By Jennifer Sperry
When it comes to windows, it is possible to have it all: energy efficiency, customization, longevity, aesthetics, and historic appropriateness. But because windows range in size and application, are informed by a spectrum of architectural styles, and have to contend with nature’s destructive forces and climate zones, there isn’t one clear choice in determining design and construction.
Experienced architects and contractors can guide a homeowner’s decision-making; however, upfront research helps unearth personal priorities. Whether the concern is efficiency, architectural accuracy, or environmental impact, information exchanges with industry professionals are extremely valuable. The following topics explore exactly where technology is taking windows, and the impact on traditional craftsmanship along the way.
The major problem with windows and heated/cooled interior environments is thermal transference. Glass transfers heat to the outdoors like a mug wicks heat away from freshly poured coffee; likewise, the flow reverses in warm climates.
To combat energy loss, window manufacturers have introduced double and triple glazing. Air spaces in between the panes lend insulating properties. In general, a double configuration is at least twice as energy efficient as single glazing. Triple glazing is roughly three times as efficient, but is also thick and heavy, traits that, on the whole, eliminate it from historic or new old house consideration.
One design detail that affects both aesthetics and performance is the edge spacer, an engineered element that holds glazing layers apart. Traditionally, spacers have been crafted of aluminum, but because of the metal’s conductive property, aluminum spacers transfer a fraction of the heat generated by insulation outside. In cooler temperatures, they can even lead to condensation buildup along a window’s bottom interior edge. The latest fix to these drawbacks is warm-edge spacers, designed to interrupt the heat transfer pathway at the glazing edge, improving a window’s U-factor (rate of heat loss).
On top of increased insulation, manufacturers are now offering low-emittance (low-E) coatings, virtually invisible metal or metallic oxide layers that suppress radiative heat flow. The type and placement of low-E coatings depend on climate and house design. For example, a low-E coating that allows for high solar gains is best for colder climates and homes that rely on passive solar heat.
Glazing and insulation are only as good as the wood sash around them. In 2003, Jeld-Wen unveiled AuraLast Wood, a trademarked system that protects against wood rot, water saturation, and termites. It treats wood completely through, unlike a dip treatment’s surface-deep protection, which can be compromised during installation. “AuraLast is colorless, odorless, water- based, and releases 96-percent fewer volatile organic compounds during manufacturing than traditional treatment methods,” says Jeld-Wen’s Brian Hedlund.
The good news for architectural enthusiasts is that many of the latest technologies can be incorporated into traditional wood window construction. “A 200-year-old window is virtually identical to a modern window with two main exceptions: weatherstripping and insulated glass,” explains Andy Keefe of Green Mountain Windows, a Vermont-based manufacturer of traditionally designed wood windows. “Weatherstripping can be concealed by any crafty manufacturer concerned with historic authenticity, and through modern technology, windows can be built with very thin insulated glass and still achieve an Energy Star rating.”
For a double-glazed window, the general rule is that insulated glass thicker than 1/2″ creates a modern appearance. “Your eye is drawn more toward the glass spacer material than the muntin and sash profiling,” says Keefe. “So seeking out 1/2″ insulated glass that yields a good energy rating is crucial.” Noting the superior performance of krypton gas in thinner airspaces, Keefe explains, “The 1/2″ glass with krypton will perform equal to or better than a window with 3/4″ glass filled with krypton.”
Low-E coatings are beneficial to a range of traditional designs. “Luckily the technology is evolving, and there are certain coatings with very low levels of discoloration that had, in the past, limited their use in historic properties,” says Keefe. “We often manufacture insulated glass windows for buildings in historic areas with mouth-blown restoration glass on the exterior light and low-E glass on the interior.”
The Woodstone company in New Hampshire specializes in handcrafted windows and historic replication—what head CAD technician Jay Eshelman calls “forensic engineering.” He explains: “We actually go into a home and mathematically find out what the original builder was thinking.” Equipped with mortise-and-tenon joinery and sashes that operate via weight and pulley, Woodstone’s traditional windows not only achieve and exceed modern standards, they are designed to be maintained over time, an important sustainability consideration.
“The reason we don’t do simulated divided-light windows is because of the maintenance issue. If a rock breaks the glass or a seal fails, the entire sash has to be replaced,” says Eshelman. Woodstone uses insulating glass in true divided-light configurations as long as aesthetic considerations are met: “We match the spacer color of the insulated glass to the color or finish of the window itself,” says Eshelman. “Even if you are one foot away, it’s very difficult to see the difference between single glazing and insulated versions.”
Architect Sandra Vitzthum of Vermont agrees that, with the right combination of factors, windows can convincingly blend history and efficiency. “Insulated glass is a better technology than true divided lights,” she concedes, joining Eshelman in asserting that warm-edge spacers must match a window’s final color. “I’ve used windows with white spacers and white painted muntins, and you really can’t tell that there are two layers of glass.” In her experience, insulated glass in divided-light applications calls for muntins that are too thick and inauthentic in appearance. Instead, the architect pairs true divided-light windows with an energy panel and weatherstripping.
Vitzthum warns against paying too much attention to a window’s energy efficient technologies without also considering its surroundings. “The worst place for leakage is between the window and house frames,” she says, adding that she now personally inspects each installed window at a job site for proper sealing.
Sustainability—Above and Beyond
Realizing that their wood windows rely on felled trees, many manufacturers source sustainably harvested lumber, and one reliable measurement is the The Forest Stewardship Council (FSC) certification. The council adheres to rigorous standards—its stamp assures the source was a responsibly managed forest. Yet another designation is the Sustainable Forestry Initiative (SFI) label, widely used across North America and also accepted in the global marketplace. Window manufacturers might reference one or both certifications.
While a wood’s origin is key, so is its manufacturing. Norwood, a window and door manufacturer based in Canada, uses excess scrap wood to heat 40 percent of its facility. Excess or broken glass is immediately recycled, and sawdust is collected in silos, compressed, and repurposed to heat the dry lumber kilns.
For homeowners interested in working with an architect toward a LEED certification, Vitzthum points out that wood windows are a natural and helpful inclusion. Windows with a low-embodied energy (the total amount of energy required to manufacture) and that are locally or regionally produced are beneficial for application.
In response to concerns over traditionally crafted wood windows and high cost, the architect makes an important point: “Instead of importing costly marble from Africa, save the money and indulge in locally made mortise-and-tenon windows. Spend on local craftsmanship rather than exotic things from far away. “Besides a door, which is opened and closed by hand, windows are one of the most tangible and human parts of a home,” she continues. “Putting money into their detail and quality is deeply satisfying.”
Jennifer Sperry is a freelance marketer and writer based in New Bedford, Massachusetts.Published in: New Old House Spring/Summer 2011