1. Heat Loss - conductive

During the summer months indoor and outdoor temperatures are often exactly equal. In this situation no heat is lost from our buildings. (A.)
XXIn winter however things are different. As the weather gets colder we turn on our heating system in an effort to maintain our rooms at near summer temperatures – but unfortunately it is never possible to do this without losing heat continuously to the colder air outside. (B.)
XXThe seepage of heat through solid partitions (walls and windows) is known as "conductive heat loss" and increases as the temperature differences on either side of the partitions become more extreme. For instance on a mild autumn evening, the outdoor temperature may only be 7ºC. lower than our room temperature, whereas on a frosty night the difference will be more like 21ºC. Where the temperature difference increases three-fold (as in this example,) conductive heat loss also increases three-fold, regardless of how well insulated our home is.
XXThe convention in the UK is to calculate heat loss on the basis of an external temperature of -1ºC. (or -2ºC. in some parts of Scotland.)
XXThis means that in freakishly Arctic conditions, your heating system simply won't provide you with enough comfortable heat. But it seldom makes sense to design for a short-term situation that crops up only once in a blue moon. A better strategy is to have one or two auxiliary heaters (perhaps mobile gas or electric fan heaters) you can call on for the odd occasions when they are needed.