Windows: The weakest link
Upgrades can do much to improve a building's airtightness.
October 1, 2019 By Cindy Macdonald
While floor-to-ceiling views of a city skyline are great for bringing in lots of natural lighting, they can be a headache from an operations point of view. In terms of heating and cooling, windows and frames are generally the poorest-performing components of the building envelope.
“Heat loss takes the path of least resistance,” says Elyse Henderson, a Vancouver-based energy and sustainability analyst with RDH Building Science. “In general, windows have worse thermal performance than opaque walls. Upgrading them and improving overall building airtightness are generally the biggest-impact solutions for saving heating—or cooling—energy.”
These two solutions are directly related, as upgrading windows can do much to improve a building’s airtightness.
From U-value to R-value
A building envelope’s performance depends on its insulating properties. The thermal performance of windows is generally presented as their U-value. When it is instead expressed as the more familiar R-value, the insulating performance of older windows is typically only around two.
“So, when you put an R2 window in an R20 or R30 wall, guess what the worst-performing part of the wall is?” says David Heska, Southwestern Ontario building sciences director for Montreal-based engineering consultancy WSP.
In response to this issue, RDH’s Henderson points out, the retrofit market is seeing the emergence of more energy-efficient glass, higher-performing framing systems and greater attention paid to proper installation practices.
‘Solar control’ and low-emissivity (low-e) coatings have been greatly improved over the past 15 years in terms of durability and effectiveness, while new ‘dynamic’ or ‘smart’ coatings can even adjust their tint in response to ambient lighting conditions, so as to block or transmit more heat. Simply upgrading from single- to double-glazed insulating glass units (IGUs) can more than double performance.
“As soon as you upgrade to an IGU, you’re going to improve your thermal performance during the cold months,” adds Stéphane Hoffman, principal and vice-president (VP) of building science analytics for engineering consulting firm Morrison Hershfield.
When considering a window upgrade, building owners and managers have the option of (a) replacing just the glass, (b) replacing both the glass and the frame system or (c) simply renewing the existing weather-sealing components to improve air- and water-tightness.
The foggiest clue
Fogging of double-glazed windows is a strong indicator of when they need to be replaced, as are ongoing water penetration, unusually high energy costs and occupant comfort complaints (e.g. too hot, too cold, too much glare.)
“When fogging starts to happen on a frequent basis, the windows are at the end of their service life,” says Morrison Hershfield’s Hoffman.
The typical life cycle for double-glazed windows is considered to be 20 to 30 years. Fogged IGUs cannot be effectively repaired.
Water infiltration is also an indicator of deterioration. In response, the facility manager may opt to renew the weather seals and sealants, which can provide another five to 10 years of service life for only about 20% of the cost of replacing the glass—but WSP’s Heska cautions there will likely not be any energy efficiency benefits, unless the building has suffered significant air leakage, too.
Multiple benefits of upgrades
Al Jaugelis, senior fenestration specialist with RDH, says advances in glass coatings—particularly in triple-pane glass units, where two coatings can be applied—have yielded the biggest improvements in windows’ energy efficiency performance.
“Low-e coatings are used to control solar heat gain and reduce cooling loads,” he says. “They are also the most effective measure to minimize indoor heat loss during the heating season.”
In the past, many buildings used tinted glass to reduce solar heat gain, so the amount of visible light transmittance was significantly less. Morrison Hershfield’s Hoffman says newer, highly transparent low-e coatings “can help reduce lighting needs and improve occupant happiness.”
Indeed, occupant comfort is an indirect benefit of a window upgrade.
“Cold spots by windows and unwanted solar heat gains through old glazing units are a key contributor to occupant discomfort, as well as overworked HVAC systems,” says RDH’s Henderson. “You have better control of indoor temperatures with high-performance glazing products.”
In fact, by reducing heating and cooling demand, window replacements may even allow building operators to downsize mechanical equipment. Other benefits for commercial office towers and multi-unit residential buildings (MURBs) include improved esthetics, which in turn can increase property value.
Alternatives for curtain walls
Another factor to consider is the nature of the existing building envelope. For buildings from the 1980s or later, C3 Specialty Glazing Solutions president Rob Wood says installers can likely just change out the glass panes from the curtain wall, whereas for older buildings, both the glass and the frames will often need to be replaced or at least modified to accept IGUs.
C3, for its part, is currently working on a 1970s-era office building at 60 Bloor Street West in Toronto, where the replacement of the curtain wall is part of a larger, LEED Gold, Class A renewal effort.
For this building, C3 is installing a unitized double-glazed curtain wall over the original single-glazed window system. Later, the original glass and portions of the frame will be removed from the interior of the building. The project also involves removing a portion of the existing metal spandrel panels. The new curtain wall is expected to increase the vision area by more than 35% and yield significantly better energy performance.
Another window replacement project with substantial energy-saving implications is the renewal of the Ken Soble Tower in Hamilton. Dating from the late 1960s, this MURB is being upgraded to the Passive House standard to reduce its greenhouse gas (GHG) emissions by 94 per cent. WSP’s Heska says the retrofit will include triple-glazed IGUs with an R-value of six.
Triple-glazed IGUs have not yet seen widespread adoption in the Canadian market, but their presence is growing, thanks in part to forward-looking efforts like British Columbia’s Energy Step Code.
Dynamic glazing, which features an adaptive coating, is also beginning to make inroads in the Canadian market. This type of ‘smart’ coating allows property managers to control transmittance of light and heat by making the glass lighter or darker at different times of day.
“This is a much more expensive technology,” says Heska. “We’re seeing it on newer construction.”
Another emerging technology of greater relevance for historic buildings is vacuum-insulated glazing, which yields a dual-pane unit of the same thickness as single-pane glass (i.e. 6 mm). Previously, upgrading from a single pane to a double-glazed IGU was hampered by the space limitations of the frame, but this way, double-glazed performance is available in a unit that fits an existing frame.
“It is in the early stages of commercialization, but has a lot of promise,” says Morrison Hershfield’s Hoffman.
Part of a system
While considering all of these options for window upgrades and planning a retrofit, of course, it is important in the context of energy management to evaluate the entire building—including walls, roof, lighting and HVAC—as one system.
“Don’t treat a window replacement as isolated,” says RDH’s Jaugelis. “It does have huge energy implications.”
Cindy Macdonald is a business writer. She can be contacted at (905) 550-6663.
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