Window frames are available in a variety of materials including aluminum, wood, vinyl, and fiberglass. Frames may be primarily composed of one material, or they may be a combination of different materials such as wood clad with vinyl or aluminum-clad wood. Each frame material has its advantages and disadvantages, as shown in the table below.
- | Advantages | Disadvantages | How to Improve |
---|---|---|---|
Aluminum | Ideal for strength and customized window design | Conduct heat and therefore lose heat faster and are prone to moisture condensation. | Anodizing or coating will prevent corrosion and electro-galvanic deterioration of aluminum frames; thermal resistance can be improved by placing continuous insulating plastic strips between the interior and exterior frame. |
Wood | Have higher R-values, are not affected by temperature extremes, and do not generally promote moisture condensation. | Require considerable maintenance in the form of periodic painting or staining. If not properly protected, wood frames can swell, which leads to rot, warping, and sticking. | |
Vinyl (typically polyvinyl chloride (PVC) | Available in a wide range of styles and shapes, have moderate to high R-values, are easily customized, are competitively priced, and require very low maintenance. | Do not possess the inherent strength of metal or wood. | Larger-sized windows are often strengthened with aluminum or steel reinforcing bars. |
Fiberglass | Some of the highest R-values; excellent for insulating; will not warp, shrink, swell, rot, or corrode. | Relatively new and are not yet widely available. Unprotected fiberglass does not hold up to the weather and therefore is always painted. |
Spacers are used to separate multiple panes of glass within the windows. Although metal (usually aluminum) spacers are commonly installed to separate glass in multi-pane windows, they conduct heat.
During cold weather, the thermal resistance around the edge of a window is lower than that in the center; thus, heat can escape, and condensation can occur along the edges.
The following have been done to alleviate the problems associated with spacers:
General guidelines for selecting the main parameters of windows based on the climate are provided in the table below.
- | Colder climate | Moderate Climate | Warm Climate |
---|---|---|---|
U-Factor | Less than 0.33 | 0.33 | 0.33 |
Visual Transmittance | 50 percent | >50 percent | >60 percent |
SHGC | 0.4-0.55 | >0.55 | <0.4 |
UV-Protection | >75 percent | 75 percent | 75 percent |
Edge Spacers | Super Spacers | Warm edge spacers | Warm edge spacers |
Frame | Non-conductive | Non-conductive | Non-conductive |
Air leakage | <0.3 cfm/sq.ft | <0.3 cfm/sq.ft | <0.3 cfm/sq.ft |
The image below shows the improvement in window performance (R-value) with advanced window glazing. It can be seen from the figure that the super windows are losing less heat and are even making progress to gain heat instead of losing heat because of solar heat gains.