Before specifying efficient heating and cooling equipment, it’s important to do what we can to reduce heating and cooling loads. Insulation is one of the key products to consider here, but because there are so many insulation products on the market, we look for additional benefits. Examples include cellulose insulation with recycled content, mineral wool insulation with no flame retardants, and fiberglass insulation with no formaldehyde binders. Other products in this area are high-performance windows and glazings, products that contribute to building airtight envelopes, products that reduce thermal bridging, and window-retrofit products.
With products in this area under constant development, we are always refining our approach. For example, as we have learned about insulation products with hazardous flame retardants and blowing agents that have high global warming potential, we have removed those products from GreenSpec, pending manufacturing changes. We encourage building professionals to pressure manufacturers for those changes through specification language and purchasing decisions.
Low-E Pyrolytic Glass
AGC Flat Glass North America manufactures low-e hard coat (pyrolytic) glass, which is used in several brands of low-e exterior storm windows. Low-e storm windows can move the thermal performance of existing windows close to that of high performance double-glazed windows. GreenSpec has contributed to more research on interior and exterior low-e panels at WindowAttachments.org.
GreenSpec lists low-e, airtight exterior storm windows because they significantly improve the thermal performance of existing windows, and they also protect existing windows from the elements.
The low-e coating on the interior of the glass is exposed and consists of a hard-coat, or pyrolytic, coating with higher solar heat gain best suited for heating-dominated climates. Lower solar heat gain, low-e storm windows are also being commercialized for warmer climates.
Although most low-e storms have metal frames (baked enamel aluminum) at least one manufacturer uses a PVC frame for reduced thermal transmittance of the frame. Most exterior low-e storms are "tracked" with two (both glass sashes) or three (two glass and one insect screen) panels.
Durability is an important factor in selecting window attachments. While there isn't much available public testing data for these products, the best exterior storms typically come with a minimum product warranty of 20 years, and GreenSpec has adopted this as a minimum standard.
For additional information on low-e storm windows, see GreenSpec's partner site WindowAttachments.org.
In commercial buildings, architects have long specified the thermal, structural, and aesthetic properties of glazing.
With residential windows, designers and builders have also become involved in specifying glazing, as advanced glazings have become available since the 1980s that reduce heat loss and regulate solar heat gain. It is even becoming common practice with high-performance buildings to specify different glazings for different orientations--choosing glazings that block most of the solar heat gain for east and west windows while choosing glazings transmit more solar gain on the south for passive solar heating.
Key properties to consider when specifying glazing include the following.
Solar heat gain coefficient (SHGC) is a value from 0 to 1 that indicates the ratio of solar heat gain coming through the window to the incident solar energy striking the window. High-SHGC glass (over 0.33 SHGC) is beneficial for passive solar heating, while lower numbers are essential for cooling-load avoidance.
U-factor is the rate at which heat moves through a window or glazing system for a given temperature difference; not including direct solar gain or air leaks. High-performance buildings are pushing for lower and lower U-factors: less than 0.30 is recommended in most situations, and whole-window values as low as 0.13 are possible.
Visible light transmittance (VLT) is a value from 0 to 1 that indicates the amount of visible light transmitted through the glazing. Higher VLT glass looks clearer—anything below 0.40 starts to appear noticeably gray.
GreenSpec lists glazing that offers strong energy performance properties for a variety of applications. Careful selection of glazing for specific applications is critical for optimum energy performance and building comfort. Some glazing listed by GreenSpec allows these performance properties to change based on user desires or ambient conditions. GreenSpec also lists glazing products that help minimize bird collisions or deliver other environmental benefits.
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