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| Sustainable Building Sourcebook | |||
| Chapter: Materials | |||
| Doors and Windows | |||
| CSI Numbers: 08000 Doors & Windows | |||
| Introduction: | |||
Windows and doors have significant roles in the energy profile of a home and are currently highly engineered to optimize energy performance. However, when designing a home, thoughtful window placement with regard to solar orientation and shading may have more impact on energy efficiency than the type of window selected. Also see the Green Building Factsheets for introductions to this and other green building topics. | |||
| At-A-Glance Notes: | |||
| Technology: | |||
| Major advances in window technology have been made in the last 15 years. Too often, contractors and some salespeople have not been able to keep up with the latest advances. For this reason it is important to specify windows with the most energy efficient properties and to ensure that the windows ordered have the necessary National Fenestration Research Council (NFRC) ratings. | |||
| Suppliers: | |||
| Well developed market with many suppliers. | |||
| Cost: | |||
| Wide variation between low cost single pane aluminum windows to wood or wood clad or vinyl with low-e double pane glazing. | |||
| Public Acceptance: | |||
| Awareness of newer window technology is low. Most are familiar with benefits of double pane windows. | |||
| Regulatory: | |||
| Windows must be tested and certified and have air infiltration qualities of less than 0.30 CFM per lineal foot of crack. New window suppliers may offer certified test data to satisfy these requirements. | |||
| Considerations: | |||
DoorsThe main energy consideration when purchasing most doors will be the insulation value of the door. Many door manufacturers now provide labels stating the insulation value of doors. Door and window insulation values are stated as U-factors. The U-factor is the total heat transfer coefficient of the system. This includes conductive, convective, and radiative heat transfer. A lower U-factor means less heat transfer. Recommended U-factor for a steel, foam core exterior door is .35. For a solid wood exterior door, the recommended U-factor is .40. WindowsFrame and Sash MaterialsWindow frames and sashes can be made of aluminum, wood, combination of wood and vinyl or aluminum, or solid vinyl. Standard aluminum components are the least expensive to purchase but will transmit the most heat. This heat transmission will raise utility costs and lower comfort levels of the occupants of the building. Aluminum frames and sash components with a thermal break will reduce the heat transmission and be more cost effective over the long run. Solid wood frames and sashes cost considerably more up front than aluminum components but provide much better thermal performance. Solid wood components require regular painting to prevent moisture damage and are prone to insect infestation. Vinyl clad wood frame and sash members provide the traditional look of wood on the interior of the window with the rot and insect resistance of vinyl on the outside surfaces. Vinyl clad wood products provide about the same thermal performance as solid wood components and will last longer due to the protection of the vinyl on the surfaces exposed to the weather. Pre-finished aluminum clad wood windows are also available in Austin. These windows offer the same thermal properties as wood windows without the maintenance issues of wood windows. The aluminum used in these windows has a high-recycled content, making them more energy efficient to manufacture. Another frame and sash option is composite recycled wood fiber and vinyl composition frame and sash material. This family of materials offers strength and insulation value comparable to wood components without the maintenance requirements of wood. Single, Double or Triple Paned Glazing:Windows can be purchased with single, double, or triple panes. Single pane windows have one piece of glass or plastic and provide almost no protection from heat transfer. The temperature on the outside of the glass will be almost the same as on the inside due to heat conduction through the glazing or glass. Double and triple pane windows have two or more layers of glazing separated by layers of air or inert gasses such as argon or krypton. These layers of gas greatly increase the insulation value of double and triple pane windows. Since the introduction of low-emittance coatings for windows, triple pane windows, which never enjoyed widespread use, are even less common. Low-emittance coatings:Low-emittance (low-E), or spectrally selective coatings are coatings that may be applied directly to glass surfaces or to thin sheets of plastic film that are suspended in the air cavity between the interior and exterior glazing layers. These coatings are highly reflective to the long wavelength infrared radiation that we feel as solar heat. This reduces heat transfer between layers of glazing by a factor of 5-10. In heating climates, low-e coatings are used to allow infrared radiation to pass through the window from the outside to the inside but not to pass from the inside out. This allows buildings to take advantage of passive solar heating by retaining the infrared radiation inside the building. In cooling climates, such as Austin, low-e or southern low-E coatings prevent the infrared radiation from entering the building by reflecting it outwards from the exterior layer of glazing. Doors and Windows, Fig. 1
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| Guidelines: | |||
DoorsExterior doors should be either solid wood flush doors, wood panel doors with panels at least 11/8" thick, or insulated steel doors. If doors have lights or glazed openings the glazing should be double pane, insulated glass. Exterior doors should have a manufacturers label indicating that the door has a U-factor of no more than .35 for a steel door or .40 for a wood door. WindowsThe first and most important guideline for energy efficiency in windows is that east exposures should be minimized and west exposures should be eliminated or shaded by porch roofs or other structures. This will avoid heat build-up in the morning and afternoon when the sun is low enough to shine directly into the windows. Another passive solar strategy for minimizing heat gain is the proper sizing of eaves or overhangs. Aluminum frame windowsAluminum frame windows are the most common windows used in new construction. Aluminum windows without a thermal break are good conductors of heat. This means that the temperature on the inside of the window frame will be very close to the temperature on the outside. In winter, the window frame and sash will be much colder than the ambient air temperature in the building. This temperature differential will cause moisture in the form of condensation to develop on the inside of the window components. This moisture may cause mold or mildew to form on the wood or drywall around the window. It may also cause the wood or drywall to deteriorate due to prolonged exposure to moisture. Therefore, when using aluminum windows, choose those with a thermal break. Wood or wood/vinyl frame windowsWood or wood/vinyl frames and sashes provide a much higher insulation value than aluminum. When used in conjunction with double pane glazing wood or wood/vinyl will greatly reduce or eliminate condensation on the inside of windows. Wood fiber/vinyl composition frames and sashes also provide good insulation values and will help prevent condensation on windows. Glazing optionsThough the payback time for double pane windows is longer for the Austin climate than for colder climates, they will provide a more comfortable environment and lower heating and cooling costs than single pane windows. Single pane windows allow significant heat transfer through the glazing of the window. In summer, this creates a pocket of hot air near the inside of the window. This hot air rises to the ceiling, cools slightly, and falls back to a lower level where it is heated and rises again. This process is reversed in winter causing cold spots. This cooling and reheating of interior air is know as a convection loop, and will cause the areas around single pane windows to be hotter or colder than the rest of the building. To combat this the thermostat is adjusted to make these areas more comfortable. This causes higher than necessary heating and cooling costs. For a highly energy efficient thermal envelope, southern low-E coatings on double pane windows are the best choice. These windows will provide good insulation value combined with reflectance of the infrared radiation that causes heat build up in. The low-E coating will also prevent premature fading of carpets and upholstery caused by the infrared radiation. Doors and Windows, Fig. 2
NFRC Window Rating SystemThe National Fenestration Research Council has developed a rating system used by many window manufacturers. The rating of each type of window may be read from an NFRC label attached to the window. The rating system covers four areas. U-factor measures the heat transfer properties or insulation value of the window. For the Austin climate a U-factor of less than 0.75 is adequate, less than 0.60 is better. SHGC (Solar Heat Gain Coefficient) measures the solar radiation that enters a building through the windows. An SHGC of less than 0.40 is recommended for Austin. VT (Visible Transmittance) measures the amount of light in the visible portion of the spectrum that passes through a glazing material. VT does not measure heating or cooling loads, but does have an impact on daylighting and occupant satisfaction with windows. A VT of not less than 0.50 is recommended. AL (Air Leakage) represents the amount of air infiltration through the cracks in a window unit. AL should be less than 0.30 cfm/sq ft. All measurements are for the entire window unit, glazing, sash and frame. | |||
| Resources: | |||
| Components/Materials/Systems: | |||
See "Windows," "Doors," "Building Materials - Retail" in Yellow Pages Andersen Corporation Austin Demolition Construction Cleanup & Demolition Discovery's "From the Past" Energy Efficient Building Products Habitat for Humanity REStore Masonite Corporation Owens Corning Fiberglass Co. Peachtree Doors, Inc. Ramirez Properties ReflecTech |
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| General Assistance: | |||
American Architectural Manufacturers Association Energy StarÆ NFRC Efficient Windows Collaborative National Fenestration Rating Council Sealed Insulating Glass Manufacturers Association (SIGMA) Window and Door Manufacturers Association PublicationsResidential Windows: A Guide to New Technologies and Energy Performance |
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