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Glass Gains Insulating Capacity Integrated Design Outcomes Can Justify Component
October, 2008
By Bruce Lang
Design choices relating to building envelope components can affect a project's ultimate energy efficiency more than internal systems and components such as lighting and HVAC, but window glass has been the weak link in conservation performance. In an era of R-19 walls and ceilings (R being a measure of insulating performance), standard insulating glass provides an insulating performance of R-2.
Until recently, insulating glass with low-emissivity (low-e) coatings has been recognized as the de facto energy-efficient standard for buildings in which both summer cooling and winter warming are important. Consisting of two pieces of coated glass separated by a sealed, gas-filled air space or cavity, it achieves a maximum R-value of 4 - providing twice the insulating performance of standard insulating glass, and simultaneously reflecting radiant solar and ambient heat.
Emerging glass technology promises even better results. For example, triple-pane low-e glass can narrow the energy performance gap between windows and walls and is increasingly a feature of green construction. By using a third pane of glass to create a second insulating cavity, triple pane low-e glass more than doubles the performance of low-e insulating glass - from R- 4 to R-9. However, triple pane glass is 50% heavier than standard insulating glass, which requires stronger window framing at increased cost.
Low-emissivity solar reflective coated film offers a less weighty alternative with performance ratings upwards of R-6. The thin film can be suspended inside an insulating glass unit to create two, three or even four insulating cavities that maximize light transmission.
Such internally mounted films do not replace low-e glass. Rather, they leverage the benefits of film-based and glass-based technologies to create a lightweight, multi-cavity insulating glass that offers a level of performance that enables designers to downsize or eliminate other building components (AC systems, perimeter heating, etc) to cost-effectively achieve maximum energy savings.
Notably, the selection of suspended film insulating glass at Manheim Township High School in Lancaster County, Pennsylvania, enabled project engineers to specify a smaller HVAC system, which provided considerable up-front cost savings that made the payback on high performance insulating glass much more attractive than otherwise would have been the case. The suspended film insulating glass was about three times the cost of generic low-e glass but helped achieve 40% savings in annual energy costs - a payback of 11.5 years. Factoring in a $90,000 up-front savings resulting from a smaller HVAC system reduces the payback to 6.5 years.
This illustrates the holistic approach to green design, in which building components are selected as part of an integrated system to achieve the greatest cost savings and total energy performance. In this case, the 432,000-square-foot high school campus of renovated and newly constructed buildings also achieved LEED Silver designation.
The thermal and solar shading performance of suspended film insulating glass as part of an integrated construction or renovation strategy can help earn LEED credits in several categories including energy and atmosphere, materials and resources, and indoor environmental quality. Yet, the objective of green design is not just to earn a particular LEED rating.
The key is to use the LEED certification process to more easily achieve the level of efficiency that the project budget will allow. When facility managers are assessing such issues as energy efficiency, greenhouse gas emissions, 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., providers of suspended film insulating glass units. For more information, see the web site at www.southwall.com.
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