1、15.1CHAPTER 15FENESTRATIONFenestration Components 15.1Determining Fenestration Energy Flow. 15.3U-FACTOR (THERMAL TRANSMITTANCE) . 15.3Determining Fenestration U-Factors. 15.3Surface and Cavity Heat Transfer Coefficients 15.5Representative U-Factors for Doors 15.11SOLAR HEAT GAIN AND VISIBLE TRANSMI
2、TTANCE 15.13Solar-Optical Properties of Glazing 15.13Solar Heat Gain Coefficient. 15.17Calculation of Solar Heat Gain . 15.28SHADING AND FENESTRATION ATTACHMENTS 15.29Shading 15.29Fenestration Attachments. 15.30VISUAL AND THERMAL CONTROLS 15.33AIR LEAKAGE . 15.49DAYLIGHTING 15.50Daylight Prediction
3、15.50Light Transmittance and Daylight Use 15.51SELECTING FENESTRATION 15.53Annual Energy Performance 15.53Condensation Resistance . 15.54Occupant Comfort and Acceptance . 15.55Durability . 15.57Supply and Exhaust Airflow Windows 15.58Codes and Standards 15.58Symbols 15.59ENESTRATION is an architectu
4、ral term that refers to the ar-Frangement, proportion, and design of window, skylight, and doorsystems in a building. Fenestration can serve as a physical and/orvisual connection to the outdoors, as well as a means to admit solarradiation for daylighting and heat gain to a space. Fenestration canbe
5、fixed or operable, and operable units can allow natural ventilationto a space and egress in low-rise buildings.Fenestration affects building energy use through four basic mech-anisms: thermal heat transfer, solar heat gain, air leakage, and day-lighting. Fenestration can be used to positively influe
6、nce a buildingsenergy performance by (1) using daylight to offset lighting require-ments, (2) using glazings and shading strategies to control solar heatgain to supplement heating through passive solar gain and mini-mize cooling requirements, (3) using glazing to minimize conduc-tive heat loss, (4)
7、specifying low-air-leakage fenestration products,and (5) integrating fenestration into natural ventilation strategies thatcan reduce energy use for cooling and outdoor air requirements.Todays designers and builders; minimum energy standards andcodes; green building standards, codes, and rating progr
8、ams; andenergy efficiency incentive programs are seeking more from fenes-tration systems and giving credit for high-performing products. Win-dow, skylight, and door manufacturers are responding with new andimproved products to meet those demands. With the advent of sim-ulation software, designing to
9、 improve thermal performance of fen-estration products has become much easier. Through participation inrating and certification programs that require the use of this soft-ware, fenestration manufacturers can take credit for these improve-ments through certified ratings.A designer should consider arc
10、hitectural and code requirements,thermal performance, daylight performance, air leakage, energy andenvironmental impacts, economic criteria, and human comfort whenselecting fenestration. Typically, a wide range of fenestration prod-ucts are available that meet the specifications for a project. Refin
11、ingthe specifications to improve energy performance and enhance a liv-ing or work space can result in lower energy costs, increased produc-tivity, and improved thermal and visual comfort.FENESTRATION COMPONENTSFenestration components include glazing material, either glass orplastic; framing, mullion
12、s, muntin bars, dividers, and opaque doorslabs; and indoor and outdoor shading devices such as louveredblinds, drapes, roller shades, lightshelves, metal grills, and awnings.In this chapter, fenestration and fenestration systems refer to thebasic assemblies and components of window, skylight, and do
13、or sys-tems that are part of the building envelope.Glazing UnitsMost fenestration currently manufactured contain a glazing sys-tem that is packaged in the form of a glazing unit. A glazing unitconsists of two or more glazings that are held apart by an edge-seal.Figure 1 shows the construction of a t
14、ypical double-glazing unit.The most common glazing material is glass, although plastic issometimes used, particularly in the form of intermediate films. Bothmay be clear, tinted, coated, laminated, patterned, or obscured. Clearglass transmits more than 75% of the incident solar radiation andmore tha
15、n 85% of the visible light. Tinted glass is available in manycolors, all of which differ in the amount of solar radiation and visiblelight they transmit and absorb. Some coated glazings are highlyreflective (e.g., mirrors), whereas others have very low reflectance.Some coatings result in visible lig
16、ht transmittance of more thantwice the solar transmittance (desirable for good daylighting, whileminimizing cooling loads). Coatings that reduce radiant heatexchange are called low-emissivity (low-e) coatings. Laminatedglass is made of two panes of glass adhered together. The interlayerThe preparati
17、on of this chapter is assigned to TC 4.5, Fenestration. Fig. 1 Construction Details of Typical Double-Glazing Unit15.2 2013 ASHRAE HandbookFundamentalsbetween the two panes of glass is typically plastic and may be clear,tinted, or coated. Patterned glass is a durable ceramic frit applied toa glass s
18、urface in a decorative pattern. Obscured glass is translucentand is typically used in privacy applications.Low-e coated glass is now used in the vast majority of fenes-tration products installed in the United States, because of its energyefficiency, daylighting, and comfort benefits. Low-e coatings
19、aretypically applied to one of the protected internal surfaces of the glaz-ing unit (surface #2 or #3 in Figure 1), but some manufacturers nowoffer double-glazed products with an additional low-e coating on theexposed room-side surface (surface #4 in Figure 1). Low-e coatingscan also be applied to t
20、hin plastic films for use as one of the middlelayers in glazing units with three or more layers. There are two typesof low-e coating: high-solar-gain coatings primarily reduce heat con-duction through the glazing system, and are intended for cold cli-mates. Low-solar-gain coatings, for hot climates,
21、 reduce solar heatgain by blocking admission of the infrared portion of the solar spec-trum. There are two ways of achieving low-solar-gain low-e perfor-mance: (1) with a special, multilayer solar-infrared-reflectingcoating, and (2) with a solar-infrared-absorbing outer glazing. Toprotect the inner
22、glazing and building interior from heat absorbed bythis outer glazing, a cold-climate-type low-e coating is also used toreduce conduction of heat from the outer pane to the inner one.In addition to low-e, fill gases such as argon and krypton are usedin lieu of air in the gap between the panes. These
23、 fill gases reduceconvective heat transfer across the glazing cavity.The main requirements of the edge seal are to exclude moisture,provide a desiccant for the sealed space, and to retain the glazingunits structural integrity. Further, the edge seal isolates the cavitybetween the glazings, thereby r
24、educing the number of surfaces to becleaned, and creating an enclosure suitable for nondurable coatingsand/or fill gases. The edge seal is composed of a spacer, sealant, anddesiccant.The edge seal contains a spacer that separates glazings and pro-vides a surface for primary and secondary sealant adh
25、esion. Severaltypes of spacers are used today. Each type provides different heattransfer properties, depending on spacer material and geometry.Heat transfer at the edge of the glazing unit is greater than at itscenter because of heat flow through the spacer system. To minimizethis heat flow, warm-ed
26、ge spacers have been developed that reduceedge heat transfer by using materials that have lower thermal con-ductivity than the typical aluminum (e.g., stainless steel, galvanizedsteel, tin-plated steel, polymers, foamed silicone) from which spac-ers have often been made.Fusing or bending the corners
27、 of the spacer minimizes moistureand hydrocarbon vapor transmission into the air space through thecorners.Several different sealant configurations are used in glazing unitconstruction. In dual-seal construction, a primary seal minimizesmoisture and hydrocarbon transmission. A secondary seal provides
28、structural integrity between the lites of the glazing unit, and ensureslong-term adhesion and greater resistance to solvents, oils, andshort-term water immersion. In typical dual-seal construction, theprimary seal is made of compressed polyisobutylene (PIB), and thesecondary seal is made of silicone
29、, polysulfide, or polyurethane.Single-seal construction depends on a single sealant to provideadhesion of the glazing to the spacer as well as minimizing moistureand hydrocarbon transmission. Single-seal construction is generallymore cost efficient than dual-seal systems. Dual-seal-equivalent(DSE) m
30、aterials take advantage of advanced cross-linking poly-mers that provide low moisture transmission and structural proper-ties equivalent to dual-seal systems.Desiccants are used to absorb moisture trapped in the glazingunit during assembly or that gradually diffuses through seals afterconstruction.
31、Typical desiccants include molecular sieve, silica gel,or a matrix of both materials.FramingThe three main categories of fenestration framing materials arewood, metal, and polymers. Wood has good structural integrity andinsulating value but low resistance to weather, moisture, warpage,and organic de
32、gradation (from mold and insects). Metal is durableand has excellent structural characteristics, but it has very poor ther-mal performance. The metal of choice in fenestration is almostexclusively aluminum, because of its ease of manufacture, low cost,and low mass, but aluminum alloy has a thermal c
33、onductivityroughly 1000 times that of wood or polymers. The poor thermalperformance of metal-frame fenestration can be improved with athermal break (a nonmetal component that separates the metalframe exposed to the outdoors from the surfaces exposed to theindoors). However, to be most effective, the
34、re must be a thermalbreak in all operable sashes as well as in the frame. Polymer framesare made of extruded vinyl or poltruded fiberglass (glass-reinforcedpolyester). Their thermal and structural performance is similar tothat of wood, although vinyl frames for large fenestration must bereinforced.
35、Polymer frames are generally hollow and thus can alsobe filled with insulation, thereby achieving a better thermal perfor-mance than wood.Manufacturers sometimes combine these materials as clad units(e.g., vinyl-clad aluminum, aluminum-clad wood, vinyl-clad wood)to increase durability, improve therm
36、al performance, or improveaesthetics. In addition, curtain wall systems for commercial build-ings may be structurally glazed, and the outdoor “framing” is sim-ply rubber gaskets or silicone.Generally, the framing system categorizes residential fenestra-tion, as shown by the examples of traditional b
37、asic types in Figure2. The glazing system can be mounted either directly in the frame (adirect-glazed or direct-set fenestration, which is not operable) or ina sash that moves in the frame (for an operating fenestration). Inoperable fenestration, a weather-sealing system between the frameand sash re
38、duces air and water leakage.ShadingShading devices are available in a wide range of products that dif-fer greatly in their appearance and energy performance. They includeindoor and outdoor blinds, integral (between the glazings) blinds,indoor and outdoor screens, awnings, shutters, draperies, and ro
39、llershades. Materials used include metal, wood, plastic, and fabric.The ability of shading devices to provide control of solar heatgains depends mainly on the location of the device. Shades on theoutdoor side of the glazing can effectively reduce solar heat gain,but need more frequent maintenance an
40、d are often difficult to adjust.Conversely, indoor devices are easier to maintain and operate, butmay not be as effective in providing any significant degree of solarheat gain control, depending on the glazing type, shade properties,and control (Barnaby et al. 2009; Lee and Selkowitz 1995; Moesekeet
41、 al. 2007; Shen and Tzempelikos 2012; Tzempelikos and Athien-itis 2007; Wright et al. 2009b). Neither indoor nor outdoor shadesproduce any significant improvement in a fenestration systemsthermal performance (Barnaby et al. 2009; Wright et al. 2009a).Shading devices are well suited to deal with dayl
42、ighting, privacy,glare and thermal comfort issues. Some products, such as properlyadjusted blind louvers, are quite versatile in this respect. Motorizedshading devices can be adjusted under changing outdoor conditionsto reduce glare, maximize daylight, reduce internal temperatures(Newsham 1994; Rein
43、hart 2004) or improve thermal comfort tobuilding occupants (Bessoudo et al. 2010).Shading of fenestration is not confined to the use of shadingdevices. Building elements such as window reveals, side fins, andoverhangs can also offer effective shading. Metal grills with fixedlouvers mounted horizonta
44、lly at the top of a fenestration can blocksolar gain while still letting some light through, thereby avoiding theFenestration 15.3negative impacts of solid structural overhangs that act as thermalbridges in the building envelope and reduce daylight. Light shelvescan provide shading and also redirect
45、 sunlight to the ceiling to pro-vide even illumination in deeper parts of the space. Outdoor vege-tative shading is particularly effective in reducing solar heat gainwhile enhancing the outdoor scene.DETERMINING FENESTRATION ENERGY FLOWEnergy flows through fenestration via (1) conductive and con-vec
46、tive heat transfer caused by the temperature difference betweenoutdoor and indoor air; (2) net long-wave (above 2500 nm) radiativeexchange between the fenestration and its surroundings andbetween glazing layers; (3) short-wave (below 2500 nm) solar radi-ation incident on the fenestration product, ei
47、ther directly from thesun or reflected from the ground or adjacent objects; and (4) airleakage through the fenestration. Simplified calculations are basedon the observation that temperatures of the sky, ground, and sur-rounding objects (and hence their radiant emission) correlate withthe outdoor air
48、 temperature. The radiative interchanges are thenapproximated by assuming that all the radiating surfaces (includingthe sky) are at the same temperature as the outdoor air. With thisassumption, the basic equation for the steady-state energy flow Qthrough a fenestration isQ = UApf(tout tin) + (SHGC)A
49、pf Et + C(AL)Apf Cp(tout tin)(1)whereQ = instantaneous energy flow, Btu/hU = overall coefficient of heat transfer (U-factor), Btu/hft2FApf= total projected area of fenestration (products rough opening in wall or roof less installation clearances), ft2tin= indoor air temperature, Ftout= outdoor air temperature, FSHGC = solar heat gain coefficient, dimensionlessEt= incident total irradiance, Btu/hft2C = constant, 60 min/hAL = air leakage at current conditions, cfm/ft2 = air density, lbm/ft3Cp= specific heat of air, Btu/lbmFHere, the first term on the right-hand side o
