Part 1: Residential Window Science Made Easy
By Rick Reynolds
Window performance can be a complex subject filled with arcane data, impossible formulas, and myriad applications. To further complicate things, there are many types of windows, from casement, to awning, to hopper, to double and single-hung, to sliders, each with its own architectural, functional, and performance characteristics. But for the focus of this article, let’s keep it simple.
Glazing evolution: a layered approach
To begin, we all know what single-glazed windows are. Chances are, the older houses many of us grew up in had them. Often, they were windows with multiple panes, or lites, divided by glazing bars called muntins. Frequently, they were drafty. Commonly, in colder climates, aluminum sliding storm windows were added to mitigate heat loss. In effect, these were the forerunners to double-glazed windows. In addition to not being terribly efficient, these storm windows made opening, closing and cleaning windows more difficult—and in free fall, smashed many a thumb!
Most code-built houses today have double-glazed windows with the glass separated by noble (un-reactive) gasses like argon, to reduce thermal bridging. The panes of glass are often set in cork, fiberglass, PVC or foam, to further inhibit thermal bridging, and are tightly sealed to prevent condensation between the panes and drafts.
Where the look of separate panes is architecturally desirable, modern multi-glazed windows are engineered to reduce both thermal bridging and air leakage. These windows come in two forms: SDLs, (Simulated Divided Lights), and ADLs (Actual Divided Lights). The distinction is purely visual, since neither style physically divides the glass into individual panes, as in the windows of yore. SDLs have grilles resembling muntins that are either sandwiched between the glass panes or installed over the outer surface of the glass to give a divided look. Regarding the latter, ADLs add an additional “spacer bar,” in between the windows, (yet narrower than the gap between the windows), to complete the illusion of separate panes (when viewed from an angle) but without the potential for thermal bridging and air leakage.
In most climates—both hot and cold—good quality, double-glazed windows, properly installed, work well to balance energy efficiency with comfort.
In high-performance homes like Zero Energy Ready homes and Passive Houses, triple-glazed windows go even further to conserve energy and promote comfort. When these highly-insulated homes are properly sealed to Passive House standards and ventilated with energy recovery ventilators (ERVs), triple-glazed windows can effectively promote or inhibit thermal gain, depending on their size, location within the home and coatings.
A word about comfort
R-value is the measure of thermal resistance used in the building and construction industry. The higher the R-value, the greater the thermal resistance. Glass has a very low R-value relative to typical walls. In cold climates, the laws of entropy dictate that heat will radiate from a person’s body to the colder interior surface of the glass, despite a comfortable indoor air temperature. That’s why we can feel cold even though we’re cranking the heat.
Contrary to perception, the cold isn’t actually tunneling through windows and into the hands and feet. Quite the opposite. Like flies to a rib roast, our body heat is voraciously attracted to the colder window pane, all thanks to the immutable laws of thermodynamics. So, keeping the interior surface of the glass as close to indoor air temperature as possible, as is the case with double and triple-glazed windows, helps in achieving personal comfort (and energy efficiency).
This brings us to the next subject: the nature of light itself—the part of the spectrum we can see and the part we can’t. Modern windows must allow the light we see by to pass unimpeded, while at the same time, manage the infrared (IR) and ultraviolet (UV) frequencies—which we either feel as heat, or with too much exposure, can cause sunburn—but are otherwise invisible.
When sunlight is streaming through a window, a net thermal gain can occur, which, depending on the season and locale, may or may not be desirable. Various types of glass and coatings can promote or diminish the amount of infrared energy coming from the sun.
On sizzling, sunny, summer days, heat-reflecting glass, with coatings consisting of microscopic, transparent layers of metals or alloys (typically some combination of titanium, zinc, copper, tin, silver, brass and stainless steel), reflect back much of the sun’s IR (infrared) heat. Conversely, on frigid winter days, heat-reflecting glass helps prevent expensive indoor heat from escaping.
By determining where and when direct sunlight will shine on your new house over the course of a year, window size, type, placement and coatings can be optimized. Reputable design/build firms can design, orient and build homes with optimal window configurations for maximum comfort and energy efficiency. But regardless of window suitability, even the best window units must be properly installed, which is an important trade discipline requiring training and building science knowledge.
See Part II: Just Window Shopping: In our next newsletter, you’ll meet Dave Levasseur, glass master for Bensonwood and Unity Homes. Part II includes the architectural aspects of windows, including the North American and European brands we’ve used over the years and custom curtain walls of windows we’ve engineered for many projects.