one for out-flow, separate from the point of view of air, but tightly coupled
with each other so that heat can easily flow between the two passages. This
is how the noses work in buildings. It’s conventional to call these noses

An energy-efficient house

In 1984, an energy consultant, Alan Foster, built an energy-efficient house
near Cambridge; he kindly gave me his thorough measurements. The
house is a timber-framed bungalow based on a Scandinavian “Heatkeeper
Serrekunda” design (figure E.10), with a floor area of 140 m2, composed of
three bedrooms, a study, two bathrooms, a living room, a kitchen, and a
lobby. The wooden outside walls were supplied in kit form by a Scottish
company, and the main parts of the house took only a few days to build.

The walls are 30 cm thick and have a U-value of 0.28 W/m2/°C. From
the inside out, they consist of 13 mm of plasterboard, 27 mm airspace, a
vapour barrier, 8 mm of plywood, 90 mm of rockwool, 12 mm of bitumen-
impregnated fibreboard, 50 mm cavity, and 103 mm of brick. The ceiling
construction is similar with 100–200 mm of rockwool insulation. The ceil-
ing has a U-value of 0.27 W/m2/°C, and the floor, 0.22 W/m2/°C. The
windows are double-glazed (U-value 2 W/m2/°C), with the inner panes’
outer surfaces specially coated to reduce radiation. The windows are ar-
ranged to give substantial solar gain, contributing about 30% of the house’s

The house is well sealed, every door and window lined with neoprene
gaskets. The house is heated by warm air pumped through floor grilles;
in winter, pumps remove used air from several rooms, exhausting it to the
outside, and they take in air from the loft space. The incoming air and
outgoing air pass through a heat exchanger (figure E.11), which saves 60%
of the heat in the extracted air. The heat exchanger is a passive device,
using no energy: it’s like a big metal nose, warming the incoming air with
the outgoing air. On a cold winter’s day, the outside air temperature was
-8 °C, the temperature in the loft’s air intake was 0 °C, and the air coming
out of the heat exchanger was at +8 °C.

For the first decade, the heat was supplied entirely by electric heaters,
heating a 150-gallon heat store during the overnight economy period. More
recently a gas supply was brought to the house, and the space heating is
now obtained from a condensing boiler.

The heat loss through conduction and ventilation is 4.2 kWh/d/°C.
The heat loss parameter (the leakiness per square metre of floor area) is
1.25 W/m2/°C (cf. my house’s 2.7 W/°C/m2).

With the house occupied by two people, the average space-heating
consumption, with the thermostat set at 19 or 20 °C during the day, was
8100 kWh per year, or 22 kWh/d; the total energy consumption for all pur-
poses was about 15 000 kWh per year, or 40 kWh/d. Expressed as an aver-

Figure E.10. The Heatkeeper Serrekunda.
Figure E.11. The Heatkeeper’s heat-exchanger.