Frame Types: Manufactured Window Units
Light, strong, durable, noncorrosive, and easily extruded into the complex shapes required for window parts, aluminum can be fabricated to extremely close tolerances to create special forms for the insertion of glazing, weatherstripping, and thermal breaks. Aluminum frames are available in anodized and factory-applied high-performance painted finishes that are extremely durable and low-maintenance.
The biggest disadvantage of aluminum as a window frame material is its high thermal conductance, which raises the overall U-factor of a window unit. Because of this, the thermal resistance of an aluminum frame is determined more by the surface area of the frame than by the thickness or projected area, as with other frame materials. Thus, an aluminum frame profile with a simple, compact shape will perform better than a profile with many fins and undulations.
In cold climates, a non-thermally broken aluminum frame can easily become cold enough to condense moisture or frost on the inside surfaces of window frames. Even more than the issue of heat loss, condensation problems have spurred development of better insulating aluminum frames. The most common solution to the heat conduction and condensation problem of aluminum frames is to provide a thermal break by splitting the frame components into interior and exterior pieces, which are joined by a less conductive material. There are many designs available for thermally broken aluminum frames.
Current technology with standard thermal breaks has improved aluminum frame U-factors from roughly 2.0 to about 1.0. Innovative new thermal break designs have been combined with changes in frame design to achieve U-factors lower than 0.5, but at a higher cost than current thermally broken frames. In commercial buildings where controlling solar gain is often more important than reducing conductive heat transfer, improving the insulating value of the frame is less important than using a solar control glazing system.
Wood is a traditional window frame material because it is widely available and easy to mill into the complex shapes required to make windows. Wood is not intrinsically the most durable window frame material because of its susceptibility to rot and its high maintenance requirements, but well-built and well-maintained wood windows can have a very long life. Cladding the exterior face of a wood frame with either vinyl or aluminum creates a permanent weather-resistant surface. Clad frames thus have lower maintenance requirements, but retain the attractive wood finish on the interior.
From a thermal point of view, wood-framed windows perform well, with frame U-factors in the range of 0.3 to 0.5 Btu/hr-sq ft-°F. The thicker the wood frame, the more insulation it provides. However, metal cladding, metal hardware, or the metal reinforcing often used at corner joints can lower the thermal performance of wood frames.
Vinyl, also known as polyvinyl chloride (PVC), is a very versatile plastic with good insulating value, high impact resistance, and good resistance to abrasion. Because its color runs all the way through the material, there is no finish coat that can be damaged or deteriorate over time. Recent advances have improved PVC's dimensional stability and resistance to degradation from sunlight and temperature extremes.
Similar to aluminum windows, vinyl windows are fabricated by cutting standard lineal extrusions to size and assembling the pieces into complete sash and frame elements. Vinyl window frames require very little maintenance, do not require painting, and have good moisture resistance. To provide the required structural performance, vinyl sections are often larger than aluminum window sections, with sizes closer to the dimensions of wood frame sections. Larger vinyl units will often need to incorporate metal or wood stiffeners.
Since vinyl has a higher coefficient of expansion than either wood or aluminum, vinyl window frame profiles should be designed and assembled to eliminate excessive movement caused by thermal cycles. In terms of thermal performance, most vinyl frames are comparable to wood. Large hollow chambers within the frame can allow unwanted heat transfer through convection currents. Creating smaller cells within the frame reduces this convection exchange. Vinyl frames can also be insulated—the hollow cavities of the frame filled with insulation—making them thermally superior to standard vinyl and wood frames.
Manufacturers have begun marketing hybrid frame designs that use two or more of the frame materials described above to produce a complete window frame system. The wood industry has long built vinyl- and aluminum-clad windows to reduce exterior maintenance needs. Vinyl manufacturers offer interior wood veneers to produce the appearance of wood, and split-sash designs may have an interior wood element with an exterior vinyl or fiberglass element.
Most people are familiar with composite wood products, such as particle board and laminated strand lumber, in which wood particles and resins are compressed to form a strong composite material. The window industry has taken this technology a step further by creating a new generation of wood/polymer composites that are extruded into a series of lineal shapes for window frame and sash members. These composites are very stable, and are comparable to or exceed the structural and thermal properties of conventional wood, with better moisture resistance and more decay resistance.
Window frames can be made of either glass-fiber-reinforced polyester or fiberglass, which is pultruded into lineal forms and then assembled into windows. Such frames are dimensionally stable and achieve good insulating value by incorporating air cavities (similar to vinyl). Like vinyl, the cavities can be filled with insulation for higher thermal performance. The strength of fiberglass allows manufacturers to maintain the same sight lines as aluminum windows while achieving significantly lower U-factors. The low coefficient of thermal expansion maintains seal integrity and minimizes warpage or leakage in high inside/outside temperature differentials. Fiberglass pultrusions have a higher heat deflection temperature than vinyl, permitting the use of dark colors unlike other thermoplastic extrusions. They can be painted, powder coated, or finished with coextruded acrylic resin.
Another alternative to vinyl is extruded engineered thermoplastics, a family of plastics used extensively in automobiles and appliances. Like fiberglass, they have some structural and other advantages over vinyl but are also more expensive and have not yet captured a large market share.
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