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In Focus: Strengthening the Weak Link in Building Conservation

Bruce Lang  (Oct/Nov 08)
This integrated systems approach to green design inherent in the Leadership in Energy and Environmental Design (LEED) certification process is best demonstrated by evaluating the role of high-performance window glass as a key component in determining the energy efficiency of an entire facility. The building envelope - foundation, roof, walls, and windows - is the interface between the building and its environment and a structure's first line of defense against the elements. Design choices regarding building envelope components affect a project's ultimate energy efficiency more than the internal systems and components (lighting, heating and cooling, etc.).

In an era of R-19 walls and ceilings (R being a measure of insulating performance), window glass has been the weak link in conservation performance. From 25 percent to 35 percent of the energy used in U.S. buildings is wasted due to inefficient windows and glass, which themselves account for 10 percent of all CO2 emissions.

Improving the performance of windows represents a significant savings opportunity both for the nation and for individual green building and renovation projects. Glass options alone will have a disproportionate impact on overall building energy efficiency compared to other building components. Let's evaluate those options:

• Single-pane glass does not adequately prevent heat transfer and is no longer acceptable for buildings in most of the nation.
• Standard insulating glass, providing an insulating performance of R-2 as compared to an R-19 wall, is obviously unacceptable although still code-compliant in many locations.
• Insulating glass with low-e coatings, providing twice the insulating performance of standard insulating glass, simultaneously reflects radiant solar and ambient heat and is the de facto energy efficient standard for buildings in which both summer cooling and winter warming are important. The "e" in low-e, which stands for "emissivity," is the ability of a surface to radiate energy.

Many might think that this is where the story ends, because 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 R value of 4. However, this level of performance is not nearly enough to achieve what green building promises in terms of energy savings and CO2 reduction. Fortunately, much of the success of green construction is due to higher performing glass technology that provides powerful energy-conserving alternatives to generic low-e glass and about which facility managers need to become familiar.

Alternatives to single-cavity low-e insulating glass are available that can narrow the energy conservation performance gap between windows and walls. One is triple pane glass, consisting of three panes of glass and two low-e coatings, which doubles the performance of low-e insulating glass from R-4 to R-9. Unfortunately, triple-pane glass is 50 percent heavier than standard insulating glass, imposing size constraints and requiring stronger window framing at increased cost.

A superior multicavity alternative consists of suspending a very thin, low-emissivity and solar-reflective coated film inside of an insulating glass unit. Without the weight disadvantages of a third pane of glass, suspended film can create two, three, or even four insulating cavities that maximize light transmission and provide conservation performance ranging from R-6 to an amazing R-20.

When facility managers are assessing such issues as energy efficiency, greenhouse gas emissions, and occupant comfort and well-being, window glass specifically, and the building envelope more generally, must be viewed together with all other building components as an integrated system to maximize desired conservation results at the most affordable cost.

Bruce Lang is Vice President of Marketing & Business Development at Southwall Technologies, Inc., in Palo Alto, CA.
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