The Basics of Window Framing

Wood. Wood windows generally have a classic appearance. Wood is a versatile material, can be painted in any color, and delivers exceptional thermal performance and energy efficiency. Softwoods have an average R-value of approximately 1.25/inch. The biggest drawback of wood is maintenance. Wood frames can have a long life span, but they are more susceptible to the elements than metal or vinyl, and consequently require ongoing repainting or refinishing. Wood must be protected from moisture because it is prone to warping, cracking, ultraviolet radiation, and rot.

Wood Composite. The wood window industry has created a new generation of wood and polymer composites that are extruded into a series of lineal shapes for window frame and sash members. These composites can compare either structurally or thermally with wood. Both, however, cannot be enhanced simultaneously. Increased density for structural performance results in lower thermal performance.

Testing indicates that composite materials may have excellent long-term durability. However, composites that are made from wood mixed with vinyl can share characteristics of vinyl, such as distortion and discoloration in hot climates. Their use for window applications is relatively new; installation in housing started in the mid-1990s.

Fiberglass. Fiberglass frames offer superior energy efficiency. Fiberglass is twice as strong as steel. While its strength and durability make it suitable for large expanses of glass, it's also virtually maintenance free. Fiberglass expands and contracts at the exact same rate as glass, reducing stress cracks and other temperature-related fractures. Fiberglass frames achieve strength without the bulk, and thus may be built with a lower frame profile like traditional aluminum frames, but with better thermal performance. However, the rigid composition of fiberglass may limit design flexibility.

Vinyl. Polyvinyl chloride (PVC) or vinyl is a versatile material with a good insulating value and available in a wide range of shapes and styles. Vinyl is not as rigid as aluminum, and is not as strong as either aluminum or fiberglass. Internal hollow chambers are often added to provide strength. These chambers also trap air, increasing the energy performance and improving the sound deadening qualities of the frame. The more internal chambers, the stronger and more energy efficient the vinyl frame will be. As with aluminum, the design of the extrusion is critical to the energy performance, structural strength, and economics of the product. Vinyl frames are generally the least expensive option.

However, vinyl expands and contracts due to heat and cold. Additionally, the damaging effects of ultraviolet (UV) light cause vinyl to eventually chalk and fade. The pigment in darker vinyl breaks down after exposure to UV light, which causes fading. Due to the likely fading problem with darker hues of vinyl, manufacturers often opt to offer lighter shades. With constant exposure to the weather, vinyl windows can become brittle with age and may not last as long as some other materials.

Aluminum. Aluminum frames are light, strong, durable, and low maintenance. The drawback of aluminum is that it causes conductive heat loss, which affects the U-factor and decreases energy efficiency. It also allows for condensation buildup, which may cause problems in the building interior. Because aluminum is about 1,000 times more heat conductive than materials like wood and vinyl, aluminum window frames are often designed using a lower profile cross-section than their wood and vinyl counterparts.

A standard aluminum frame window will allow heat to travel freely from the warm inner side of the frame to the cold exterior side. This heat loss path can be short-circuited by separating the aluminum in a section of the frame and replacing it with a non-conductive material. A thermal break separation of only one-quarter inch can substantially improve window performance. It is important that all frame parts which extend metal from inside to outside be broken and that the breaks are placed correctly to avoid thermal bridges or short circuits for heat loss. This type of frame requires additional manufacturing steps and typically costs more than standard aluminum windows. It is advisable to check the National Fenestration Rating Council (NFRC) window U-factor rating since not all thermal break windows perform better than well-designed standard aluminum products.

Aluminum Clad Wood Frames. Window frames that are wood on the inside but have a protective coating outside are known as clad. When wood frames are clad with vinyl, aluminum, fiberglass, or composites, they combine wood's insulation with the durability of man-made materials. Clad-wood products feature a wood interior that improves thermal efficiency. Thermal energy performance of properly designed wood clad products can approach the efficiency of wood windows.

While typically more costly than vinyl, aluminum cladding is stronger and more flexible. Aluminum cladding can be painted, with manufacturers offering many different colors. As another benefit, aluminum cladding is only applied to the window exterior, enabling the rich traditional appearance of wood to be visible on building interiors.

Although aluminum is a conductor of heat and cold, because the cladding does not extend to the interior, the wood minimizes the transfer of heat and cold, making the windows energy efficient. Many aluminum window manufacturers offer a higher nickel content to the aluminum, thereby dramatically reducing the risk of corrosiveness.

The aluminum cladding can be thin or thick, based on how it is manufactured. Roll-form aluminum is a thin sheet that is bent to cover and protect the exterior of the window. Roll-form aluminum isn't much thicker than a beverage can, and is easily bent and damaged. Most standard windows using roll-form aluminum use a less expensive paint coating that can easily fade, chalk, and scratch.

The aluminum used on clad windows can also be extruded. Extrusion, or forcing solid metal through an opening under compressive forces, is somewhat like forcing toothpaste through a tube. Extruded aluminum is made from heated ingots forced through a shaped opening with the aid of a powerful hydraulic press to produce a wide variety of useful products in almost any shape. Extruders maintain standard shaped dies-angles, channels, bars, tubes, pipes, among them-but custom dies can be made to any design specification.

Extruded aluminum is heavier in weight than roll-formed aluminum. About as thick as a quarter, extruded aluminum is a strong and durable cladding material. Extruded aluminum is generally more wear-resistant than roll-formed aluminum and because it is a thicker substrate, offers a more conducive surface for a thicker, more durable paint coating. Before it is painted, extruded aluminum is profiled, or contoured to fit the outline of the molding or other architectural element. This adds to extruded aluminum's ability to support a thicker, more even coating.

Complex shapes are more efficiently produced by extrusion than by other methods, such as stamped, formed, rolled, cast, welded or machined processes. That is because with extrusions, rather than joining different elements, a custom extrusion design can combine all elements into a single component in a single economical die. Extrusions also yield a stronger product that offers tighter dimensional stability. In addition, aluminum extrusions can offer architects the flexibility to design a complex profile.

Window manufacturers offer a wide variety of extruded products to meet the rising demand for unique fenestration products. Round tops, or circular windows, as well as octagons, hexagons, trapezoids, pentoids, triangles, or virtually any geometrically shaped window can add dimension and drama to a building exterior. In some cases, manufacturers offer a combination of roll-form and extruded aluminum techniques according to the function of each frame component. For example, aluminum-clad windows can be made with extruded frames and roll-form sashes. This can result in the finish on the two different substrates exhibiting different performance over time.