What is the R value?

R is the measure of the correlation between degree days and energy used. An R value of zero shows that external weather conditions do not affect the energy used in the building whilst an R of 1 shows that the energy used in the building perfectly follows the weather (and that all the data points lie on the line). A high value of R is evidence of a good control system in a building and a low value can imply control problems. Recent work has shown that for an R value above 0.7, two months of data is sufficient to gain a yearly benchmark within 10% of actual annual energy use.

The mean R value for a building is 0.87 with 30 % having an R value greater than 0.93 and the best controlled 10% of buildings having an R value greater than 0.97.

R values can be calculated from 3 weeks or more data points (4 or more meter readings).

Buildings with the highest R values use considerably less energy than those with lower R values, due to the better level of control and less wasted heat. Buildings with an R value over 0.9 use 10% less energy on average, those over 0.95 use 15% less and those over 0.97 use 20% less energy on average.

R < 0.87 below average
R > 0.87 above average
R > 0.93 well above average
R > 0.97 excellent