2.4 How much insulation does a building need?

A further related question is ‘how large a heating system will it need?’ Obviously the severity of the winters at the building location is a key factor. There are two measures for this: the winter design temperature and the number of degree days.

The winter design temperature is that of the coldest weather likely to occur on the worst winter days at a particular location. It is used to size the heating system. For example the design temperature for London is –2°C, while for Berlin it is –11°C.

The number of degree days is a measure of the average external temperature over the winter months and can be used to estimate the heating fuel bills.