The 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.
Frame and Sash Materials
Window 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 (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
|EEBA recommends the above components in a glazing system for this climate.