Low-Energy Windows for Multifamily Buildings in British Columbia

Canada’s Pacific province does not lack its fair share of optimism. “The Best Place on Earth,” as British Columbia’s government used to call it, has an Energy Efficient Buildings Strategy with the goal of reducing the average energy demand per home by 20% by 2020. Now, 20% by 2020 may at first sound like a neat play with numbers, but British Columbia is indeed taking very concrete steps toward this goal. Among these steps are comprehensive utility demand-side management programs and provincial government “LiveSmart” incentives, a revenue-neutral carbon tax of $25 per metric ton of CO2 and codes for lowenergy buildings, as well as standards for energy-efficient products, including windows.

British Columbia’s energy efficiency efforts include a recent focus on multifamily buildings. It’s an important market segment: In Vancouver, for instance, three-quarters of new housing has been multifamily over the past 30 years. To assess how the energy performance of multifamily buildings can be improved, building science researchers Graham Finch, Dave Ricketts and Warren Knowles of RDH Building Engineering studied the challenging segment of mid- and high-rise multifamily buildings. Their analysis of envelope heat losses in existing high-rises found that windows are a major factor. While RDH also identified several other important factors, let’s take a closer look at their findings on windows and at the steps British Columbia is taking to improve window energy efficiency.

Windows in Current British Columbia High-Rises 

Titled “The Path toward Net-Zero High-Rise Residential Buildings”, the study by RDH looks at the construction practice for mid- and high-rise multifamily buildings in British Columbia and the potential for improvement. Even in Vancouver’s relatively mild climate, space heating is typically the largest energy end-use in multifamily buildings.

To assess heat loss, the thermal envelope performance of 13 representative buildings was modeled in detail using the thermal simulation programs THERM 5.2 and WINDOW. This included hundreds of wall, roof and window models based on original drawings while taking into account actual construction details, thermal bridging, and window and door sizes and frame configuration. This analysis includes both original 1980s enclosure conditions as well as the envelopes that have been rehabilitated in
recent years.

Due to thermal bridging from steel stud framing, exposed slab edges and balconies, the overall performance of the original walls of the studied buildings is only about R-3.5 to R-5.5 (U-factor 0.18 to 0.29). Even after rehabilitation, the typical wall performance is no more than R-4.5 to R-7 (U-factor 0.14 to 0.22). As expected, though, the heat loss through windows is substantially higher. The typical 1980s buildings were built with dual-pane aluminum windows with neither low-E nor thermal breaks (U-factor around 0.75).

Among the rehabilitated buildings, thermal breaks are common, but low-E was not ubiquitous before the BC window standards (see below). U-factors for newer aluminum frame windows range between about 0.45 for low-E windows and 0.6 for clear-glass windows. With window areas of 34% to 65% of the wall area, several of the study buildings lose about three times as much heat through their windows as through their walls.

Those buildings that were rehabilitated without low-E glass were deprived of an important energy savings opportunity. To ensure that such opportunities are not missed, British Columbia has recently started to require that all new windows must meet energy efficiency standards that essentially require the use of low-E glass (see info box). Simulations indicate that windows meeting the new standards should reduce space heating energy use in high-rise residential buildings by 5 to 10 percent if replacing typical clear glass windows without thermal breaks (or 2.2 to 4.4 percent of total building energy use, up to three times larger than replacing the lighting to meet the advanced standards). This does not take into account the air leakage reduction potential from new windows.

Toward Net-Zero Energy Buildings

Leading-edge engineering firms in British Columbia are exploring pathways toward the construction of net-zero-energy buildings by 2020. By then, the City of Vancouver intends that all new buildings use 50% less energy than today and meet the remaining energy demand from renewable sources. Stronger energy efficiency is supported by a foundation of policies contained in the 2010 Clean Energy Act, with oversight by the BC Ministry of Energy and Mines. 

Furthermore, the Homeowner Protection Office supports research and education initiatives for the residential construction sector, including the RDH research. High-performance windows are an important element of deep energy demand reduction in high-rise residential buildings with high window-to-wall ratios. As suitable windows with low-conductance frames and triple glazing are becoming available for these building types, significant jumps in envelope performance can be achieved. RDH Building Engineering estimates that a thermally improved enclosure design with U-0.17 windows, walls with an effective R-value of about 18, airtight construction and 80% heat recovery on ventilation air can result in 50% to 90% heating energy use reductions as compared to typical current high-rise residential buildings. While not the only one, windows with superior performance are a crucial element of energy efficiency.

New British Columbia Window Standards

Over the last two years, British Columbia phased in maximum U-factor standards for new windows installed in existing and new buildings under the Energy Efficiency Act. This act started in 2009 with a 0.35 U-factor limit for windows in small buildings. In January 2011, similar standards were added for large buildings, such as residential buildings with five or more stories. These require U-0.45 for metal-framed windows and U-0.35 for non-metal windows

Efficient Windows for Multifamily Housing

On average, the windows in existing multifamily buildings are less energy efficiency than those in single-family homes. Among multifamily housing units, more than half still have single-pane windows, as opposed to less than 40% in single-family homes. Surveys by the Energy Information Administration found that only about 10% of multifamily housing units have had some or all of their windows replaced.

Why this lag in window performance among multifamily homes? In many older, centrally heated buildings without adequate temperature controls, operable windows may be the only available thermostat during the winter, making the case for efficient windows less compelling. Moreover, most occupants of multifamily homes have little say over their windows. More than 80% of multi-family housing units are rented, leaving window choices up to property owners whose incentive to maximize window performance is less than that of a homeowner. Also, capital is often in short supply for multifamily buildings with lowand moderate-income renters.

Even in owner-occupied condo buildings, the process for replacing windows requires much more than just a trip to the local lumberyard. Particularly in larger buildings, the interests of multiple parties, as well as structural and potential historic requirements, must be taken into account. Yet these barriers do not diminish the role that window efficiency plays for heating and cooling energy use and for occupant comfort.

A wide range of window types are used across the multifamily sector depending on building location, size, height, façade types and architectural preferences. Energyefficient options are available regardless of window type. In this newsletter, we introduce some energy efficiency programs such as ENERGY STAR Multifamily High Rise and the DOE High Performance Windows Volume Purchase Program, that encourage the use of energy-efficient windows across different types of multifamily buildings.

For More on Windows

This article was adapted from the Summer 2011 Word on Windows, which is the Efficient Windows Collaborative's newsletter. The Efficient Windows Collaborative is the Alliance’s consortium dedicated to increasing the market share of high-efficiency windows in both the residential and commercial sectors.