Why did Mr. Green have a terrible fuel bill last winter?

Mr. Green: While you’ re here. I wish yon’ d have a look around the house. I had a terrible fuel bill last winter. My mother-in-law was staying with us and she wanted the temperature around 80℉ all the time.
Engineer: Yes, lots of people overheat their houses. Actually 70℉ is comfortable, if you let yourself get used to it.
Mr. Green: What’ s the most popular heating system used nowadays?
Engineer: The warm air system, the kind you have, has become more common because of economy and convenience. A heat chamber heats air, which passes through ducts and openings into the rooms by blowers or by convection. These openings are called outlets. Fitters are installed in the ducts to collect dust. They can also control humidity to some extent. The fuel is usually coal, oil, or gas.
Mr. Green: What kind of cooling systems are used in homes?
Engineer: Either central or localized air conditioning is used. With the central system, air duets bring cool air into the rooms. Sometimes, if the house has warm air heating , the same duets may be used for air conditioning. This system provides an even temperature throughout the house. However, in homes which are not constructed for the central system, small individual air conditioners are installed at several windows. This is probably the easiest way. But the temperature of the rooms is quite uneven.
Mr. Green: How important is the insulation of the house?
Engineer: Very important, and sometimes this is not appreciated. The roof, for example, should have adequate insulation with glass fiber,  aluminum foil, or something similar. It is important to have vapor seals on the warm side, to avoid moisture when the air cools. Double glazing of windows is also very important.