In almost all buildings people value a strong connection to the outdoors, whether it’s a view through a window, natural daylight, or easy access to outdoor space on a nice day. Doors and windows provide those critical connections—the challenge is to have them do that without sacrificing energy efficiency or durability.
This category includes doors (both interior and exterior), windows, skylights, and curtainwall or storefront glazing systems. In addition to energy-efficient windows and doors, you’ll find revolving doors (they save a lot of energy by controlling air leakage) and solar cells embedded in glass to make building-integrated photovoltaics.
Exterior shading devices, such as awnings or shutters are covered under Exterior Specialties, and interior window treatments are under Furnishings. The louvers that are included here under Openings are those that are part of a glazing system, often fitting in between the two panes of a double-glazing system.
Energy-efficient exterior doors are usually polyisocyanurate foam wrapped in metal or fiberglass. Insulating values of R-5 to R-7 are common in doors made that way. Storefront commercial door systems are not typically as energy efficient, although revolving doors help to reduce air leakage in tall buildings.
Interior doors are typically solid wood, molded hardboard, or plywood veneers over a hollow core. Since lauan plywood comes from poorly managed rainforest wood, it should be avoided. Molded hardboard is often made with some recycled content and pressed into shape, but most hardboard is made with unhealthy urea-formaldehyde binders. While solid wood is beautiful and a natural, minimally processed product, clear stock is becoming harder to get and may come from old-growth forests.
An increasing number of manufacturers are offering doors using wood from Forest Stewardship Council (FSC)-certified sources. Salvaged doors, or doors made from salvaged wood, can also be good choices.
Since the early 1980s, the energy performance of typical windows has increased by more than 50 percent, the result of both improvements in glazing and in frame construction.
Double-glazed, low-e coating insulated glazing units (IGUs) are now standard in many parts of North America, although they are still described as “high-performance glazing.” Truly high-performance glazing systems, with a suspended clear film between the glass providing three layers of protection, are increasingly affordable, and becoming available in large units for commercial facades as well.
By careful selection of low-e coatings, windows can be “tuned” to optimize the performance of a structure—balancing heat loss, solar gain, and visible light transmission through the glass.
Use of an inert, low-conductivity gas in the space between layers of glazing is another way to improve thermal performance. Most low-e windows have argon gas fill. Some super-energy-efficient windows have krypton or a mix of argon and krypton between the glazing layers.
About window frame materials
Although standard for many years, aluminum windows are disappearing from most cold-climate markets. Vinyl frames are much better than aluminum in terms of thermal performance, but there are some environmental concerns associated with the production and eventual disposal of PVC (polyvinyl chloride). Vinyl windows vary greatly in quality. A particular weakness of vinyl windows is that vinyl expands considerably (more than wood or fiberglass) when heated by sunlight, and many consumers complain about weather-sealing problems over the life of the window due in part to this issue.
Wood windows are still the standard for energy efficiency. Vinyl or aluminum cladding adds value because of its low-maintenance qualities. Wood-window manufacturers are facing increasing difficulty in finding affordable, knot-free material from which to manufacture their product, and some are using finger-jointed material with an interior coating and exterior cladding. Other energy-efficient frame materials include fiberglass, with or without foam insulation in the hollow channels, and composites such as a combination of recycled vinyl and wood fibers.
All glass buildings are rarely the most efficient design; nor the best for daylighting—optimal window to wall ratios should be established with care. However high-efficiency curtainwall systems are available. Storefront and curtainwall systems are all steel or aluminum, both of which conduct heat well so energy efficiency in those systems is all about effective thermal breaks in their design. These systems should also be designed to serve as effective pressure-equalized rainscreens to prevent rain penetration.
The edge seals and spacers that separate the layers of glass are another important element in any window assembly. These have to be energy efficient (providing an effective thermal break) but also very durable, as failure of this seal will cause condensation inside the window (fogging), and the loss of any low-conductivity gas fill.