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Going medieval

The window-to-wall ratio debates pits energy efficiency against esthetics and functionality.

June 4, 2014  By Irwin Rapoport

The window/wall ratio (WWR) controversy in the ICI building community is
an ongoing one and a proposal to reduce the percentage was defeated
last January.

The window/wall ratio (WWR) controversy in the ICI building community is an ongoing one and a proposal to reduce the percentage was defeated last January. The current ratio is a 40 per cent WWR limit on the prescriptive path in ASHRAE 90.1 and the International Energy Conservation Code. It is only for the prescriptive path, and any amount of glazing can be used in the performance path where one uses energy modeling to show that your proposed building has the same or better energy performance as a reference building that meets the prescriptive path.

Mona Campbell Building  
LEED doesn’t have to mean less glass. The Mona Campbell Building at Dalhousie University is certified LEED Gold with a generous, wrap-around glass facade. Designers achieved the designation with a garden roof, high-tech interior air quality sensors, pre-heated ventilation air and a water loop heat pump system.



Thus, architects and designers can use any amount of glazing, but the comparison is made to a 40 per cent WWR building, which forces engineers to use other features such as high efficiency HVAC equipment and daylighting to meet the target.


The proposal for a new standard, 189.1, had it passed, would have dropped the prescriptive limit from 40 to 30 per cent WWR (a 25 per cent reduction). Again, any amount of glazing would have been allowed in the performance path, but the WWR reduction was fought by many who considered it to be a bad precedent for glass industry and counter to high-performance design. The revision would have cost glass suppliers and contractors a lot of business, but the industry got together and successfully lobbied the committee to have it removed. That battle was won, but glaziers can only hope it’s the end of the war.

Making buildings more energy efficient with a smaller environmental footprint has been a hot topic for some time, and many green building advocates have been upset with glass curtain wall building envelopes. They share a basic understanding that a glass barrier between the outdoors and indoors can never be as insulating as a brick or stone wall. This has resulted in a push by some green building advocates to require buildings to have smaller windows in relation to the wall space in order to qualify for certain green certifications, like LEED. Curtain wall manufacturers, architects and designers have pushed back, pointing out that a healthy balance can be achieved and that the energy inefficiency of glass construction has been frequently overstated.

ASHRAE is one key organization that sets standards for energy efficiency of commercial buildings (not just HVAC standards). ASHRAE 90.1 is its main energy standard, while ASHRAE 189.1 is a green standard meant to include both energy efficiency and other green attributes (for instance, land use, material use and indoor environmental quality) similar in concept to LEED or GreenGlobes, but written as a standard or code format instead of a point system.

While USGBC supports ASHRAE 189.1, it is not required to be used in LEED buildings. They are still two separate programs. LEED is still the main system, and it uses ASHRAE 90.1 as the basis for energy efficiency (but awards more points for higher levels of performance). However, 189.1 has gained some acceptance and use with military and publicly funded buildings.

Thomas Culp is the president of Birch Point Consulting and an energy code consultant for the Glass Association of North America and the Aluminum Extruders Council. He presented his clients’ case against the proposal at last January’s 189.1 committee hearings. “The original proposal came out of an analysis for ASHRAE 90.1-2010 comparing the energy use of a medium office building with different window-to-wall ratio,” says Culp. “That study showed energy use could be decreased a few per cent if the prescriptive limit was reduced from 40 to 30 per cent WWR.” Culp argued the analysis they used was not applicable. “It was only for the 2010 version of ASHRAE 90.1, and did not include the higher performance windows and daylighting required by ASHRAE 189.1. The requirements would not even comply with ASHRAE 90.1-2013; it was only for one building type (medium office), it only looked at site energy and it did not consider impacts on indoor environmental quality and occupant well-being.”

Regarding the last point, Culp stresses that “as part of the debate (which lasted from last June through this January), we presented dozens of studies that demonstrated all the positive impacts that windows have on high performance buildings. For spaces with access to high quality daylighting and/or views, there are studies that show increased real estate value, higher rental rates, a 20 per cent increase in office worker cognitive test rates, 39 additional work-hours per year in office worker productivity, nine to 16 per cent improved performance on visual memory tests (but glare decreased it by 17 per cent), and 15 per cent decreased absenteeism in office workers.”

Studies also found decreased office worker turnover and a 21 per cent increase in student test scores, a six per cent increase in retail sales and, for hospital and health care facilities, a 22 per cent reduced development of surgical post-op delirium, a reduced length of hospital stay by 2.6 days, 22 per cent less pain medication prescribed to post-spinal surgery patients, reduced depression and improved sleep.

Culp points out that “overall, productivity-related costs of a building’s use are over 100 times greater than the energy-related costs. So even if there is a small energy savings from a proposed change, if it adversely harms the indoor environment, productivity, and use of a space, the negative impact and increased costs of the full use of the building will dwarf any potential energy savings. Energy and health should not be a trade-off.

“Plus,” he adds, “you really don’t have to choose one or the other. With recent advances, including high-performance framing, new and multiple low-e coatings, dynamic glazing, improved daylighting and shading designs, both high energy performance and high quality spaces can be achieved.”

Interventions by Culp and others succeeded as the ASHRAE 189.1 committee, at its January meeting in New York City, voted 26-4 (with three abstentions) to discontinue the proposal.

Steve Kemp, manager of buildings sustainability for the MMM Group, says there is a “wealth of evidence” by ASHRAE and others that modest glass ratios are best for building energy performance, and high glass ratios don’t necessarily result in good daylighting. “The sweet spot tends to be in the 30 to 40 per cent range,” Kemp says, “so I am bit surprised by the 30 per cent number.

“Right now,” he adds, “the best performing windows (triple-glazed) are at best 10 per cent of the market. Most windows are double-glazed, low-e Argon and whether in an aluminum or fiberglass window frame, you are talking R3 to R4 as the insulating value, where even a modest wall is R15 to R20. A lot of glass sometimes ends up in window wall and curtain wall, and the opaque sections are at best R4 to R6. In a curtain wall, you can put more glass in because it doesn’t matter from a heating perspective because the glass has similar performance to the opaque wall it is replacing; however, doing so will increase summer cooling loads.”

Kemp adds that buildings with punched-wall windows are “almost always a worse performer than the wall they replace,” and that “mountains” of research have demonstrated that, with well-insulated walls, 30 to 40 per cent glass ratios is the optimum WWR, balancing daylighting, passive solar gains and building heat loss. “You get the gains,” says Kemp, “but you need the insulation in the walls to keep them in the building to get the benefit.”

Kemp suggests that ASHRAE 189.1 is an aspirational standard that is typically not enforced in any jurisdiction. “ASHRAE 90.1 is designed to be a minimal code,” he says, noting that people appreciate daylight. “Even at 25 per cent, if a building is well designed, there is a lot of daylight. Providing daylight is not about the amount of glass, but about the floorplate design and how far you are from the windows.” Kemp, an engineer, has been in the energy efficiency field for 20 years and has witnessed the trend for more glass, pointing out that it is an aesthetic choice “and, for better or worse, window wall and curtain wall condos are the trend. The architects and engineers that design buildings know that it is harder to air condition and that you have to upgrade the HVAC systems to comply with building energy codes to meet the market demand for the glass aesthetic.” All-glass buildings and those with large percentages of window wall are best suited for warmer climate areas. “But the flip side is they require a lot of air-conditioning,” says Kemp. “You could do a tint, but the market likes clear glass.”

Manitoba Hydro  
Another example of a building achieving high sustainability standards with no shortage of glass.This is Manitoba Hydro Place, the first office tower in Canada to achieve LEED Platinum.


Energy efficiency and having reduced energy bills is a concern for building owners and that impacts the final design. Kemp says that an alternative path that is “slowly” being implemented is what New York City has done and what Toronto is considering: the requirement for all buildings to publish their energy bills. “Not many do,” he says, “but we have a database of about 80 buildings and the trend is clear: energy bills go down with more modest glass ratios. We’ve been involved with LEED Platinum buildings with lots of glass. The corollary is that they have exceptional mechanical systems to make up for the envelope. There are lots of places in buildings to improve energy efficiency, but the hope and goal of the industry in 20 to 30 years is almost zero energy use, in which case it becomes more difficult to rely only on one technology. We have to rely on all the bits and pieces of the building.”

Kemp says that the 1989 version of ASHAE 90.1 was the first that he applied and has influenced his work ever since. “Glass technology has improved,” he says, “but not as dramatically as some people believe.” To demonstrate this point, Kemp refers to a recent study from British Columbia that examined the energy efficiency of condos in the lower mainland – those built in the 1970s and those in early 2000s. “They tend to heat the central corridor with natural gas, pumping air to pressurize it so you can’t smell what your neighbour is cooking,” he says, “but the heat loss for the perimeter is terrible – and all the buildings in this study had electric baseboards. The trend in the 1970s was single-paned glass and the new ones were double-glazed low-E argon and the perimeter energy use didn’t move over that 30-year period. The better-performing glass was immediately offset by adding more glass, so nothing changed.”

If the WWR ratio was reduced to 30 per cent maximum, Kemp suggests that the glass industry would be pressured to develop more cost-effective solutions for triple-glazed windows. “They are significantly more expensive than the standard product,” he says, “but you can get the equivalent performance at higher window-to-wall ratios. It would be interesting to make the default that you could sell a triple-glazed window.”

On this point, Culp notes, “Obviously reducing the window area limit, even if in just the prescriptive path of one standard, would have been bad for business. But more to the point, it would have delivered the wrong message to designers about the proper role of glazing in high performance buildings.”

Having more energy-efficient buildings has multiple benefits and for Kemp, dealing with the impact of climate change is important as well as reducing energy costs and securing LEED points. Renovating all-glass buildings such as Montreal’s Westmount Square complex with triple-glazed windows can be done, but it only makes financial sense when they need to be replaced.

“At the current costs of energy, especially now with the extra low costs of natural gas,” says Kemp, “it almost never makes to replace windows. You replace windows when they’re leaking or have failed in another way. It can cost about $1,000 a window just to take them out. Until the glass is replaced, you should install more energy-efficient systems in the buildings in terms of control systems, lights and boilers.”

Culp agrees that addressing existing buildings is a challenge, but adds “in addition to replacement options, there are also new systems to add low-e glazing units or panels to existing windows to upgrade the envelope performance at a fraction of the cost of full rip-out and replacement.”

Kemp says that curtain wall panels (opaque-spandrel) also need to be efficient. “It’s got so much thermal bridging that the best performance out there is an effective R5 and R6 and most of it is R4,” he says. “You can put in R20 and R30, but they can’t get the performance out of it because it has so much aluminum or steel thermal bridging around it.” However, Kemp is impressed by the possibilities of vacuum insulated glazing, in which the pane of glass allows for R12 and even R20 efficiencies. “But you still have to stick the glass in a real window frame and heat always goes along the path of least resistance,” he says. “We have to figure out how to improve the frames because right now you can install this R20 glass and all of a sudden, you get R11.”

Concerning advances in glass technologies, Culp says “the industry has developed new high performance framing (double thermal breaks, wider thermal breaks), as well as vacuum insulation spandrel panels for very high energy efficiency. Additionally, use of multiple low-e coatings (either two low-e coatings in triple glazing, or two low-e coatings in double glazing with a durable fourth surface coating), dynamic glazing, and integrated design (combining high performance products with daylighting, shading, glare control and ventilation) offer significant positive impacts.”

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