UPDATED on March 18, 2015
The common perception that windows are “energy holes” is a bad rap. Since today’s high-solar-gain triple-glazed windows gather more heat than they lose, good windows perform better than an insulated wall. After all, a wall can only lose energy, while windows can gain energy during the day to balance energy lost at night.
The Canadian window-rating system
In Canada, windows are rated according to the ER (Energy Rating) method. For those who live in cold climates, the Canadian ER system is arguably an easier-to-understand method of rating windows than any system used in the U.S., where NFRC window labels cause a fair amount of head-scratching.
According to a Natural Resources Canada document explaining the original ER rating system, “A window’s ER rating is a measure of its overall performance, based on three factors: 1) solar heat gains; 2) heat loss through frames, spacer and glass; and 3) air leakage heat loss. A number is established in watts per square meter, which is either positive or negative, depending on heat gain or loss during the heating season.”
Under the original ER rating system, windows with a negative ER were “energy holes,” while windows with a positive ER acted like heaters. Poorly designed windows had ER ratings as low as -25, while the best-performing triple-glazed windows had an ER of about +1 (for operable windows) or +8 (for fixed windows).
Window manufacturers weren’t happy to discover that many of their windows ended up with negative ratings. Canadian authorities decided to respond to manufacturers’ complaints by instituting a type of grade inflation: 40 points were simply added to every window’s old ER score. With a stroke of the pen, -10 became 30, and 1 became 41.
South-facing windows produce the most energy
Consider a 1-square-meter fixed window equipped with the best available triple glazing installed in southern Canada. Over the course of a heating season, this window will, on average, perform like an 8-watt heater. This level of performance is an average based on all four orientations; if the window faces south, it will contribute much more than 8 watts.
Good residential designers shouldn’t be choosing window sizes without performing energy calculations. To get an idea of the necessary arithmetic, consider this summary from Jesper Kruse, an energy consultant in Greenwood, Maine: “I’m working on a project for a future Passive House in Maine. … I’m using triple-pane Thermotech windows with a low-e coating on surfaces #3 and #5, 90% argon, 10% air filled. On my south-facing wall I’m using glazing with a solar heat-gain coefficient (SHGC) 0.61 and a [metric] U-value of 0.9 [equal to a U.S. U-factor of 0.16]. For a total area of 12.7 square meters (around 110 sq. ft.), my total heat losses [attributable to south-facing windows] are 2,005 kWh per year, and gains are 3,880 kWh per year — a net gain of 1,775 kWh per year.”
North-facing windows need high-solar-gain glazing
Since American window manufacturers prefer to sell the same kind of glazing from Florida to Maine, it’s hard to buy high-solar-gain glazing in this country. That helps explain why few designers realize that high-solar-gain glazing is the best cold-climate choice — even for north windows.
According to Morgan Hanam, the manager for window services at Enermodal Engineering in Ontario, “In Canada, even if windows are evenly distributed between the north and south, it’s still worth putting in high-solar-gain windows on all sides – even considering air-conditioning loads.”
The reason is simple: even if sunlight never strikes a window directly, a north-facing window can gather a little bit of heat from reflected and ambient light. “We get a lower heating energy cost with a high-solar-gain low-e glazing on the north rather than the intuitive choice — the better insulating, lower SHGC low-e,” explained Stephen Thwaites, the technical director of Thermotech Windows. “Interestingly, the answer is the same when we use London, England, weather data instead of Ottawa, Canada weather data.”
Although high-solar gain glazing beats low-solar gain glazing for north windows, the amount of energy involved is small. Thwaites admits that it’s “a pretty minor difference.”
Can north-facing windows be net energy gainers?
In theory, if a window can be designed with a very low U-factor and a high enough SHGC, even a north-facing window can gain more energy than it loses.
Almost two decades ago, Dariush Arasteh, a staff scientist at Lawrence Berkeley National Laboratory, started calling high-performance triple-glazed windows “superwindows.” Along with co-authors Brent Griffith and Paul LaBerge, Arasteh produced a 1994 paper, “Integrated Window Systems,” that predicted, “Highly insulating windows with moderate solar heat gain coefficients will transmit more useful solar radiation during the day than they will lose, even during cloudy days, on north orientations, and with significant sky obstructions.”
A year earlier, Arasteh wrote “Monitored Thermal Performance Results of Second Generation Superwindows in Three Montana Residences,” in which he stated, “Simulation studies have shown that highly insulating windows with moderate solar transmittances (R-values greater than 6 [in other words, U-0.166 or less] and shading coefficients greater than 0.5) can outperform insulated walls on any orientation, even in a northern U.S. climate. Such superwindows achieve this feat by admitting more useful solar heat gains during the heating season than energy lost through conduction, convection and infrared radiation.”
Unfortunately, Arasteh was unable to verify his energy calculations with field measurements because he couldn’t locate or build any windows meeting his rigid specifications. Arasteh wrote, “Testing of first generation superwindows in three new homes in northern Montana during the winter of 1989-1990, reported in an earlier study, indicated that the glazed areas of superwindows” — that is, the windows not including the frames — “can in fact outperform insulated walls on obstructed off-south orientations. However, this same study also showed that further improvements in the thermal performance of window edges and frames are necessary if the entire window is to outperform an insulated wall.”
To sum up, good triple-glazed windows facing south, east or west gain more heat than they lose during the heating season in a cold climate. Windows facing north may lose just a little more heat than they gain — at least until window manufacturers develop a window meeting Arasteh’s specs. As it turns out, Passivhaus-certified windows from Europe may already be able to meet Arasteh’s threshold.
Perhaps a little gain, perhaps a little loss
Although the possibility that north-facing windows could be energy-positive is intriguing, the potential energy gains, if any, would be small. Whether a north-facing window with excellent glazing is slightly energy-positive or slightly energy-negative will depend on several factors, including the glazing specifications, the building’s location, and the time frame under investigation. A north-facing window will perform better over a seven-month time frame (say, from October to April) than over a four-month time frame (say, November to February). Of course, the number of months that a building requires space heat varies with climate and with building envelope quality.
According to calculations performed by energy consultant Marc Rosenbaum, a north-facing window with excellent glazing in Boston will be slightly energy-negative from November 1 to February 28. If the window has what Rosenbaum “calls super-duper German glazing” — U-0.11, SHGC 0.51 — each square foot of glazing (not considering frame performance) will gain 9,894 BTU and lose 10,608 BTU during the four months under consideration. Rosenbaum notes that these calculations do not include an adjustment of the SHGC for off-angle radiation.
A house without walls
If it’s possible to buy windows that perform better than an insulated wall, why not design all-glass houses?
In addition to the obvious drawbacks — triple glazing is expensive, and most people feel that a glass house doesn’t provide enough privacy — there are two basic problems with the all-glass house idea: overheating on sunny days, and excessive heat loss on cold nights.
Although good windows gather more heat than they lose, the gains and losses happen at different times. In a well designed house, this isn’t a problem, because the house includes enough thermal mass to absorb heat gained during the day, and because the amount of glazing is appropriate — not too much, not too little.
Although interior thermal mass can help dampen the see-saw effect between daytime heat gain and nighttime heat loss, there is an upper limit to the amount of effective thermal mass that can be included in a passive solar house. Generally, thermal mass thicker than 4 inches won’t absorb useful amounts of heat during the time frames under discussion.
The Canadian ER window-rating system assumes that no heat gains are wasted. Although this is a reasonable assumption in a well designed house, it won’t be true in a house with excessive glazing.
Anyone who has ever lived in one of the experimental passive solar houses built during the 1970s knows that houses with too much south-facing glazing overheat on sunny afternoons in February and March, forcing the residents to open the windows to stay comfortable. Of course, once the windows are opened, some of the heat gathered by the windows is wasted.
Fortunately, the principles of passive solar design are now better understood than they were in earlier decades, so newer passive solar homes are much less likely to overheat.
The bottom line
Although windows were “energy holes” forty years ago, the best modern windows are now energy assets. They not only provide a view and a method of ventilation — they also help heat your house.
Last week’s blog: “Roofing and Siding Jobs Are Energy-Retrofit Opportunities.”