The economic aspect of using more insulation is difficult to establish precisely because the financial saving associated with reduced heat losses depends on the cost of heating for the lifetime of the building. This will certainly depend on the kind of fuel that will be used, although it seems reasonable to anticipate that the cost of all fuels will continue to increase. It is much more likely that district heating will only help to hold down the rate of increase in the cost of heat. In spite of the uncertainty about the future cost of heat, there is little doubt that it is in the national interest to use more insulation than is commonly used at present. Similarly, it is advisable to use up to 6 inches of insulation in the spaces between ceiling joints.
Windows have much lower resistance to heat transfer than do equal areas of insulated wall.
A single pane of glass has a resistance of only about 1 unit compared to more than 10 units for insulated walls. In fact, an ordinary double-glazed window in the south wall of a building in the southern part of Canada has about the same net heat loss over a full heating season as an equal area of insulated wall. This is due to the relatively large amounts of solar energy that enter through south windows in winter. Windows facing directions other than south receive less solar irradiation and, consequently, have higher net heat losses. It can be concluded, therefore, that it is advisable to use double-glazing and to keep the size of windows as small as will be consistent with their function of admitting daylight and permitting a view of the outdoors.