool would have an R-value of 0.250 / 0.040 or 6.25, with a U-value of 1/6.25 or 0.16 W/m²/K. Now, this just shows that the term 'K-value' probably comes from the US or pre-dates SI. In countries using SI, the technical temperature unit is Kelvin (K), not to be confused with K-value for conductivity! It may be less confusing to use °C rather than K (same difference) and I could have used 0.16 W/m²/°C.
Things get more complicated by layers of more than one material (e.g. insulation between joists), you need the average K-value for the layer.
Take a typical 10% timber and 90% Vermiculite area ratio:
Average K-value for the layer is 0.115 x 10% + 0.063 x 90% = 0.068. Average R-value for 75mm joists with full-depth Vermiculite is 0.075/0.068 = 1.10. However, it doesn't stop there! I haven't included R-values for 12.7mm plasterboard (0.08 ), the warm (0.13) and cold (0.04) surfaces. The overall R-value for the ceiling would be 0.13 + 0.08 + 1.10 + 0.04 = 1.35. Now I am in a position to calculate the U-value for the ceiling as 1/1.35 = 0.741 W/m²/K.
If you use space blanket instead of bare mineral wool, say, the low-emissivity (reflective) top surface will add another 0.40 R-value for a vented reflective cavity to the overall equation. However, as the top reflective surface gets dirty, this benefit is lost.
From
wikipedia:
"The conversion between SI and US units of R-value is 1 h·ft²·°F/Btu = 0.176110 K·m²/W, or 1 K·m²/W = 5.678263 h·ft²·°F/Btu"
