It should be borne in mind that any calculations dependent on experimental values of air leakage around windows are subject to great uncertainty on account of the variability of the factors involved. A well-built house without weather stripping may when new show less heat loss by air leakage than has been assumed in calculating the fuel savings given in Table XII. The gain resulting from weather stripping such a house would be correspondingly less. It should also be realized that infiltration of air is not necessarily disadvantageous, since a certain amount of ventilation is necessary. In the ordinary dwelling house air leakage is relied upon to furnish part of the ventilation, and it is unwise to attempt to prevent such leakage altogether. It does not appear, however, that ordinary weather stripping will reduce the air leakage to an excessively low value. The calculations involving insulation are much more definite and certain than those involving air leakage. The application of insulation resuits in a certain absolute saving which is independent of heat loss through or around windows and doors. The per cent saving of fuel, however, is still dependent upon the heat loss through the uninsulated openings.

Table XII. Approximate Fuel Savings in Dwelling Houses

(Expressed in percentage of fuel which would have been required for similar house without insulation or weather stripping)

Saving Per Cent

No insulation, weather stripped..........................

15 to 20

Same, with double (storm) windows.......

25 to 30

-inch insulation, not weather stripped.....

20 to 30

-inch insulation, weather stripped........................

About 40

-inch insulation, with double windows.....

About 50

1-inch insulation, not weather stripped..........................

30 to 40

1-inch insulation, weather stripped......................

About 50

1-inch insulation, with double windows.....

About 60

(Expressed in percent age of fuel which would have been required for similar house without insulation, but with weather stripping)

Saving Per Cent

With double windows, no insulation..................................

10 to 15

-inch insulation only.......................

25 to 35

-inch insulation, with double windows.....

40 to 45

1-inch insulation only....................

35 to 45

1-inch insulation with double windows............................

50 to 55

In summer the effect of insulation is beneficial, but too much should not be expected in this respect. Increasing the total insulating value of a wall or roof will always tend to keep the building cooler during the hot part of the day, but many other factors in addition to the insulating value enter into the question in a rather complicated way. In general, thick masonry walls having large heat-storing capacities are better than relatively thin insulated walls. The insulation of roofs is probably much more effective than the insulation of walls, since the former have much greater exposure to the sun.

[Note. - Some of the most common raw materials from which insulation board is made are cornstalks, wheat straw, sugar cane, flax straw, wood-pulp tailings, wood waste, spent licorice root.]