How to Specify a Window Frame
Windows provide a host of benefits, from daylight, to views out, to natural ventilation. They also expose an otherwise solid building envelope to potentially significant energy losses and solar heat gain. Glazing aside, a quality window frame can improve a structure’s energy performance—particularly as commercial buildings increasingly resemble glass boxes. But what differentiates one window assembly from the next?
Standard materials for commercial window frames are vinyl, aluminum, fiberglass, wood, and wood clad. Vinyl, or PVC, is the most cost-effective option, with units (including glazing) ranging from €150 to €2,000, depending on quality and size.
Vinyl frames can be manufactured relatively economically and have low upkeep needs, as vinyl does not have to be painted or sealed. Extruded aluminum frames are typically chosen for their strength and durability; units can cost between €400 to €1,200 each.
Suitable for temperate climates, aluminum frames are highly conductive, which can exacerbate heat loss and gain; a thermal break helps reduce heat transfer.
Fiberglass, or fiber-reinforced plastic, boasts high durability and can be eight times stronger than vinyl, which means larger windows can have thinner and lighter frames. Ranging from €400 to €1,400 per unit, the material requires little maintenance.
Wood window frames are less common in commercial buildings due to their high cost—between €270 to €2,100 per unit—and high maintenance requirements.
A wood frame’s exterior should be painted throughout its life to preserve its look and durability. A natural insulator, wood can be suitable for projects with specific aesthetic or historical preservation requirements.
Wood-clad frames have a resilient and low-maintenance material, such as a fiberglass, aluminum, or vinyl layer, on its exterior face while leaving the wood frame exposed on the interior side. This composite window frame can provide the look of wood but at a lower cost.
If maintained properly, vinyl windows can last up to 40 years.
Two values to note for a window unit are its U-factor and air leakage rating. A lower value is more desirable for both. The U-factor indicates the rate of heat transfer through a window assembly.
U-factors generally range from 0.25 to 1.25 Btu per hour per square foot per degree Fahrenheit. Look for air leakage ratings of 0.3 cubic feet per minute per square foot of window area or less.
Typical U-factors for window assemblies made of vinyl—a nonporous and moisture-resistant material—range from 0.30 to 0.50. Insulation added into a vinyl frame’s air cavities can further reduce its U-factor.
Aluminum frames require a robust thermal break—an element with high insulating capabilities, which can be made of polyamide strips or reinforced fiberglass—between the exterior and interior faces to minimize energy loss.
Standard U-factors for aluminum windows range from 0.50 to 1.0. Wood and wood-clad frames typically have U-factors between 0.30 and 0.50. Fiberglass frames can be both durable and energy efficient, also with help from internal cavities that can be filled with insulation.
Because fiberglass comprises in part glass, its rate of thermal expansion and contraction will be similar to that of the window glazing, reducing the possibility of a broken seal.
Seals and Weatherstrips
Window seals keep insulating gases, such as argon and krypton, between double- or triple-paned windows. Multilayered windows require two seals: an inner sealant, commonly made of polyisobutylene (PIB); and a secondary sealant applied around the perimeter of the exterior glazing lite, which can be made of silicone, polyurethane, butyl rubber, or polysulfide.
Weatherstrips further reduce air leakage and heat loss, and can include tension seals, foam tape, felt strips, and tubular vinyl or rubber. Tension seals come in vinyl, bronze, aluminum, copper, or steel strips that can be installed on the inner track of a window.
This option is durable and invisible while in place, but it must be tightly installed in corners, and might make a window more difficult to open. Foam tape comes in nonporous, closed-cell foam, open-cell foam, or EPDM rubber. With its affordable price point, this option comes in a variety of lengths and thicknesses; however, it is not particularly durable and should be installed where little wear is expected, such as in fixed windows.
Felt strips are installed solo or paired with a metal strip, and can be self-adhered, stapled, or glued to a window frame. Felt strips are susceptible to water damage. Meanwhile, tubular vinyl or rubber strips come at a low-to-moderate price point and can be applied to the top and bottom edges of a window sash. This option is less discreet, but the strips come in a host of colors to help minimize its appearance.
If maintained properly, vinyl windows can last up to 40 years. The material’s weakness is its susceptibility to deterioration caused by ultraviolet (UV) light, which can also lead to fading, as well as broken insulation seals due to freeze-thaw cycles.
Aluminum frames can better withstand severe weather events, and can last for up to 20 years with regular upkeep and inspection, but they are susceptible to rusting and dents. Wood and fiberglass can both last up to 50 years.
Wood is vulnerable to rot from prolonged exposure to UV and precipitation, as well as insect damage; fiberglass will also naturally deteriorate and fade if exposed to UV light over time.
Vinyl windows do not require additional finishes and cannot be painted. The standard vinyl color is white, but other darker colors are offered at a premium. Aluminum frames are typically anodized to help delay corrosion.
Similar to vinyl, fiberglass also does not require further coating for durability; however, it can be painted in different colors to meet specific aesthetic needs. Wood frames need painting and sealing regularly throughout the product’s lifespan.
Pine is a standard wood species for windows, but premium woods like mahogany are also available.