Document 5nYZ8NdXVg0mNkBbeJVjKwaN

132 CHAPTER 9 Table 17------ Effective Resistance of Ventilated Attics'--(Summer Condition)11 PART A. NON-REFIECTIVE SURFACES No VenfSotron J Hatvraf Ventilation Power VentfefMxr* 1960 Guide Air temp., F Sofrafr* temp., F Ventilation rate, cfm/ sq ft 0 | 0.1* 0-5 1 1.5 1/U Ceiling resistance. nr* 10 20 10 20 10 20 10 20 10 20 80 120 140 160 1.9 1.9 1.9 1.9 1.9 1.9 2.8 3.4 2.8 3.5 2.8 3.6 6.3 9.3 9.6 16 n 6.5 10 9.8 17 . 12 6.7 11 10 18 13 20 21 22 90 120 140 160 1.9 1.9 1.9 1.9 1.9 1.9 2.5 2.8 2.6 3.1 2.7 3.4 4.6 6.7 6.1 10 5.2 7.9 7.6 12 5.8 9.0 8.5 14 6.9 13 8.6 15 10 17 100 120 140 160 1.9 1.9 1.9 1.9 1.9 . 1.9 2.2 2.4 2.6 2.3 2.7 3.2 3.3 4.4 4.2 6.1 5.0 7.6 4.0 6.0 5.8 8.7 7.2 11 4.1 6.9 6.5 10 8.3 13 PART B. REFLECTIVE SUREACES' 80 120 140 160 6.5 6.5 8.1 8.8 13 6.6 6.5 8.2 9.0 14 6.5 6.5 8.3 9.2 15 17 18 18 17 18 19 25 26 27 19 20 21 30 31 32 90 120 140 160 6.5 6.5 7.5 8.0 10 6.5 6.5 7.7 8.3 12 6.5 6.5 7.9 8.6 13 13 15 16 12 14 16 17 20 22 13 16 18 19 22 25 100 120 140 160 6.5 6.5 6.5 6.5 6.5 6.5 7.0 7.3 7.6 7.4 8.0 10 7.8 10 12 8.2 11 14 8.5 11 13 12 15 18 8.8 12 15 12 16 20 .............................-- __ .uBto la BMia wmunwn. a wm tor tat Teatuttiott is *1* i&duded- Tbe effective rcsasttoee of tbs attic may be Added to the rnntennft (1/(0 of tbe ceiling (Table 18, Fart D) to obtain, tbe effective tesmtaaoe gf tbe combiiwiHn-a baaed cm sol-air (Chapter 13) and room temperature. These valise apply to wood frame construction with a roof deck and roofias bavin* a conductance of ID Btu/(aq ft) (hr) (F dec). b When attic ventilation meeta the requirementa of Table 3 in Chapter 10,0.1 fm/sq ft may be wunmtd u the natural summer ventilation rate for design purpcoea. * 8nii(w Vail, abbreviated r*< none (far) (aqft) (Fdeg) perBto. Determine -riling;rceatanoafnunTables II and 18, and cometfee framing by Figure 7. Do aot add tbeeffect of a reflective surface facing the attic to tbs cefliax resistant* from Table 18, Part D.ee ft ts accounted for in Table 17, Art B, d Roof surface temperature rather than aol-air temperature (aee Chapter 0) may be used if 0.8 i# subtracted from the attic resistance shown. * Baaed an air discharging outward from attic. * SurfaCae with effective emisririty S of 0-05 between ceiling joists facing tbe attic apace. by the roof area. Values of Vr and {/ should be calculated using a value of 2.5 (the reciprocal of one-half the air space resistance, 0.80) rather than the conductances of surfaces facing tbe attic, since the attic is assumed to be equivalent to an air space. If the attic contains windows, dormers, and vertical wall spaces, and if their area is small compared to that of the roof, they may be considered part of the roof area. For accuracy, the gum of the coefficients of each individual section, multi plied by its percentage of the total area, should be used as Ur. Where attic wall areas are large or where louvers or vents are used, it is preferable to estimate tire attic temperature as illustrated in Chapter 12, and calculate tbe heat loss through the ceiling by multiplying the value of Um for the ceiling by the difference in temperature above and below the ceiling. BASEMENT FLOOR, BASEMENT WALL, AND CONCRETE SLAB FLOOR COEFFICIENTS The heat transfer through basement walls and floors to the ground is dependent on the temperature difference between the air within and that of the ground, on the material con stituting the wall or floor, and on the conductivity of the sur rounding earth. The conductivity of the earth will vary with local conditions, and is usually unknown. Tests0 at the ASHAE Research Laboratory indicate a heat flow of approxi mately 2.0 Btu per (hr) (sq ft) through an uninsulated con crete basement floor, with a temperature difference of 20 deg between ground temperature and the air temperature 6 in. above the floor (see Table 18). For basement walls below grade only, the temperature dif ference for winter design conditions will be greater than for the floor. The test results indicate a unit area heat loss, at mid height of the basement wall portion below grade approxi mately twice that of the same floor area. For concrete slab floors laid in contact with the ground at grade level, tests0 indicate that for small floor areas (equal to Table 18 .... Coefficients of Transmission (U) of Concrete Basement Floors on Ground with Various Types of Finish Flooring U " 0.10 Btu per (hr) (sq ft) (Fahrenheit degree temperature difference between the ground and the air over the floor). * Sicca authentic data an not available, this coefitdcat a sometimes used for ecocntafloorson pound- Far coon recentprocedures'1 refer to hrettoBa! Btauau / Standard* fit-port BUS-103. Heat Transmission Coefficients of Building Materials 133 that of a house 25 feet square) the heat loss may be calculated as proportional to the length of exposed edge rather than total area. This amounts to 0.81 Btu per (hr) (linear foot of ex posed edge) (Fahrenheit degree difference between the indoor air temperature and the average outdoor air temperature). It should be noted that tins may be appreciably reduced by in sulating under the ground slab, and also along the edges be tween the floor and the abutting walla. See also sections on Basement Temperatures and Heat Loss, and on Floor Heat I/wa in Basementless Houses, in Chapter 12. In most calcula tions if the perimeter loss is calculated accurately, no other floor loss need be considered. GLASS AND DOOR COEFFICIENTS The U values for glass sheets and hollow glass block, given in Sections A, B, and C of Table 19, have been computed by methods ind data given in an ASHVE Research Paper.* It is assumed that the surface conductance for convection loss to the air is 4.0 Btu per (hr) (sq ft) (F deg). It is also as sumed that the glass loses heat by radiation to the ground and to the clear sky, which together have an effective radiat ing temperature below the air temperature. It is therefore necessary to determine, by trial and error, the temperature of the outdoor glass surface such that the sum of the radiation and convection losses equals the heat conducted through the glass section, and equals the heat delivered to the glass from the heated space. This heat flow, divided by the air-to-air temperature difference, results in a U value that is used in the usual manner. The equivalent surface conductance for radiation and convection combined, based on oir-to-surface temperature difference, therefore varies from about 5.5 for single glass to about 6.6 for double glass for exactly the same environmental design conditions. Curtains, draperies, Vene tian blinds, etc., will result in lower glass surface temperatures than when the windows are not covered. It is asumed that the room air temperature equals the aver age temperature of the room surfaces seen by the glass. Special consideration should be given to those cases where the glass tees interior surfaces at temperatures differing greatly from the room air temperature, i.e., such cases as in sun rooms, greenhouses, and some panel heated rooms, or where there is an unusual amount of air motion in the vicinity of the glass. Although based on zero outdoor air, the values change only slightly with different design temperatures, being about 5 percent greater for a SO F outdoor design temperature. In computing the Table 19 values, consideration of the de pendence of the indoor surface conductances upon tempera ture and direction of heat flow leads to surface conductances averaging about 1.50 for block and vertical glass, and about 1.80 for horizontal glass, as compared to the value of 1.46 used in computing U values given in other tables in this chapter. These values should therefore be used in estimating the temperature at which condensation on glass surfaces will occur. The application factore given in Section D of Table 19 are based upon hot box tests summarized in a research bulletin,14 and are approximate only. In practice, some variation in heat flow through windows having the same ratio of glass to sash area, may be expected because of difference in construction . details and in air-space edge effects. The high conductance of aluminum and steel sash must be taken into consideration where excessive amounts of metal sash and frames arc in volved. This is particularly important when they are in close proximity to radiation heat sources and are consequently subjected to high differential temperatures. Table 19 .... Coefficients of Transmission (U) of Windows, Skylights and Glass Block Walls Coeftkteafr era tiepronod in Btv par (hour) {square fool) (fohrtnfceff degree difftronco in tteiparotvre between fte air on (be two tided. Those for oat* door exposure* or* bated upon (be flowing outdoor conditions 0 F air tumpercturu, dear diet, no tolar radioftoe, and 15 aph outdoor wind velocity SECTION A--VERTICAl GLASS SHEETS Number of Shooti................... One Two Three Air space, inches............. None 1* H H 1" Outdoor exposure........ 1.13 0.610.55 0.53 0.41 0.36 0.34 Indoor exposure........... 0.75 0.50 0.46 0.45 0.38 0.33 U.32 SECTION B--HORIZONTAL GLASS SHEETS Heot Bow Up Hoot flow Down Number of Sheets......... One Two One Two Air space, inches. .. None X M 1* None X M 1* Outdoor exposure. 1.40 D.Vli 0.66 0.63 -- Indoor exposure.. 0.96 0.59 0.56 U.b6 0.60 0.43 0.39 0.38 SECTION C--WALLS OF HOLLOW CLASS BLOCK Description Outdoor Indoor Exposure Fortitioo 5K x 5K x 3X in. thick..................................... 7% x 7% x 3% in. thick.................................. UK x UKx3X in. thick.............................. x 7K x 3% in. thick with giass fiber di viding the cavity........................................... 11% x 11% x 3% in. thick with glass fiber dividing the cavity....................................... 0.60 0.56 0.53d 0.48 0.44 0.46 0.44 0.40 0.38 0.36 SECTION D--APPROXIMATE APPLICATION FACTORS FOR WINDOWS--MULTIPLY FLAT GLASS U VALUES BY THESE FACTORS Window Description Single Glass Double Gloss* Windows with Storm Scsb1 Percent Glass* Factor Percent Glass* Factor Percent Glass* Factor 100 i no wo 1.00 Wood sash.................. 80 0.90 80 0.95 80 0.90 60 0.80 60 0.85 60 0.80 80 1.00 80 1.20 80 1.00* Aluminum.................. 80 1.10 80 1.30 80 1.10 * For 1 in. ar greater. b Sm Chapter 0, 'Tables M and 15, tor summer lead. * See Chapter 0, Tables 14, IS, and 18, tor summer load. d From unpublished data noommesded bp ASHAE Tech. Adv. Comm, on Heat Flow Through Fcoea{ration. * Unit type double gtasing (two lights or paces ia same opening). r Use with U rahme lor two sheets with I ia. air apace. * Based go area of exposed portion of sash; docs not include frame or portions of sash concealed by frame. k For metal atom sash gr metal sash with attached storm pane.