Document B8VQr1XzzY0bQEmr6Ojp0Nk0w

278 CHAPTER 13 : 1953 Guide Table 9. Total Equivalent. Temperatube Differentials fob Calculating Heat Gain Through Sunlit and Shaded Roofs Sun Time Description op Roof Construction* A.M. P.M. 8 | 10 | 12 2 |4 |6 |8 10 12 Light Construction Roofs--Exposed to Sun l* Woodb or 1* Woodb + 1" or 2` Insulation 12 38 . 54 62 50 26 10 4 0 Medium Construction Roofs--Exposed to Sun 2" Concrete or 2" Concrete + 1* or 2" Insulation or 2" Woodb 6 30 48 58 50 32 14 6 2 2" Gypsum, or 2Gypsum + 1' Insulation 1" Woodb or 1 2" Woodb or 1+4' Rock Wool 2* Concrete or fin Furred Ceiling 2" Gypsum J 0 20 40 52 54 42 20 10 6 4' Concrete or 4" Concrete with 2" Insulation 0 20 38 50 52 40 22 12 6 Heavy Construction Roofs--Exposed to Sun 6' Concrete 6* Concrete + 2" Insulation. 4 6 24 38 46 44 32 18 12 6 6 20 34 42 44 34 20 14 Roofs Covered with Water--Exposed to Sun Light Construction Roof with 1" Water 0 4 16 22 18 14 10 2 Heavy Construction Roof with 1" Water -2 -2 -4 10 14 16 14 10 Any Roof with 6* Water -2 0 0 -6 10 10 8 4 0 6 0 Roofs with Roof Sprats-- Exposed to Sun Light Construction Heavy Construction 0 4 12 18 16 14 10 2 -2 -2 2 8 12 14 12 10 0 6 Light Construction Medium Construction Heavy Construction Roofs in Shade -4 0 6 12 14 12 8 2 0 -4 -2 2 8 12 12 10 6 2 -2 -2 0 4 8 10 10 8 4 * Includes I in. felt roofing with or without sing. May also be used for shingle roof. ^Nominal thickness of the wood. NOTES FOR TABLE 9 Total heat transmission from* solar] Explanation: radiation and temperature difference I between outside and room air. Btu [ _ per (hr) (sq ft) of roof area J _] Equivalent temperature I differential from above/ table j (Heat transmission (cmoeerffiBciteunt pfoerrsu(hmr) (sq ft) (F deg) 1. Source. Calculated by Mackey and Wright method (see reference list) and adjusted after studying ASHVE original test data. Estimated for July in 40 deg north latitude. (For sol-air temperatures used in calculations see Table 8.) For typical design day where'the maximum outdoor temperature is 95 F and minimum temperature at night is approximately 75 F (daily range of temperature, 20 F) mean 24 hr tem perature 84 F for a room temperature of 80 F. All roofs have been assumed a dark color which absorbs 90 percent of solar radiation, and reflects only 10 percent. 2. Application. These values may be used for all normal air conditioning estimates; usually without correction, in latitude 0 deg to 50 deg north or south when the load is calculated for the hottest weather. Note 5 explains how to adjust the temperature differential for other room and outdoor temperatures. 3. Peaked Roofs. If the roof is peaked and the heat gain is primarily due to solar radiation, use for the area of the roof, the area projected on a horizontal plane. 4. Attics. If the ceiling is insulated and if a fan is used in the attic for positive ventilation, the total tem perature differential for a roof exposed to the sun may be decreased 25 percent! Cooling Load 279 5. Corrections.'. For temperature difference when outdoor maximum design temperature minus room is dif ferent from 15 deg. If the outdoor design temperature minus room temperature is different from the base of 15 deg, correct as follows: When'thedmefence is greater (or less) than 15 deg add the excess to' (or subtract the deficiency from) the above differentials. For outdoor daily range of temperature other than 0 deg. If the daily range of temperature is less than 20 deg, add 1 deg for every 2 deg lower daily range; if the daily range is greater than 20 deg. Bubstract 1 deg for every 2 deg higher daily range. For example, the daily range in Miami, Florida is 12 deg or 8 deg less than 20 deg, therefore, thecorrection is + 4 deg at all hours of the day. Light Colors. Credit should not be taken for light colored roofs except where the permanence of the light color is established by experience, as in rural areas or where there is little smoke. When the exterior surface of roof exposed to the sun is a light color, such as white or aluminum (which absorb approximately 50 percent and reflect 50 percent of the solar radiation) add to the temperature differential for roof in shade 55 percent of the difference between the roof in sun and roof in shade. When the roof exposed to the sun is a medium color such as light grey, blue or green, or bright red, add 80 percent of this difference. For solar transmission in latitudes other than 40 deg north, and tn other months. The table values of tem perature differentials will be approximately oorrect for a roof in the following months: North Latitude Laritude (deg) Months 0 All Months 10 All Months 20 All Months except Nov, Dec. Jan 30 Mar. Apr. May. June. July, Aug.'Sept 40 April. May, June, July. Aug 50 May, June, July South Latitude Laritude (deg) Months 0 All Months 10 All Months 20 All Months except May, June, July 30 Sept, Oct, Nov, Dec, Jan, Feb, Mar 40 Oct, Nov, Dec, Jan, Feb 50 Nov, Dec, Jan For other months, the total temperature differential (fz) may be approximated by the use of the following formula: tx -- fg + (fv -- <*) where t9 = temperature differential for the same roof in shade for desired time of day; obtained from Table 9. lj =* maximum solar transmission through glass, Btu per (hr) (sq ft) for flat skylight in July, 40 deg north latitude (Note: this is maximum value irrespective of time). I* -- same as Iy except use the maximum value for flat skylight, for month, and latitude desired for txtr -- temperature differential for particular roof exposed to sun for the desired time of day from Table (ATote that this makes adjustment only for solar radiation and that there may be additional correction for out door temperature.) air temperature t, is the temperature of the outdoor air, which, in the absence of all radiation exchanges, would give the same rate of heat entry into the surface as would exist with the actual combination of incident solar radiation, radiant energy exchange with the sky and other outdoor sur roundings, and convective heat exchange with the outdoor air. The sol-air temperature data6'610 as developed by Mackey and Wright for an industrial atmosphere were used as a basis for preparing Table 8 showing summer design sol-air temperatures. Sol-air temperatures may also be estimated from experimental observation of surface temperatures of walls and roofs which appear in the literature.11 12 Both analytical and expenmental studies have been made on the problem10 of heat flow through walls and roofs. Those concerned with a further study of the details of cooling-load estimates in particular relation to periodic heat flow will find much of value and interest in the reports of experimental studies of these problems.10 -11 12 13 14 16 The reader may also refer to the Cooling Load chapter of The Guide 1952 for the theory of heat flow through ..walls and roofs.