From the point of view of insulation only, the most important question is the thickness of insulating material to be applied, rather than what material to select, provided the choice is restricted to the class of cellular or fibrous materials. No known material in a very thin layer can be expected to provide an appreciable amount of insulation. On the other hand, a relatively thick layer may not be economical, since relatively little additional gain is made over some layer of intermediate thickness. The selection of a material for a particular purpose must be governed largely by the requirements of that purpose in a way of structural strength, cost, fire hazard, etc. The real cost of an insulating material is obviously not the cost per square foot of commerical thickness but rather the cost per unit insulating value of the commerical thickness.
If a layer of insulating material is added to a wall, the insulating value of the wall will be increased by an amount equal to the insulating value of the layer of material added. The thicker the layer the greater will be the insulating value of the resulting wall. The percentage increase in the insulating value of the wall, however, will depend upon the original insulating value of the wall without insulation. The percentage increase in the insulating value of an actual wall containing windows will also obviously depend upon the amount of glass surface and the air leakage around windows and doors, since these factors are unaffected by the addition of insulating material.
A great many types of walls and roofs are to be found in present-day dwelling-house construction. The insulating value of one type or individual may be considerably different from that of another, but in an actual building heat losses through and around windows and doors tend to level out the effect of these differences in the properties of the walls themselves to such an extent that there are no wide variations in the amounts of fuel required to heat houses of various types of the same size in the same locality, unless air leakage around windows and doors or through very poorly constructed walls is excessive.
An estimate of the probable savings in fuel resulting from insulating or weather stripping an ordinary dwelling house is given in Table XII. The first part of the table gives the fuel saving expressed in per cent of fuel which would have been required for a similar house without insulation or weather stripping. In the second part of the table the savings are expressed in per cent of fuel required for a house without insulation but with weather stripped windows. The calculations were based on data on heat transfer in building construction taken from the "Guide," published by the American Society of Heating and Ventilating Engineers. An average insulating material (K = 0.31) is assumed, but no commercial fibrous or cellular insulating material departs far enough from this average value to make a significant difference in the approximate figures in Table XII. In taking into account the effect of windows and doors, it is assumed that the aggregate area of such openings is equal to one-fifth of the total side-wall surface, and that the heat loss through such openings is that corresponding to a 5-mile wind striking perpendicular to the wall. This corresponds roughly to average conditions over a large part of the country. Whenever insulation is involved, it is assumed that the insulation is applied to both walls and roof, and that the insulation is not substituted for some other member which is present in the uninsulated construction.
The ranges in values correspond to the extremes in wall constructions usually encountered in average dwelling houses. As a general rule, ordinary walls of solid masonry are somewhat less effective in retarding heat loss than well-constructed frame or hollow tile walls. A somewhat greater percentage saving in fuel is therefore obtained by insulating a solid masonry wall than by applying the same insulation to a frame or hollow tile construction. Any house representing a considerable initial investment, particularly one with solid masonry walls, should be insulated, since the cost of insulation is a small proportion of the total, and the resulting additional comfort and fuel saving is considerable.