Document x5GQGnzpgkkaBQ3RbajwYm900

276 CHAPTER 12 1958 Guide " necessary to,make an allowance for floor heat loss from rooms located over the basement. The temperature in crawl spaces below floors will vary widely depending on the number and size of wall vents, the amount of warm piping present and type of piping insulation. It is necessary, therefore, to evaluate the conditions and to select an appropriate temperature by judgment. HEAT LOSSES FROM FLOOR SLABS Two types of concrete floors used in basementless houses are (a) the unheated floor, relying for warmth on heat delivered above floor level by the heating system, and (b) the heated floor containing heated pipes or ducts that constitute a radiant slab or portion thereof for complete or partial heating of the' house. For type (a) the floor heat loss, economically considered, is of minor importance since it comprises generally about 10 percent of the total heat loss of the house. From the comfort standpoint, however, it may be most important, since houses with cold floors are not successfully heated. In this connection, it should be remembered that a well insulated floor does not assure comfort if down-drafts from windows or exposed walls create pools of chilly air over considerable areas of the floor. For this reason a floor of type (a) should not be used in a severe climate except with a heat ing system that delivers enough heat near the floor to counteract the downdrafts of the exterior walls and the heat transmission through the floor. The results of some experiments10'11 with type (a) unheated floor slabs indicate that the heat loss from a concrete slab floor on grade is more nearly proportional to the perimeter than to the area of the floor, and that the heat loss can be estimated by means of the equation: H, = FP (<i - i,,) (3) where Hr *= heat lose of the floor, Btu per hour. P = perimeter or exposed edge of the floor, linear feet. F = heat loss coefficient, Btu per (hour) (linear foot of exposed edge) (degree dif ference in temperature between the inside air and the outside air). (F ranges... " between 0.81 for a floor with no edge insulation to 0.55 for a floor with edge in-;. sulation). (i = indoor air temperature, Fahrenheit. 1 to = outdoor air temperature, Fahrenheit. In most instances the values given in Table 5 for edge loss are of sufficient^. precision for use.12 The insulation shown extending under the floor hon-.\ zontally for 2 ft can also be located along the foundation wall with equal.. effectiveness if the insulation extends 24 in. below the floor level. v; Example S: Calculate the heat loss from the floor of a 12 ft x 15 ft room with two. exposures. The floor is an unheated concrete slab which is. insulated at the edge with 2 in. of insulation extending horizontally for a distance of 2 ft from the edge, and the house is located in an area with an outside design temperature of --15 F. i.% Solution: From Table 5 the heat loss per foot of exposed edge is 45 Btu per_hf-. The length of exposed edge is 12 ft + 15 ft = 27 ft, and the total edge loss 27 x 45,S- * 1215 Btu per hr. Floors of type (b) containing heating pipes or ducts, are now in commo . Heating Load 277 use. The heat loss downward into the ground and outward through the edges of the floor slab is called the reverse loss. The results15 of an investigation in which a warm-air perimeter duct was embedded in four types of concrete floor slab and foundation constructions has verified the indication that Equation 3 can be used to calculate the reverse loss when warm-air perimeter heating ducts are used. To make the results of this application more usable, values corresponding to those shown in Table 5 for unheated floors are given in Table 6 for concrete floors with a warm-air perimeter duct. The desirability of edge insulation is apparent. One inch of waterresistant material is the minimum* thickness of edge insulation that should be used, but a 2-in. thickness is recommended.12'I4> 16 The values of edge loss in Table 6 indicate that the reverse heat loss of heated slabs is likely to be about 20 percent of the total 'heat loss of many types of present day Table 5. Heat Loss op Concrete Floors at or Near Grade Level per Foot op Exposed Edge houses, and may exceed 20 per cent if only one inch of insulation is used at the edge of the floor. The concrete floor slab is usually placed on a gravel fill 4 in. thick or more both to insulate the floor from the earth and to retard the rise of ground water by capillarity. A waterproof membrane should be in- 6(1 ove.r.^e gravel fill. Obviously, it is important that such floors be vid j6Yeral ln.c^les above grade, and that effective subsoil drainage be pro- e to avoid slabs soaked by rain or melting snow, and consequent excessive heat loss. .H TRANSMISSION HEAT LOSS surfof formula for the loss of heat by transmission through any surface is given in Equation 4: where Ht = AU (t, - Q (4) heat loss transmitted through the wall, roof, ceiling, floor, or glass, Btu per hour. A area of wall, glass, roof, ceiling, floor, or other exposed surface, square feet.