What does the slope of the best fit to the performance curve mean?
The slope of the best fit to the performance curve gives the energy loss per degree day, i.e. how much the building fabric and air tightness contributes to the energy used by the building. It is measured in kWh/m2/degree/week. Higher values means more energy is being lost through the building shell for every degree colder the external temperature is. The value of the slope is only meaningful if the R value of the fit is well above average. 50% of buildings have a slope value below 0.1 kWh/m2/degree, 30% less than 0.08 and 10% less than 0.04. slope > 0.1 worse than average – tend to use 30% less energy overall than the average slope < 0.1 better than average – tend to use 35% less energy overall than the average slope < 0.08 much better than average – tend to use 45% less energy overall than the average slope < 0.04 excellent – tend to use 55% less energy overall than the average A slope less than zero implies the building uses less energy as the temperature is colder and must be an art
