It’s frustratingly hard to make a really big dent in the leakiness of an
already-built house! As we saw a moment ago, a much easier way of
achieving a big dent in heat loss is to turn the thermostat down. Turning
down from 20 to 17 °C gave a reduction in heat loss of 30%.

Combining these two actions – the physical modifications and the turn-
ing-down of the thermostat – this model predicts that heat loss should
be reduced by nearly 50%. Since some heat is generated in a house by
sunshine, gadgets, and humans, the reduction in gas consumption should
be more than 50%.

I made all these changes to my house and monitored my meters every
week. I can confirm that my heating bill indeed went down by more than
50%. As figure 21.4 showed, my gas consumption has gone down from
40 kWh/d to 13 kWh/d – a reduction of 67%.

##### Leakiness reduction by internal wall-coverings

Can you reduce your walls’ leakiness by covering the inside of the wall
with insulation? The answer is yes, but there may be two complications.
First, the thickness of internal covering is bigger than you might expect.
To transform an existing nine-inch solid brick wall (U-value 2.2 W/m2/K)
into a decent 0.30 W/m2/K wall, roughly 6 cm of insulated lining board is
required. [65h3cb] Second, condensation may form on the hidden surface
of such internal insulation layers, leading to damp problems.

If you’re not looking for such a big reduction in wall leakiness, you can
get by with a thinner internal covering. For example, you can buy 1.8-cm-
thick insulated wallboards with a U-value of 1.7 W/m2/K. With these over
the existing wall, the U-value would be reduced from 2.2 W/m2/K to:

Definitely a worthwhile reduction.

## Air-exchange

Once a building is really well insulated, the principal loss of heat will be
through ventilation (air changes) rather than through conduction. The heat
loss through ventilation can be reduced by transferring the heat from the
outgoing air to the incoming air. Remarkably, a great deal of this heat
can indeed be transferred without any additional energy being required.
The trick is to use a nose, as discovered by natural selection. A nose warms
incoming air by cooling down outgoing air. There’s a temperature gradient
along the nose; the walls of a nose are coldest near the nostrils. The longer
your nose, the better it works as a counter-current heat exchanger. In
nature’s noses, the direction of the air-flow usually alternates. Another
way to organize a nose is to have two air-passages, one for in-flow and