Energy Efficient Windows
by Bruce Lang
Why should homebuilders care about installing energy efficient windows in the homes they build? The short answer is because buyers care. Plus, new homes with high performance windows will differentiate your project from the competition. No longer can builders pass off generic low-e glass as the last word in energy efficiency. Enhanced glass technology is taking windows to a whole new level of performance.
Optimal windows for Southwest markets must reflect solar energy and provide high insulation performance to reduce AC costs effectively. But not all homebuilders have knowledge of all the available glass and window options. For example, homebuilders in the Sunbelt know that windows must block solar heat to reduce air conditioning operating costs. They also know that high light transmission is important to homeowners who don't want to trade off darkened interiors for heat reduction. What they may not know is that glass that is capable of superior insulation performance can reduce the cost of air conditioning.
How? Home cooling requirements are not just a function of reducing radiant solar energy. The same insulation performance in windows that keeps heat indoors during winter by impeding heat transfer via conduction and convection also keeps heat out year-round in warm climates.
It's easy to think that direct solar energy from the sun radiating on windows is the only source of heat. But Physics 101 tells us that heat always moves from where it is hot to where it is not. In the Southwest, ambient air with a temperature of 90 degrees will pass through single-pane glass to warm interiors that are being kept 15 degrees cooler through air conditioning.
Heat transfer will also take place via poorly insulated window frames. To the extent windows permit this heat transfer via conduction and convection, they will not maintain the adequate temperature difference required to optimally reduce air conditioning operating costs, no matter how well they block radiant solar heat.
Why is this important? Because as much as 35 percent of the energy used in homes and buildings is wasted due to inefficient glass, which is responsible for over 10 percent of the total carbon emissions in the U.S. annually. Given this reality, homebuilders should know that a double-pane window with either visible tint or an invisible solar heat-blocking coating will perform better at reducing heat transfer from solar radiation, conduction and convection than a single-pane window tinted to reflect solar energy.
Air conditioning efficiency is a function of a home's windows' ability to both reflect radiant solar heat and insulate against heat transfer via conduction and convection. Of course, all windows, glass and heat reflective coatings are not alike.
Windows, from Worse to Better
Let's review glass options in terms of which best supports a Sunbelt home's cooling requirements:
Single-pane glass without a coating to reflect solar energy and providing an insulation value, or U value of 1, leaves the homeowner defenseless against all types of heat. Given today's need to save energy on AC and reduce carbon emissions, a single-pane window is totally unacceptable.
Single-pane glass with a solar control coating is better in terms of reflecting direct solar radiation; however, as noted previously, it does not adequately prevent heat transfer caused by conduction and convection.
Standard insulating glass with a solar control coating and a U value of approximately 0.5 should be the minimum baseline. The coating reflects solar energy and the sealed air space impedes heat transfer.
Insulating glass with dual coatings that simultaneously reflect radiant solar heat and ambient heat via conduction and convection is even more effective in saving energy and improving occupant comfort. For most of the U.S., this version of low-e glass with a U value of 0.35 is the Energy Star standard for homes in which both summer cooling and winter warming are important. However, considering the complexities of optimal cooling, this type of window should be de facto requirement for Arizona and Texas homes as well.
And the story doesn't end here. Revisions to the Department of Energy's Energy Star glass performance standards scheduled to debut in January 2010 should make it clear that even the most energy efficient low-e glass no longer represents a level of energy efficiency required to "transform the market," a key charter of the agency's Energy Star program.
The Next Level
Generic, low-e insulating glass consisting of two pieces of coated glass separated by a sealed, gas-filled air space or cavity achieves a maximum U value of 0.25. With further advances in glass coating technology expected to provide minimal performance improvement, the focus has now shifted from coatings to cavities.
Just as the introduction of single-cavity insulated glass provided a breakthrough in performance beyond monolithic glass, the introduction of multi-cavity constructions, consisting of two or even three insulating cavities, is providing the next performance breakthrough for insulating glass.
A superior alternative to the generic low-e glass now available consists of suspending a low emissivity and solar reflective film inside an insulating glass unit. Film can create two, three or even four insulating cavities that maximize light transmission and provide conservation performance ranging from U 0.16 to an amazing U 0.05 to provide optimal cooling for both residential new construction and renovation projects.
Such internally-mounted film does not replace low-e glass; rather, it leverages the benefits of film-based and glass-based technologies to create a lightweight, multi-cavity insulating glass that offers a new level of performance. Most units fabricated today utilize low-e coated glass to minimize solar heat gain, while using film to maximize insulation performance, block UV radiation, reduce noise, and increase occupant comfort more effectively than low-e glass alone. For protection against hurricane force winds, laminated glass can be used in conjunction with internally mounted film to achieve both energy conservation and enhanced security.
Windows, window glass and the building envelope must be viewed as an integrated system when assessing issues such as energy efficiency, greenhouse gas emissions, occupant comfort and well being. An optimal window should reflect a maximum amount of radiant solar heat, transmit high levels of visible light, provide maximum insulation against heat transfer via conduction and convection, and enhance protection against windblown debris. Film-based, multi-cavity insulating glass can do all of that and more to maximize energy savings in homes throughout the Sunbelt.
Bruce Lang is Vice President of Marketing and Business Development at Southwall Technologies, Inc., in Palo Alto, California, a company that manufactures suspended 'lm insulating glass units available from over 50 window and insulating glass manufacturers worldwide.
650.798.1200
www.southwall.com
