The energy efficiency of the window area in the bathroom is extremely important for a number of reasons. More efficient windows will improve energy efficiency and increase comfort. Efficient windows also have less moisture accumulation on the window (those frosty windows we sometimes see on cold winter days) because warm, moist air is naturally attracted to cold surfaces like those of poorly insulated windows.
Windows in older homes can be extremely inefficient. First of all, they may have loose-fitting frames and poorly working components that allow air infiltration. Another reason for poor efficiency is the type of frame and glazing present. Uninsulated metal frames and single glazed (one pane of glass) windows will allow warmth to be conducted through the materials very easily. If the client wants to keep the current windows in place, the windows can be tightened up with caulking and weatherstripping. Adding full storm windows is another way to improve efficiency, but your client may not want to deal with storm windows every season. Windows that are energy efficient will have at least two layers of glazing (glass); a low-e coating (low-emittance) (see Figure 2.6); a good-quality, insulating frame; tight-fitting parts; and sometimes argon or another gas between the panes of glazing. One way to identify the most efficient windows on the market is to look for the Energy Star label. (Refer to chapter 3, "Environmental and Sustainability Considerations," for more information about Energy Star.)
The National Fenestration Rating Council (NFRC) is a nationally recognized organization that has developed a rating system for windows and skylights. This rating system allows the purchaser to compare different attributes of windows related to energy performance, solar gain, visibility, air leakage, and condensation. Figure 2.7 shows an NFRC sample label.
The NFRC label contains ratings on the following factors:
• U-factorsfor windows and skylights. U-factor is a measurement of heat conductivity or thermal transfer. This can be heat loss or heat gain. The lower the U-factor, the more energy efficient the window. The U-factor is especially important in heating-dominated climates, but it is also beneficial in cooling-dominated climates.
• solar heat gain coefficient (sHGC). SHGC is a measure of how much solar radiation passes through the glass. This is expressed as a number between 0 and 1, and the lower the number the less solar heat it transmits. The desired rating depends on the location of the window and the climate. For example, a high SHGC is desirable for passive solar heat gain in a cold climate while a low SHGC is preferred in a hot, sunny climate. West-facing windows can be an issue in most any climate, so a low SHGC is important for these windows, blocking the intense western sun.
• Visible transmittance (VT). VT indicates the amount of visible light transmitted through the glass. The NFRC’s VT is a whole window rating and includes the impact of the frame. The VT rating varies between 0 and 1 and the higher the VT the more light transmitted, thus maximizing the amount of daylight passing through the window. Most double – and triple-pane windows are between 0.30 and 0.70.
• Air leakage (AL). The air leakage is the amount of heat loss or gain that occurs by infiltration through cracks and openings in the window assembly. The AL is measured in cubic feet of air through a square foot of window area (L/s/m2 or m3/h/m). The lower the AL the less air will pass through the window assembly, and this rating is optional on the NFRC label. This rating does not measure air leakage between the window assembly and the wall, after the window has been installed.
• Condensation resistance (CR). The CR measures how well a window resists the formation of condensation on the inside surface of the window and is expressed as a number between 1 and 100. The higher the number the better the resistance to condensation, and this is also an optional rating on the NFRC label.
Window energy efficiency requirements may vary by climate zone, but also by jurisdiction if a
variation of the IRC code is adopted.
Choosing energy-efficient, durable, low-maintenance framing materials is important for windows.
Following the recommendations of the Energy Star program would be a good suggestion. Energy
Star-qualified windows come in a variety of framing materials as follows:
• Fiberglass frames are strong, durable, low maintenance, and provide good insulation. Fiberglass frames can be either hollow or filled with foam insulation.
• Vinyl frames are low maintenance and provide good thermal insulation. Sections may be hollow or filled with foam insulation. Wide vinyl sills may be reinforced with metal or wood.
• Aluminum frames are durable, low maintenance, recyclable, and typically have at least 15 percent recycled content. Frame design typically includes thermal breaks to reduce conductive heat loss through the metal.
• Wood frames are strong, provide good insulation, and are generally favored in historical neighborhoods. The exterior surfaces of many wood windows are clad (or covered) with aluminum or vinyl to reduce maintenance.
• Combination frames use different materials separately throughout the frame and sash to provide optimal performance. For example, the exterior half of a frame could be vinyl while the interior half could be wood.
• Composite frames are made of various materials that have been blended together through manufacturing processes to create durable, low-maintenance, well-insulated windows.
Another way to add additional daylight to a bathroom is through the use of a skylight. They are also a good option for adding light to a windowless room or area, such as an inside dressing room or a hallway. Skylights bring in natural light, usually eliminating the need for artificial light during the daylight hours. Skylights can be Energy Star-qualified and they carry NFRC ratings as well. When selecting a skylight for a bathroom, consider many of the same factors you would for other windows. Skylights can come in two basic styles (see Figure 2.8):
• Traditional. A traditional skylight typically uses the same basic technologies to construct the window as a standard window. Many skylights are fixed, but some are operable to let warm air out of the house in the summer. These operable skylights can be installed horizontally, at an angle, or perpendicular to the roof joists. Another form of skylight is a plate of glass fixed to an opening in the roof and often covered by an opaque bubble to help shield the glass and add some insulating value. Traditional skylights have an opening in the ceiling that extends from the glass area on the roof through the attic to the bathroom below. The opening can remain the size of the window glass or flare out as it enters the room space to add light to a larger area.
• Tubular. The tubular daylight devices (TDD) are tubes that extend from the roof down through the roof /attic to the bathroom space. These devices gather light at the roof and transmit it down to a diffusing lens mounted in an interior surface, usually the ceiling. The opening for these tubes is usually smaller than a typical skylight so it is a means to add daylight to many areas of the home, including closets, bathrooms, or halls, that would not have access to sunlight.
The quality of the skylight or tube is important because skylights are usually exposed to the extremes of sun and weather for longer periods of the day than standard windows of the home, especially those that are installed horizontal to the roof. They could even be covered with snow for periods of time. Skylights have had the reputation of leaking because anytime you penetrate the roof, water leaks are a possibility. Proper installation and using high-quality products are essential to prevent such water damage.