temperature demand in 2006 was 2866 degree-days. The average rate of
heat loss, if the house is always held at 19 °C, is therefore:
7.7 kWh/d/°C × 2866 degree-days/y/(365 days/y) = 61 kWh/d.
Turning the thermostat down to 17 °C, the average rate of heat loss drops
to 48 kWh/d. Turning it up to a tropical 21 °C, the average rate of heat loss
is 75 kWh/d.
During 2007, I made the following modifications to the house:
What’s the predicted change in heat loss?
The total leakiness before the changes was 322 W/°C.
Adding cavity-wall insulation (new U-value 0.6) to the main walls re-
duces the house’s leakiness by 20 W/°C. The improved loft insulation (new
U-value 0.3) should reduce the leakiness by 14 W/°C. The glazing modi-
fications (new U-value 1.6–1.8) should reduce the conductive leakiness by
23 W/°C, and the ventilation leakiness by something like 24 W/°C. That’s
a total reduction in leakiness of 25%, from roughly 320 to 240 W/°C (7.7
to 6 kWh/d/°C). Table E.9 shows the predicted savings from each of the
The heat-loss parameter of this house (total floor area 88 m2) is thus
hopefully reduced by about 25%, from 3.7 to 2.7 W/°C/m2. (This is a long
way from the 1.1 W/°C/m2 required of a “sustainable” house in the new
|– Cavity-wall insulation (applicable to two-thirds
of the wall area)
|– Improved roof insulation||3.5 kWh/d|
|– Reduction in conduction from double-glazing
two doors and one window
|– Ventilation reductions in hall and kitchen from
improvements to doors and windows