Green with Envy Home Store is always answering the hardest questions about using green products and services. This week has been very interesting because of the looming deadline of the energy tax credits. After attending Greenbuild in Chicago last week, it is quite evident that most manufacturers believe that the tax credits will NOT be extended into 2011. This week’s questions were very interesting but windows seemed to be the main topic as clients rush to purchase replacement units before the end of the year. The main questions that we answered related to descriptions about window specifications. We like really good explanations about products and the Efficient Windows Collaborative provided the best information. Here we go!
U-Factor
The rate of heat loss is indicated in terms of the U-factor (U-value) of a window assembly. The lower the U-factor, the greater a window's resistance to heat flow and the better its insulating properties. The nationally recognized rating method by the National Fenestration Rating Council (NFRC) is for the whole window, including glazing, frame and spacers. Center-of-glass U-factor is also sometimes referenced, and describes the performance of the glazing alone without the effects of the frame. For most energy efficient windows, the whole window U-factor is higher than the center-of-glass U-factor.
High-performance double-pane windows can have U-factors of 0.30 or lower, while some triple-pane windows can achieve U-factors as low as 0.15.
Low U-factors are most important in heating dominated climates, although they are also beneficial in cooling dominated climates. ENERGY STAR provides recommended U-factors for your climate.
R-Value.
A measure of the resistance of a glazing material or assembly to heat flow. It is the inverse of the U-factor (R = 1/U). A high-R-value window has a greater resistance to heat flow and a higher insulating value than one with a low R-value.
Solar heat gain coefficient (SHGC). The fraction of solar radiation admitted through a window or skylight, both directly transmitted, and absorbed and subsequently released inward. The solar heat gain coefficient has replaced the shading coefficient as the standard indicator of a window's shading ability. It is expressed as a number between 0 and 1. The lower a window's solar heat gain coefficient, the less solar heat it transmits, and the greater its shading ability. SHGC can be expressed in terms of the glass alone or can refer to the entire window assembly.
Increased Light and View
Daylight and view are two fundamental attributes of a window. Unfortunately, windows are also the source of significant solar heat gain during times when it is unwanted. Traditional solutions to reducing solar heat gain, such as tinted glazing or shades, yields a result that reduces the amount of light. New glazings with low-solar-gain low-E coatings can provide better solar heat gain reduction than tinted glass, with a minimal loss of visible light. This also means that views can be clearer and unobstructed.
Greater Protection from UV Fading
Many organic materials, such as carpet, fabrics, paper, artwork, paints, and wood may fade upon exposure to sunlight. Window selection can influence the type and intensity of transmitted radiation. The most harmful radiation in sunlight is ultraviolet (UV) rays, which are the most likely to break chemical bonds, leading to fading and degradation. Coatings on glass can reduce the transmission of UV radiation by up to 75%. UV absorbers can also be incorporated into thin plastic films in multi layer windows or as an interlayer in laminated glass. In both cases, the UV transmission can be reduced to less than1%. However, it is important to note that the remaining visible light that is transmitted can still cause fading in some materials. But low-E coated glass or plastic films reduce fading to a minimum for many modern interior furnishings.
We attempted to match performance measures for many major window manufacturers with the brand that we recommend, Serious Windows. The process was overwhelming and caused many calls to technical service at the manufacturers. Most large manufacturers do not provide full frame values but only center of glass performance measures. This contradicts the information provided by the Collaborative stating that manufacturers are switching over to full frame measurements. Keep sending your questions and we will try to keep up!!!