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Window Technologies: Frame Types - Non-metal Frames

Non-metal Frames

Wood
The traditional window frame material is wood, because of its availability and ease of milling into the complex shapes required to make windows. Wood is favored in many residential applications because of its appearance and traditional place in house design. 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. Wood is not intrinsically the most durable window frame material, because of its susceptibility to rot, but well-built and well-maintained wood windows can have a very long life. Paint protects the exterior surface and allows an easy change in color schemes.

Wood

Wood Clad
A variation of the wood-framed window is to clad the exterior face of the frame with either vinyl or aluminum, creating a permanent weather-resistant surface. Clad frames thus have lower maintenance requirements, while retaining the attractive wood finish on the interior. While vinyl and enameled metal claddings offer much longer protection to wood frames, they are generally available in a limited number of colors.

Wood

Vinyl
Vinyl, also known as polyvinyl chloride (PVC), is a very versatile plastic with good insulating value. Vinyl window frames do not require painting and have good moisture resistance. Because the color goes all the way through, there is no finish coat that can be damaged or deteriorate over time—the surface is therefore maintenance-free. Some vinyl window manufacturers are now offering surface treatments like laminates (wood veneer, paintable/stainable, maintenance free) and coatings. These products increase color selection and surface appearance options. Recent advances have improved dimensional stability and resistance to degradation from sunlight and temperature extremes.

Wood

In terms of thermal performance, vinyl frames are comparable with wood, while there are minor differences, depending on the frame construction. Small hollow chambers within the frame reduces convection exchange, as does adding an insulating material.

Hybrid
Manufacturers are increasingly turning to hybrid frame designs that use two or more frame materials to produce a complete window system. The wood industry has long built vinyl- and aluminum-clad windows to reduce exterior maintenance needs. Vinyl manufacturers and others offer interior wood veneers to produce the finish and appearance that many homeowners desire. Split-sash designs may have an interior wood element bonded to an exterior fiberglass element. We are likely to see an ever-increasing selection of such hybrid designs as manufacturers continue to try to provide better-performing products at lower cost. It may be important for a homeowner to learn about these materials from the perspective of maintenance requirements and options for interior finishes. However, it becomes increasingly difficult to estimate the thermal properties of such a frame from simple inspection. The best source of information is an NFRC label that provides the thermal properties of the overall window.

Hybrid/Composite

Composite
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 wood window industry has now taken this 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 have the same or better structural and thermal properties as conventional wood, with better moisture resistance and more decay resistance. They can be textured and stained or painted much like wood. They were initially used in critical elements, such as window sills and thresholds in sliding patio doors, but are now being used for entire window units. This approach has the added environmental advantage of reusing a volume of sawdust and wood scrap that would otherwise be discarded.

Whole Window Properties - Non-metal Frames
  Single Glazing Double Glazing
Glazing Single Double Clear Double Tint
U-factor 0.71-0.99 0.41-0.55 0.41-0.55
SHGC >0.60 0.41-0.60 0.41-0.60
VT >0.60 0.51-0.60 ≤0.40
Note: The thermal performance properties of specific glazings and frames can vary depending on product design and materials.
The results presented here are averages. Consult specific manufacturers for NFRC rated U-factors and SHGCs for products of interest.

Non-metal Thermally Improved Frames

Thermally improved windows may include a combination of features resulting in a lower U-factor such as high-performance frame design and low conductance spacers in combination with high-performance glazing.

Thermally Improved or Insulated Vinyl
While most vinyl frames are comparable to wood in terms of thermal performance, many manufacturers have developed ways to improve the insulating value of their vinyl frames even further. One option is to avoid large hollow chambers within the frame that can allow unwanted heat transfer through convection currents. Creating smaller cells within the frame reduces this convection exchange. Another option is to fill the hollow cavities of the frame with insulating material. Usually these high performance frames are used with high performance glazings.

As with standard vinyl frames, thermally improved or insulated vinyl frames do not require painting and have good moisture resistance. Because the color goes all the way through, there is no finish coat that can be damaged or deteriorate over time—the surface is therefore maintenance-free. Recent advances have improved dimensional stability and resistance to degradation from sunlight and temperature extremes.

Insulated Vinyl

Thermally Improved Wood or Composite Frames
From a thermal point of view, wood-framed windows perform well with frame U-factors in the range of 0.30 to 0.50 Btu/hr-sq. ft-°F. Although the absence of frame cavities limits the options to further boost wood frame insulating value, thermal improvements can be achieved through thicker frame design, by avoiding thermal shortcuts through metal parts, and with low conductance spacers.

Wood is not intrinsically the most durable window frame material, because of its susceptibility to rot, but well-built and well-maintained wood windows can have a very long life. Paint protects the exterior surface and allows an easy change in color schemes.

The wood window industry has also created 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 have the same or better structural and thermal properties as conventional wood, with better moisture resistance and more decay resistance. They can be textured and stained or painted much like wood. They were initially used in critical elements, such as window sills and thresholds in sliding patio doors, but are now being used for entire window units. This approach has the added environmental advantage of reusing a volume of sawdust and wood scrap that would otherwise be discarded.

From a thermal point of view, wood-framed windows perform well with frame U-factors in the range of 0.30 to 0.50 Btu/hr-sq. ft-°F. Although the absence of frame cavities limits the options to further boost wood frame insulating value, thermal improvements can be achieved through thicker frame design, by avoiding thermal shortcuts through metal parts, and with low conductance spacers.

Wood is not intrinsically the most durable window frame material, because of its susceptibility to rot, but well-built and well-maintained wood windows can have a very long life. Paint protects the exterior surface and allows an easy change in color schemes.

The wood window industry has also created 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 have the same or better structural and thermal properties as conventional wood, with better moisture resistance and more decay resistance. They can be textured and stained or painted much like wood. They were initially used in critical elements, such as window sills and thresholds in sliding patio doors, but are now being used for entire window units. This approach has the added environmental advantage of reusing a volume of sawdust and wood scrap that would otherwise be discarded.

Thermally Improved Wood
Whole Window Properties - Non-metal Frames
  Single Glazing Double Glazing Triple Glazing
Glazing Double HSG Low-E Double MSG Low-E Double LSG Low-E Triple HSG Low-E Triple MSG Low-E Triple LSG Low-E
U-factor 0.23-0.30 0.23-0.30 0.23-0.30 ≤0.22 ≤0.22 ≤0.22
SHGC 0.41-0.60 0.26-0.40 ≤0.25 0.41-0.60 0.21-0.40 ≤0.20
VT 0.51-0.60 0.51-0.60 0.41-0.50 0.41-0.50 0.41-0.50 ≤0.40
Note: The thermal performance properties of specific glazings and frames can vary depending on product design and materials.
The results presented here are averages. Consult specific manufacturers for NFRC rated U-factors and SHGCs for products of interest.