Windows Versus Walls: Debunking the energy myth

By Bruce LangCanada's climate is one of the more diverse on the planet. It varies based on geography, ranging from long, cold winters and sunless days in the Far North to four distinct seasons along the U.S. border, and typically mild winters in the B.C. Lower Mainland. Temperatures can climb to more than 40 C (104 F) in the summer and drop below –50 C (–58 F) in the winter. This diverse and extreme climate can have significant ramifications for commercial building design, especially when it comes to energy efficiency and occupant well-being and productivity.

The building envelope—roof, walls, and windows—is the interface between the building and its environment, and a structure's first line of defence against the elements. Envelope design and product choices have a significant impact on energy efficiency and occupant well-being. Well-insulated ‘solid’ walls are typically the priority for a specifier when designing for cold climates, but they do not offer the esthetic appeal and natural daylighting benefits of glass. What if glass could offer similar insulation and energy efficiency demanded from walls?

The low-down on high-performance glassThe best-kept secret about improving commercial building energy efficiency is high-performance window glass. In fact, glass use as a percentage of the building envelope is rising as architects look to harness its esthetic appeal and daylighting benefits. Much of this increase has been enabled by advancements in low-emissivity (low-e) coating technology over the last two decades.

However, compared to insulated walls and ceilings, typical windows are a serious energy-loser. Insulation is measured in terms of resistance to heat flow or R-value––the higher the R-value, the better the insulating performance. Walls with an insulating performance of R-30 (i.e. RSI-5.3) are considered normal for most Canadian buildings today, while the insulating performance of windows typically tops out at only R-4 (i.e. RSI-0.7). Why settle for R-4 windows in homes and buildings with R-30 insulated walls? This energy conservation double standard exists because it is easier to be a wall than a window. Walls only have to insulate well, while windows must do a lot more.

Windows (specifically window glass) must:

Additionally, many windows must also open to provide ventilation and egress in case of emergency. With windows accounting for up to 30 per cent of the heat loss of conventional buildings and homes, they represent low-hanging fruit that can make a dramatic––and immediate––impact on energy efficiency.

One radical solution might be to board up many of the existing windows. This could save some energy, but it hinders the transmission of natural light into a building. The increasingly recognized benefits of bringing in daylight include:

There is clearly an incentive to make windows perform better. Simply reducing their size and number is not feasible, especially in cold Canadian climates, where ‘cabin fever’ can be a reality.

High-performance glass optionsSince glass is the heart of a window, specifiers should know about high-performance options. Single-pane glass may keep out the weather, but it does little to insulate against heat loss or reflect the sun's heat—its performance is about R-1 (i.e. RSI-0.18). The air space inside dual-pane insulating glass (i.e. two glass panes with a low-e coating separated by a sealed air space), particularly when filled with an inert gas such as an argon, enhances insulation, and the coating reflects the sun's heat––maximum performance up to about R-4.

Unfortunately, since coating technology has now reached practical limits with emissivity as low as 0.003, people can no longer rely on better low-e coatings to improve glass performance as it has for the past two decades. To break through the glass performance barrier, one must now make a shift from coatings to ‘cavities,’ which are heat-impeding air spaces inside of an insulating glass (IG) unit. Unlike dual-pane glass (which is limited to a single cavity), multi-cavity glass uses multiple insulating air spaces to achieve a new level of energy efficiency.

Triple-pane insulating glassTriple-pane insulating glass consists of three panes of glass and two low-e coatings separated by two air spaces. It improves insulating performance up to R-10 (i.e. RSI-1.8)—with krypton gas fill. The bad news is triple-pane glass is 50 per cent heavier than dual-pane glass, requiring stronger window framing and adding significant structural load to the building. It is also more difficult to handle and install.

Suspended-film insulating glassSuspended-film insulating glass consists of a coated film suspended between two panes of glass. It improves insulating performance up to R-20 (i.e. RSI-3.5)—with krypton gas and three suspended films—at the same weight as dual-pane glass. Up to three coated films can be suspended inside the unit to create up to four insulating cavities. Adding a heat-impeded gas to the internal cavities can achieve centre-of-glass insulating performance of up to R-10 (with argon) and R-20 (with krypton) as illustrated in Figure 1.

High-insulating glass outperformsSuspended-film insulating glass uses multiple films to achieve at least R-8 (i.e. RSI-1.4) insulating performance and moderate solar heat gain. Windows equipped with suspended-film insulating glass can actually be more energy-efficient than insulated walls when daylight passive solar gain is considered in addition to the glass's insulation properties. Unlike walls, suspended-film insulating glass can achieve a net energy gain by admitting more heat from the sun than is lost through conduction. It is at this point a glass system is capable of outperforming the surrounding wall.

For example, as stated above, suspended-film insulating glass can reach a performance of up to R-20. At this point, the glass is stopping 95 per cent of the potential heat loss (U-factor 0.05). This means there is less than a two per cent differential in heat loss between R-20 glass and a surrounding R-30 wall. When considering there is also solar gain in a 24-hour, 365-day cycle, the glazing system's passive gain may ultimately offset its heat loss. This means despite having a lower R-value, an R-20 glass unit may actually outperform an R-30 wall.

Additional benefits of suspended-film insulating glassSuspended-film, multi-cavity insulating glass leverages the benefits of film- and glass-based technology to create a lightweight IG unit. Low-e coated glass is used to minimize solar heat gain, while suspended coated film is used to maximize insulating performance, block UV radiation, reduce noise, and increase occupant comfort more effectively than coated glass alone.

However, additional benefits can be realized when the higher performance of suspended-film insulating glass is considered as part of a holistic approach to optimize overall building performance and cost. For example, a building designed with low-performance glass will likely require additional systems, such as perimeter heating and a larger HVAC system. However, a ‘tight’ building envelope design can eliminate perimeter heating and downsize the HVAC system. Not only does this reduce the building's initial price, but it also lowers annual operating costs.

Glass that insulates like a wallIn an era of R-30 walls, glass has been the energy efficiency ‘weak link’ in the building envelope. However, this is no longer the case. It is important specifiers know the performance limitation of dual-pane glass, or the weight limitation of triple-pane glass, need no longer be accepted.

Superior multi-cavity solutions that incorporate suspended coated film have changed the rules and can achieve up to R-20 glass performance without additional structural weight. Specifiers have a great opportunity to use these multi-cavity solutions to not only dramatically increase energy savings, but also reduce the overall costs by leveraging this glass's higher performance to eliminate or downsize other building systems. In other words, design professionals no longer have to think of walls for insulation—they can think windows.

Bruce Lang is the vice-president of marketing and business development for Southwall Technologies, a supplier of high-performance films and glass products. He is also the president of Southwall Insulating Glass, a company that manufactures energy-efficient, suspended-film insulating glass. Lang has a bachelor of science in electrical engineering from Stanford University and a master's degree in business administration from Santa Clara University in California. He can be reached via e-mail at [email protected].