where the “Something” is measured in watts per °C. As day turns to night,
and seasons pass, the temperature difference ΔT changes; we can think of
a long period as being chopped into lots of small durations, during each
of which the temperature difference is roughly constant. From duration
to duration, the temperature difference changes, but the Somethings don’t
change. When predicting a space’s total energy loss due to conduction and
ventilation over a long period we thus need to multiply two things:

1. the sum of all the Somethings (adding area × U for all walls, roofs,
floors, doors, and windows, and 13 NV for the volume); and
2. the sum of all the Temperature difference × duration factors (for all
the durations).

The first factor is a property of the building measured in watts per °C.
I’ll call this the leakiness of the building. (This leakiness is sometimes
called the building’s heat-loss coefficient.) The second factor is a property
of the weather; it’s often expressed as a number of “degree-days,” since
temperature difference is measured in degrees, and days are a convenient
unit for thinking about durations. For example, if your house interior is at
18 °C, and the outside temperature is 8 °C for a week, then we say that that

Figure E.3. U-values required by British and Swedish building regulations.