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.

Effects of extra insulation

During 2007, I made the following modifications to the house:

  1. Added cavity-wall insulation (which was missing in the main walls
    of the house) – figure 21.5.
  2. Increased the insulation in the roof.
  3. Added a new front door outside the old – figure 21.6.
  4. Replaced the back door with a double-glazed one.
  5. Double-glazed the one window that was still single-glazed.

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
building codes.)

– Cavity-wall insulation (applicable to two-thirds
    of the wall area)
4.8 kWh/d
– Improved roof insulation 3.5 kWh/d
– Reduction in conduction from double-glazing
    two doors and one window
1.9 kWh/d
– Ventilation reductions in hall and kitchen from
    improvements to doors and windows
2.9 kWh/d
Table E.9. Break-down of the predicted reductions in heat loss from my house, on a cold winter day.