Generic low-e glass provides insulating performance of about R-4 in a world in which R-19 insulated walls are the norm. Lab 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 U.S. alone, 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 more than 10 percent of total carbon emissions annually and is a major contributor to global warming.
Generic low-e insulating glass—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 meet increased requirements for 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 and improving insulation performance. Low-e coatings are rated for the amount of heat they radiate, 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.
Coated glass is available today with emissivity ratings below 0.03, and lowering emissivity from 0.03 to 0.00 will have a negligible incremental effect 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 Low-e coated glass has become a minimum performance baseline and no longer represents a path to “improved” energy performance.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 to be 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, in the U.S. the Department of Energy (DOE) is implementing changes to its Energy Star program that will require windows to exceed current performance requirements. DOE’s phased revisions to Energy Star’s window performance standards debuted in January of this year, with more demanding follow-on standards to come. The new performance standards should make it clear that generic low-e insulating glass no longer provides the level of energy efficiency required to “transform the market,” as is expected 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 American history, single pane glass was the norm, providing protection from weather more than it did insulation 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 glass and became the glass of choice in heat-intensive climates. In the early 1980s low-e coatings designed to increase insulation performance by impeding transfer of nonsolar heat, such as heat generated by a heating system or in the ambient air, were introduced to the market.
Enhanced low-e coatings that reflect direct solar radiation, which often is a problem for lab facilities with significant amounts of glass on the south and west sides, also became available. Fortunately glass can be equipped with both types of coatings for dual performance, which should be the de facto standard for facilities 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 standard for energy efficiency for new construction and renovation.
With further advances in glass coating technology expected to provide minimal improvement in performance, 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 multicavity constructions with two or even three insulating cavities is providing the next performance breakthrough for insulating glass.
Two multicavity alternatives to generic low-e insulating glass are currently available. One is triple pane glass: three panes of glass and two low-e coatings. The good news is that by using a third pane of glass to create a second insulating cavity, triple pane low-e glass improves generic low-e insulating glass performance from R-4 to R-9. The bad news is that triple pane glass is heavier than standard insulating glass, requiring stronger window framing and increasing cost accordingly.
A superior alternative is suspending a low-emissivity and solar-reflective film inside an insulating glass unit. Without the weight disadvantages of a third pane of glass, 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 to meet the unique requirements of new construction and renovation projects.
Princess Elisabeth polar research station in Utsteinen, Antarctica, equipped with Heat Mirror insulating glass.
Cutaway view of Heat Mirror® insulating glass, showing one Heat Mirror suspended film and two cavities.
Such internally mounted low-e coated films do not replace low-e glass. They leverage the benefits of film- and glass-based coatings to create a lightweight, multicavity insulating glass that offers a new level of performance. Most units incorporating film today utilize low-e coated glass to minimize solar heat gain while using coated film technology to maximize insulation performance. A variety of inert gases are used to fill the internal air space and further impede heat transfer.
Cutaway view of Heat Mirror insulating glass, showing two Heat Mirror suspended films and three cavities
Performance at the edge of the insulating glass unit, traditionally where insulation is least effective as compared to the center of the glass, is improved by using thermally insulated spacer materials to separate the glass, often referred to as “warm edge” construction. In addition to superior insulating performance, suspended film insulating glass blocks UV radiation, reduces noise, and increases occupant comfort more effectively than low-e glass alone.
Clearly, film-based, multicavity insulating glass is tomorrow’s stateof- the-art window glass available today. It has been saving energy in multiple buildings on the campus of Durham College in Whitby, Ontario; in St. Gabriel’s Church in Toronto; and at B.C. Cellular in Burnaby, British Columbia.