energymanagement
Energy Saving Alternatives to Generic Low-E Glass 25 to 35% of energy wasted due to inefficient glass By Bruce Lang
Generic low-e glass provides insulating performance of about R-4 in a world in which R-19 insulated walls are the norm. Property managers should know that there’s a dramatic performance gap between what low-e glass provides and what green building practices promise in saving energy and reducing heating and air conditioning operating cost. Despite heavily insulated walls and ceilings and the popularity of low-e glass in the United States and Canada, 25 to 35 percent of the energy used in buildings is wasted due to inefficient glass. So, it should come as no surprise that glass is responsible for up to 10 percent of the total carbon emissions annually and is a major contributor to global warming. Generic low-e insulating glass— consisting of two pieces of coated glass separated by a sealed air space—achieves a maximum thermal insulation value of R-4. Even low-e insulating glass in which argon or other inert gases are used to fill the internal air space and further impede heat transfer will not enable low-e glass-equipped windows to achieve increased requirements in conservation performance. The “e” in low-e, which stands for “emissivity”, is the ability of a surface to radiate energy. A low-e glass coating reflects heat, reducing heat transfer between panes of glass, improving insulation performance. Low-e coatings are rated for the amount of heat they radiate by using a scale of measurement called U values—the lower the number, the less heat is radiated and the better the insulation performance of the glass. 40 Canadian Apartment Magazine
Coated glass is available today with emissivity ratings below 0.03. Lowering emissivity from 0.03 to 0.00 will have a negligible incremental improvement on glass performance. The truth is that low-e glass thermal performance has reached practical limits. Low-e coated glass has become a minimum performance baseline and no longer represents a path to “improved” energy performance. The incremental performance benefit of using low-e glass is ZERO, because it is already assumed as a required product. Clearly, further improvements in glass thermal performance will not come from enhancements in low-e coatings. With window insulating performance at its current levels, improving window performance represents a significant opportunity for tremendous energy saving. Accordingly, with input from Canadian authorities, the US Department of Energy (DOE) is implementing changes to its Energy Star program which will require windows to exceed current performance requirements. DOE’s phased revisions to Energy Star’s window performance standards are scheduled to debut in August, 2009 with more demanding followon standards in 2013. The new performance standards should make it clear that generic low-e insulating glass no longer provides a level of energy efficiency required to “transform the market”, an expectation of products validated by the Energy Star program. To appreciate the energy-saving capability of currently available alternatives to generic low-e glass, it
is necessary to understand a bit about the use of glass in the modern era. For most of North America’s history, singlepane glass was the norm, providing protection from weather more than insulating against heat loss in winter and heat gain in summer. Though patented in 1865, insulating glass—two panes of glass separated by a sealed air space—was not widely adopted until the middle of the 20th century. Insulating glass came to be considered the energy-efficient alternative to single pane and became the glass of choice in heating-intensive climates. In the early 1980s low-e coatings were introduced to the market. They are designed to increase insulation performance by impeding non-solar heat transfer such as heat generated by a heating system or in the ambient air. Enhanced low-e coatings reflecting direct solar radiation, that often is a problem for properties with significant glass on their south and west sides, also became available. Fortunately glass can be equipped with both types of dual performing coatings which should be the defacto standard in properties in which both heating and cooling are necessary. Windows with one or both types of low-e coatings have supplanted windows with traditional insulating glass to become the “energy-efficient” standard for multi-residential housing construction and renovation. 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