More recent construction and remodeling of exist ing construction is likely to use insulation in the stud spaces. As an example, good quality batt or loose fill insulation, properly installed, may have a thermal resistance of 3.7 ru per inch of insulation thickness. Each inch added would add a resistance of 3.7 ru to the resistance of the building section. If all 35% inches of available stud space were filled with such insulation, 13.4 ru would be added to the 4.05 of the original construction for a total of 17.45 ru. But with the insulation occupying the air space, the air space no longer furnishes extra resistance and, subtracting the 0.95 ru of the air space from the total, we get 16.50 ru, the actual resistance of the section with full insulation. It can readily be seen that the original resistance is less than one quarter that of the insulated resistance. It can further be seen that with substantial amounts of insulation the importance of the resistances due to the structural and finish members of the section becomes relatively unimportant. In walls, the total resistance will be the resistance of the insulation plus about three additional resistance units for the air films and structural and finish materials. Similar reasoning can be applied to floor and ceiling construction to arrive at similar conclusions and the appropriate value of resistance of the structural members of such sections. D. Windows Windows consist of one or more thin sheets of glass, a material of high thermal conductivity. The resistances to heat transfer are the inside and outwhere spaces side films, plus the resistance of air more than one layer of glass is used. If the glass had no resistance whatever to heat flow. the resist ance of the two air films (inside and out) would total 0.85 ru, and the U-value would be 1.18. Actually, the moderate resistance of glass lowers the value to 1.13. If two sheets of glass are used, there is in addition an air space, and the U-value is respace and duced to about 0.60 (0.65 for 4-inch air 0.56 for 1 inch or more). Similarly, if three sheets are used, two air spaces are involved, and the U-value is about 0.41 (a little more for 1/4-inch air space and a little less for 1/2-inch or greater air space). Consideration of these numbers leads to the conclusion that double glazing reduces the heat loss through glass to about half that of single glazing; and triple glazing (three layers of glass) reduces the heat loss to about one third that of single glass. be reduced to markedly less than the inside. The A G. All-weather Thermal Performance 1. Heat Loss For economical operation, the maximum heat loss per square foot of heated area should not exceed the value given in Table 1. The maximum recommended heat loss limitation varies with degree days. These maximums are based on changing one half and three quarters of the air per hour. The values are expressed in watts (or Btuh) per square foot of floor area of the space to be heated, measured to the outside of the exterior walls. Heat loss calculations should be made in accordance with procedures outlined in Part 2. Degree Square Days Over 8000 10.6 36 9.9 34 8.8 8.6 8.3 7.8 2 2 2 2 2 2 30 2. Insulation a. The recommended maximum heat loss limits will generally be met by using insulation values given in Table 2. b. For heated and partially heated basements, it is suggested that walls separating heated areas from unheated areas be insulated. Wall insulation of at least R/6 is suggested for this application. Glazing practices cited in par. d should also be followed. Insulation should not be used between the first floor and heated portions of the basement. Any area of the first floor over an unheated portion of the basement should be insulated. c. Slab edge heat loss in Btuh per exposed linear foot may be determined from data described in the FHA "Minimum Property Standards (FHA No. 300) For One and Two Living Units." These data in terms of watts linear foot are shown in Table 3. d. Weather-stripping should be used on all doors and windows. Multiple glazing should be used on doors and windows for improved comfort, economy, and as required to meet the heat loss limits shown in Table 1. Double glazing is suggested where winter design temperatures are 7 For lower, or where the degree days exceed 4500. Triple glazing should be consid ered in areas of extreme winter weather severity. 3. Ventilation a. Positive mechanical exhaust to the outside should be provided in bathrooms and kitchens (except for systems with positive humidity control). It is suggested that a humidistat in the living area (but not in bathrooms or kitchens) be installed paralleling the manual switch on the kitchen fan. b. Natural ventilation of structural spaces should meet or exceed the minimums illus trated in Figures 4a and 4b. R/7 R/7 R/7 The insulation R/ values shown refer to the installed resistance of the insulation when applied according to the manufacturer's instructions in the building section described. The total section resistance (Rr value) is the R/ value plus the resistance of the section construction materials. (See par. C, page 7.) |