R-value is measured in meter²·°C/Watt (in the stone-age units used in the US, that's ft²·°F·h/Btu). Materials may be rated in R/inch, R/meter, etc. To calculate the R-value of a wall, you calculate the R-value of each layer by multiplying the R rating of the material times the thickness; then you add the R-values of the various layers. The system is not perfect. In particular it has trouble with combinations of conduction, convection and radiation barriers.
· 10 years ago