When it comes to energy efficiency in commercial buildings, windows can often be pinpointed as a setback. Twenty-five (25) to 35 percent of the energy used in commercial buildings is wasted because of old or inefficient glass, and glass in commercial buildings is to blame for more than 10 percent of carbon emissions. And, there’s often a large energy-performance gap between the R-values of walls and windows, devaluing a wall’s high R-value.
Low-E glass was once the answer to energy-efficient windows. Single-pane glass was no longer adequate for most commercial/institutional buildings, and standard insulating glass only provided an R-value of 2. Enter low-E glass: It provided twice the insulating performance of standard insulating glass, and it reflects radiant solar and ambient heat.
Although it provided great insulating performance when it was first introduced, generic low-E glass doesn’t represent the level of efficiency necessary to keep moving forward – especially considering the U.S. Department of Energy’s ENERGY STAR® new window performance standards, which will take effect as of Jan. 1, 2010. Even though it was once the standard, low-E coated glass has become a minimum performance standard. These days, there are some new options.
Two choices that meet ENERGY STAR’s proposed Phase 2 window performance standards, which could take effect as early as 2013, stand out:
- Triple-pane glass. Triple-pane glass consists of three panes of glass and two low-E coatings. By using a third pane of glass, a second insulating cavity is produced (multi-cavity window construction). But, triple-pane glass is much heavier than standard insulating glass, which means that a stronger window framing is required (and costs increase, too). If this glass is used in doors/entrances, it can make door use tricky for seniors, children, and the physically disabled. Also, the additional coated glass can weaken visible light transmission by 20 percent or more, possibly reducing the comfort and efficiency benefits of natural light.
- Suspending a low-E, solar-reflective film inside an insulating glass unit. Film can create two, three, or four insulating cavities that maximize light transmission and provide a range of conservation performance. Internally mounted film has the benefits of film-based and glass-based technologies. It forms a lightweight, multi-cavity insulating glass that minimizes solar heat gain, blocks UV rays, reduces noise, etc. This glass also allows you to cut back on (or do away with) other building components, such as air conditioning, perimeter heating, etc.
Calculating energy efficiency in terms of glass is more meaningful when it’s seen as part of an integrated, holistic approach. When you’re evaluating energy efficiency, comfort, and emissions, windows should be viewed as part of all other building components – part of an integrated system.