where the “Something” is measured in watts per °C. As day turns to night,

and seasons pass, the temperature difference Δ*T* 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:

- the sum of all the Somethings (adding area ×
*U*for all walls, roofs,

floors, doors, and windows, and^{1}⁄_{3}*NV*for the volume); and - 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