Those making an investment in energy efficient windows should carefully consider the possibilities and limitations of their decision. New windows are expensive, but offer long term and noteworthy impacts such as increased comfort and money savings. Today, a prospective buyer has several issues to consider besides the aesthetic ambient. Scientific advancements and improved rating systems demand that we pay most attention to performance-based factors. Here are some ideas and concepts to be aware of as we try to create the view that is just right.
NFRC (National Fenestration Rating Council) is a non-profit organization that is responsible for window certification and labeling program, being the “Blue Book” of the window industry. Trustworthy manufacturers of windows use NFRC values to establish the effectiveness and performance of windows. Windows can be distinguished by different types of calibration and ratings that tell consumers how well they perform.
U-Factor indicates how well the window insulates. It ranges between 0.25 and 1.25. The lower the value, the better the insulating effectiveness.
SHGC (Solar Heat Gain Coefficient) measures solar energy transmittance of a window. SHGC values range between 0 and 1, where a lower value represents less “solar gain”. The lower the solar gain, the less solar heat will pass through the window. A high SHGC window will be especially useful in winter by collecting solar radiation (more solar heat will be transmitted through the window). A low SHGC window blocks the heat and will reduce cooling demands during summer (less solar heat will be transmitted through the window).
VT Value (Visible Transmittance) is the amount of light that passes through the window. Values typically go from 0.1 to 0.9. The higher the value, the more light you’ll see. Visible transmittance of a window can exceed 0.9 (uncoated water-white clear glass) and drop below 0.1 (highly reflective windows with coatings on tinted glass).
AL Value (Air Leakage) measures the rate at which air leaks through cracks of the window components. The lower the rate, the less air will escape through cracks in the window.
CR Value (Condensation Resistance) is a measure of the ability of a product to resist the formation of condensation on the interior surface of that product. CR values range between 0 and 100. The higher the CR value, the better that product is at resisting condensation formation.
Strategies to Boost Energy Efficiency of Windows
Strategically placed windows (in addition to heavy insulation, heat-retaining walls and floors) are an important part of a passive solar home design.
The strategy for passive solar window design is closely tied to window ratings. It focuses on orientation and size of glass to benefit from solar heat gain in winter and minimize it in summer. As a rule of thumb, the south wall should have windows that let in a plenty of light, while the other walls should be glazed with windows that have better insulation.
Significance of Orientation and Climate
Windows that are facing south should have a SHGC greater than 0.6 to maximize solar heat gain during the winter and U-factor of 0.35 or less to decrease heat transfer.
Windows that are placed on the west and east sides receive maximum solar gain during summer. Such windows should have low SHGC. Low SHGC values can be achieved with tinted glass, selective glass and reflective coating.
Windows on north, west and east should be minimized while still allowing for adequate daylight. Since there is more difficulty associated with heat and light control, east and west facing windows when the sun is low in the sky, windows with low SHGC should be chosen.
North-facing windows gather modest amounts of solar heat, so they are used for lighting primarily. In cool climates, buildings should be oriented so their longest wall faces directly towards south with plenty of windows to let the sunshine in. Meanwhile, the north wall should have very few windows, since it practically never receives direct sunlight.
Homeowners in cool climates are advised to buy windows with a high SHGC (high solar gain), that help them reduce heating bills. In warmer climates, glazing areas should face south to collect solar heat during the winter when the sun is low in the sky. Homeowners in warmer areas are advised to choose windows with low SHGC (low solar gain) that help reduce cooling loads.
To improve overall home’s energy efficiency, windows can be designed with different elements or finishes that help performance and reduce energy costs.
Glazing simply means that windows have two or more glass panes. Glass panes are spaced apart and hermetically sealed, with an insulating air space in between. The benefit of glazing is that it minimizes the thermal transmission through the glass, thereby improving the energy efficiency of the building. Window glazing primarily lowers the U-factor, but it also lowers the SHGC.
The space between the window panes can be filled with inert gas — usually krypton or argon, which has a higher resistance to heat flow than air. The benefit of gas fills is that it additionally improves the thermal performance of windows with insulated glazing.
Low Emissivity Coatings
Low emissivity (low-e) coatings are economical choice in cutting air-conditioning costs in the summer. Low-e coatings are microscopically thin and transparent. Low emissivity coatings may be applied to the inside of the panes during the manufacturing process or afterward. Windows with low-e coatings normally cost about 10% to 15% more than regular windows, but save money in the long run since they reduce energy loss by 30% to 50%.
Reflective coatings are thin, metallic layers that come in a variety of colors, including silver, gold, and bronze. Reflective coatings reduce the transmission of solar radiation, blocking more light than heat. Reflective coatings greatly reduce a window’s VT and glare, but they also reduce a window’s SHGC.
Spectrally Selective Coatings
This type of coating filters out 40% to 70% of the heat normally transmitted through insulated window glass but allow the full amount of light transmission. Spectrally selective coatings create a window with a low U-factor, low SHGC and a high VT.
Improving the thermal resistance of the window frame can contribute to a window’s overall energy efficiency, particularly its U-factor.
These frames are stable and have air cavities that can be filled with insulation, giving them improved thermal performance compared to wood or vinyl.
Vinyl frames have good moisture resistance. Their cavities can be filled with insulation, which makes them thermally advanced to standard vinyl and wood frames.
Aluminum is very strong and light, but has a poor insulating properties because aluminum window frames conduct heat quickly. To reduce thermal flow and the U-factor, aluminum frames must have an insulating plastic strip (thermal break), placed between the inside and outside of the frame and sash.
These frames consist of composite wood products, such as particleboard and laminated strand lumber. Composite frames have the same or better structural and thermal properties than conventional wood.
What else to look for?
In addition to the NFRC rating, energy efficient windows have an Energy Star label. This means they’re certified to meet the U.S. Energy Star program’s standards for superior products. Energy Star windows take into account U-Factor and SHGC.