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 Mobile ViewEXTERIORS of buildings ADHESIVES, EXTERIOR CONSTRUCTION AGE of a BUILDING - how to determine ALGAE, FUNGUS, LICHENS, MOSS ARCHITECTURE & BUILDING COMPONENT ID ATTIC CONDENSATION CAUSE & CURE BARK SIDE DOWN on DECKS, TRIM, STEPS BASEMENT WATERPROOFING BEST CONSTRUCTION PRACTICES GUIDE BRICK VENEER WALL Loose, Bulged BRICK WALL DRAINAGE WEEP HOLES BOOKSTORE - EXTERIORS BUILDING SAFETY HAZARDS GUIDE BUILDING SETTLEMENT CHIMNEY INSPECTION DIAGNOSIS REPAIR DECK & PORCH CONSTRUCTION DECK COLLAPSE Case Study DECK FINISHES COATINGS PRESERVATIVES DECK FLASHING LEAKS, ROT Case Study DEFINITIONS of ENGINEERED WOOD OSB LVL etc EARTHQUAKE DAMAGED FOUNDATIONS ENERGY SAVINGS in buildings ENGINEERED WOOD Flooring ENGINEERED WOOD Products EXTERIOR WALL SIDING TRIM & FINISHES FIBERGLASS INSULATION FIBERGLASS HAZARDS FIBERGLASS INSULATION MOLD FLASHING MEMBRANES PEEL & STICK FLASHING for METAL ROOFS FLASHING ROOF WALL DETAILS FLASHING ROOF-WALL SNAFU FLASHING SIDING DETAILS FLASHING WALL DETAILS FLASHING WOOD ROOF DETAILS FLOOD DAMAGE ASSESSMENT, SAFETY & CLEANUP FOOTING & FOUNDATION DRAINS FOUNDATION CRACKS & DAMAGE GUIDE FOUNDATION WATERPROOFING FRENCH DRAINS GALVANIC SCALE & METAL CORROSION GLUES ADHESIVES, EXTERIOR CONSTRUCTION GRADING & SITE WORK, EXTERIOR GUTTERS & DOWNSPOUTS HEAT TAPES & CABLES on Roofs for Ice Dams HOUSEWRAP AIR & VAPOR BARRIERS KIT HOMES, Aladdin, Sears, Wards, Others LEAD POISONING HAZARDS GUIDE LEAD TEST KIT for HOME USE LEED GREEN BUILDING CERTIFICATION LOG HOME GUIDE LIGHTNING PROTECTION MOISTURE CONTROL in BUILDINGS MOLD DETECTION & INSPECTION GUIDE MVOCs & MOLDY MUSTY ODORS ODORS & SMELLS DIAGNOSIS & CURE PAINT ANALYSIS, DIAGNOSTIC USES PAINT & STAIN GUIDE, EXTERIOR PORCHES & Sunrooms PORCH CONSTRUCTION & SCREENING RAILINGS, DECK & PORCH RAILINGS, STAIRWAY RETAINING WALL DESIGNS, TYPES, DAMAGE RETAINING WALL GUARD RAILINGS ROOF ARCHITECTURAL STYLES - PHOTO GUIDE ROOF CLEANING RECOMMENDATIONS ROOF COLOR RECOMMENDATIONS ROOF DORMER TYPES - PHOTO GUIDE ROOFING DIAGNOSIS INSPECTION & REPAIR ROOF VENTILATION SPECIFICATIONS SHEATHING, Gypsum board SHEATHING Homasote & Other Board SHEATHING, OSB SHEATHING, Plywood SHEATHING, FOIL FACED - VENTS SIDING, Sheathing Identification - Photo Guide SIDING, ALUMINUM SIDING, ASBESTOS FIBER CEMENT SIDING ASPHALT ROOF SHINGLES on WALLS SIDING ASPHALT SHINGLE or SHEET SIDING DAMAGE by SPLASHBACK SIDING EIFS & STUCCO SIDING, FIBER CEMENT SIDING HARDBOARD SIDING STEEL SIDING VINYL SIDING, WOOD PRODUCT CHOICES SIDING, WOOD INSTALLATION SIDING WOOD, FAILURES OVER FOAM BOARD SIDING WOOD, FLASHING DETAILS SIDING WOOD SHINGLE INSTALLATION SOLAR ENERGY SYSTEMS STAIN DIAGNOSIS on BUILDING EXTERIORS STAIN DIAGNOSIS on BUILDING INTERIORS STAINS & FINISHES, INTERIOR STAIRS, RAILINGS, LANDINGS, RAMPS STONE CLEANING METHODS STRESS SKIN INSULATED PANELS STUCCO WAll FAILURES DUE TO WEATHER STUCCO WALL METHODS & INSTALLATION STUCCO OVER FOAM INSULATION STUCCO PAINT FAILURES SURFACE GRADING, SITE DRAINAGE TERMITES, ROT THERMAL EXPANSION of MATERIALS THERMAL MASS in buildings TRIM, INTERIOR INSTALLATION TRUSS UPLIFT, ROOF TRUSSES, Floor & Roof VAPOR BARRIERS & CONDENSATION in BUILDINGS VENTILATION in buildings VINYL Siding or PLASTIC Window ODORS in buildings VINYL CHLORIDE HEALTH INFO WALL SIDING TRIM & FINISHES WALL FINISHES INTERIOR WATER BARRIERS, EXTERIOR BUILDING WATER ENTRY in buildings WIND ENERGY SYSTEMS WIND TURBINES & LIGHTNING WINDOWS & DOORS Best Practices, Windows & Doors CLIMATE, WINDOW CHOICES FOR CONDENSATION on WINDOWS & SKYLIGHTS DOORS, ENERGY EFFICIENCY DOORS, EXTERIOR, Selecting & Installing DOORS, EXTERIOR FRAMES DOORS, INTERIOR DOOR FLASHING DETAILS DOOR SOURCES GLASS vs HEAT MIRROR SOLAR GAIN/Loss HURRICANE, WIND, & STORM-Resistant WINDOWS LOW-E WINDOW GLAZING LOW-E VS QUAD-GLAZING LOW-E RETROFIT ADD-ON FILMS MATERIALS & CONSTRUCTION of WINDOWS POLYCARBONATE GLAZING SITE BUILT DOUBLE GLAZED WINDOWS SKYLIGHTS, GUIDE SKYLIGHT CONDENSATION PROBLEMS SKYLIGHT DESIGN ISSUES SKYLIGHT ENERGY EFFICIENCY SKYLIGHT INSTALLATION Procedures SKYLIGHT LEAK DIAGNOSIS & REPAIR SKYLIGHT VENTILATION DETAILS SLIDING GLIDING WINDOW DEFECTS SLOPED GLAZING DETAILS SOLAR SHADES & SUNSCREENS STORM WINDOW INTERIOR STORM WINDOW PLASTIC CHOICES STORM WINDOW WEEP HOLES SUNGAIN, FILMS, LOW-E GLASS SUNSPACE GLAZING for SUNTANNING VERTICAL GLAZING DETAILS VINYL WINDOWS VINYL / PVC WINDOW WARPING WINDOWS, ALUMINUM WINDOW CERTIFICATION WINDOW EFFICIENCY Features & Ratings CONDENSATION on WINDOWS & SKYLIGHTS Energy Certification for Windows NFRC Label for Windows Whole Window Energy Ratings Glass-Only Ratings on Window Energy Energy Star Label for Windows Window U-Factor Gas Filled Window Glazings Sash and Frame vs Window Energy & U-Value Warm-Edge Spacers& Window Energy Ratings Low-E Coatings for Windows Spectrally Selective Low-E Windows WINDOW GLAZING Based on Climate Solar-Heat-Gain Coefficient, Windows Visible Transmittance Ratings for Windows Window Air Leakage Fading due to UV Light Window Orientation WINDOW SHADING, SCREENING WINDOWS, FIBEGLASS WINDOW FLASHING & SEALING Guide WINDOW GLAZING Based on Climate Solar-Heat-Gain Coefficient, Windows Visible Transmittance Ratings for Windows Window Air Leakage Fading due to UV Light Window Orientation WINDOW HARDWARE PHOTOS WINDOW INSTALLATION, Flange-Type WINDOW INSTALLATION, Integral Brickmold WINDOW / DOOR ENERGY EFFICIENT, DOE Select & Install Energy Efficient Windows Selecting Energy Efficient Skylights Window Energy Efficiency Designs Window Glazing Types Window Operating Types Window Frame Materials Window Installation Improving Existing Windows Energy Performance Ratings Window Daylighting Requirements WINDOW / DOOR AIR LEAK SEALING HOW TO WINDOW LEAKS INTO BASEMENT WINDOW MATERIALS & CONSTRUCTION WINDOW SHADING, SCREENING WINDOW SOURCES WINDOW TYPES, Guide WINDOW TYPES - Photo Guide Window & Door Sources WINTERIZE A BUILDING WOOD Burning Heaters Fireplaces Stoves More Information |
Window efficiency ratings: this article explains window efficiency and efficiency ratings, as well as specific features in window selection and installation that affect the window's energy efficiency. InspectAPedia tolerates no conflicts of interest. We have no relationship with advertisers nor with topics or services discussed at this website.© Copyright 2012 InspectAPedia.com, Daniel Friedman, Steve Bliss, Wiley & Sons, All Rights Reserved. Information Accuracy & Bias Pledge is at below-left. Use page top links to major topics or use links at the left of each page to navigate within topics and documents at this website. Green links show where you are in a document series or at this website. Guide to Window Efficiency Features & Ratings We review the proper installation details for windows and doors, and we compare the durability of different window and door materials and types. This article includes excerpts or adaptations from Best Practices Guide to Residential Construction, by Steven Bliss, courtesy of Wiley & Sons. Also see WINDOW / DOOR ENERGY EFFICIENT, DOE, see Skylight Energy Efficiency, and to improve existing windows, see WINDOW / DOOR AIR LEAK SEALING HOW TO. See WINDOWS & DOORS our home page for window and door information, and also see WINDOW TYPES - Photo Guide for a photographic guide to window and door types and architectural styles. Our links at page left provide in-depth articles on window and door selection, inspection, installation, problem diagnosis, and repair. Windows have a large impact on a home’s energy consumption, accounting for up to 25% of a typical home’s heating bills in cold climates and up to 50% of cooling bills in hot climates, according to the Environmental Protection Agency’s (EPA) Energy Star program. Even the best windows, with an R-value of 3 to 4, are thermal holes compared to today’s average R-19 wall. In addition to conductive heat losses, older windows add substantially to a home’s air leakage. Beyond fuel bills, windows can also have a dramatic effect on occupant comfort. Sitting next to a leaky single glazed window in winter will make an occupant feel cold regardless of the thermostat setting, due to both cold drafts and to radiant heat losses from his or her body to the window surface. Cold window surfaces also cause condensation, potentially leading to mold, peeling paint, and wood decay of window components. (Also see CONDENSATION or SWEATING PIPES, TANKS and DEW POINT TABLE - CONDENSATION POINT GUIDE as well as HUMIDITY LEVEL TARGET.) Energy Certification for WindowsEvaluating a window’s energy performance is a complex task that has been made a lot simpler by two programs developed in a collaborative effort between government and industry. The groups have developed standardized testing procedures and ratings, and provide simple recommendations based on climate zone. The NFRC Label for WindowsThe National Fenestration Rating Council (NFRC), with support from the U.S. Department of Energy, created test procedures and rating systems for the energy performance of windows, glazed doors, and skylights. Any window making energy claims without an NFRC label should be avoided. For every window, the NFRC label rates the U-factor, Solar-Heat-Gain Coefficient (SHGC), and Visible Transmittance (VT). Air Leakage (AL) and Condensation Resistance (CR) are optional ratings. The ratings are explained briefly in Figure 3-7 and in more detail in the following sections. Ratings should appear on the window label when delivered but can also be found on the NFRC website at www.nfrc.org.
Whole Window Energy RatingsIt is important to note that NFRC ratings apply to the entire window, including the sash and the frame. NFRC uses a single standard size to simplify testing and to make it easier for consumers to make apples-to-apples comparisons between windows. The actual energy performance of windows significantly larger or smaller than the standard test size will vary somewhat from the label since the relative effect of the glass edge and frame is greater on smaller windows. Glass-Only Ratings on Window EnergyWhere glass-only ratings are needed, for example, for passive solar design, these can usually be obtained from the window manufacturer or the manufacturer of the insulated glass unit (IGU) installed in the window or door. Energy Star Label for WindowsLaunched by the U.S. EPA to promote the use of energy-efficient appliances and equipment, the Energy Star label was added to doors, windows, and skylights in 1998. An Energy Star Label certifies that the window or skylight meets the U.S. Department of Energy’s (DOE) energy guidelines for the climate zones listed on the label (see Table 3-3 below).
For windows without the label, you can still use the Energy Star guidelines in Table 3-3 (above) as a selection guide. Remember that these guidelines are based on NFRC whole-window ratings, not just the glass. For cold-climate buildings designed to use passive-solar heating, look for a whole-window SHGC of .55 or above. Window U-FactorA window’s ability to conduct heat (not including solar effects) is usually given as a U-factor or U-value. The lower the U-value, the more insulation value a window provides. Low U-values have the biggest impact in heating dominated climates but help reduce cooling loads as well. For climates with substantial heating or cooling loads, choose a total window U-value of .35 or less. The U-value is the inverse of the more familiar R-value. For example, standard double glazing has a center- of-glass U-value of about .5, which equals an R-value of 2 (1/0.5). Typical glazing U-values are shown in Table 3-4 (below). U-values for the entire window, however, must take into account the edge spacers, sash, and frame, as discussed below.
The NFRC (National Fenestration Council) in discussing solar heat gain at windows, describes the U-Factor (U) as follows:
Also see DEFINITION of Heating & Cooling Terms for discussion and definitions of R, U, and K Factors in building heat loss, heat loss resistance, and insulation values. Gas Filled Window GlazingsFilling low-E coated glass with the inert gas argon or krypton will reduce heat loss through the glass by 10 to 15%. Krypton outperforms argon somewhat, but it is usually not enough to justify the higher cost. Since argon fill is now available on most low-E windows for little or no cost, getting the boost in R-value is always a good idea. In addition to reducing heat loss, it increases the temperature of the inside surface of the window, improving comfort and reducing condensation. Studies indicate that about 10% of the gas will leak out of a well-built sealed glass unit in about 20 years. Also see SUNGAIN, FILMS, LOW-E GLASS. (Also see CONDENSATION or SWEATING PIPES, TANKS and DEW POINT TABLE - CONDENSATION POINT GUIDE as well as HUMIDITY LEVEL TARGET.) Sash and Frame Effects on Window Energy Ratings & U-ValueThe U-value of the entire window, as reported on NFRC labels, includes the effects of the glass edge, sash, and frame.With high-R glass, standard edge and frame materials often lower than the overall R-value compared to the center-of-glass measure. Aluminum sash and frames without thermal breaks are the worst, contributing to both thermal losses and condensation in cold climates. Thermally broken metal frames are better but should still be avoided in cold climates.Wood and hollow vinyl or fiberglass components all have moderately good thermal properties. Insulated vinyl and fiberglass frames offer the best thermal performance (Table 3-5).
Warm-Edge Spacers Effect on Window Energy RatingsBecause of their high thermal conductivity, standard aluminum edge spacers lower the insulating value of insulated glass units (IGUs) and often cause condensation along the bottom of the window. The loss of insulation value is more pronounced in very high R-value windows and small windows where the window edge accounts for a larger proportion of the window area. Starting in the mid-1980s, manufacturers have responded with a variety of innovative solutions that are now used in about half of all new IGUs. Solutions include using less conductive metals with an improved shape (PPG’s Intercept Spacer) or switching to a plastic or synthetic rubber spacer with little or no metal content (TruSeal’s Swiggle Seal or EdgeTech’s Super Spacer). Warm-edge windows can raise the glass temperature at the perimeter of the window by 6°F to 8°F, significantly reducing the condensation potential. The biggest risk in using a new edge technology is that the seal will fail prematurely, resulting in a fogged unit. To guard against this, it is best to stick with a technology that has proven itself in the marketplace and is backed by a good warranty and a reliable window manufacturer. (Also see CONDENSATION or SWEATING PIPES, TANKS and DEW POINT TABLE - CONDENSATION POINT GUIDE as well as HUMIDITY LEVEL TARGET.) What are Low Emissivity (Low-E) Coatings for Windows?
Hard-coat, or “pyrolitic,” low-E, which has a slightly lower R-value, is used in high-solar-gain glass and can also be used on storm windows and other removable glass panels exposed to the air. Low-E window glass coatings can also be applied to a clear polyester film, called Heat Mirror, which is suspended between two panes of sealed glass, yielding insulation values as high as R-5 with one layer of film or R-8 with two layers. Also see SUNGAIN, FILMS, LOW-E GLASS. Spectrally Selective Low-E WindowsThe newest generation of low-E glazing, often referred to as “spectrally selective,” provides an ideal combination of high R-values, low heat gain, and high-visible-light transmittance. Spectrally selective windows generally outperform all other window types in mixed and hot climates, but they reap the greatest benefit in homes with significant cooling loads. Because of their high insulation value, spectrally selective windows even perform well in cold climates, particularly in homes with significant air-conditioning loads or large amounts of west facing glass (see Window Orientation). One exception is a house designed to use passive solar gain in winter, which would perform better with high-solar-gain glass. See SUNGAIN, FILMS, LOW-E GLASS. Industry Associations for Windows & DoorsAmerican Architectural Manufacturers Association (AAMA) www.aamanet.org Efficient Windows Collaborative www.efficientwindows.org National Fenestration Rating Council (NFRC) www.nfrc.org Sustainable by Design www.susdesign.com Shareware calculators for sun angles, solar heat gain, and shading Window and Door Manufacturers Association (WDMA) www.wdma.com -- Adapted and paraphrased, edited, and supplemented, with permission from Best Practices Guide to Residential Construction. Questions & Answers regarding this articleQuestions & answers about window efficiency ratings - how to translate the numbers on building window stickers and labels into efficiency or energy performance. Ask a Question or Search InspectAPediaHTML Comment Box is loading comments...
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