Following on from last months article, window frames are an important part of a window’s overall thermal performance, and improving the thermal resistance of the Frame can contribute to a window’s overall energy efficiency, particularly its U-factor. There are advantages and disadvantages to all types of frame materials: however, window manufacturers have generally migrated from traditional wood to more rot-resistant versions, including aluminum, vinyl, fiberglass, and composite. Fiberglass, composite and aluminum are likely to be the most durable choices, but from an energy perspective, fiberglass and composite are preferable to aluminum that is highly conductive. Some manufacturers offer foam-filled fiberglass and composite frames that perform even better.
Wooden frames insulate relatively well, but require a lot of maintenance. Vinyl frames are usually made of polyvinyl chloride (PVC) with ultraviolet light (UV) stabilizers to keep sunlight from breaking down the material. Vinyl windows do not require painting and have good moisture resistance. The hollow cavities of vinyl frames can be filled with insulation, which makes them thermally superior to standard vinyl and wood frames. Fiberglass frames are dimensionally stable and have air cavities that can be filled with insulation, giving them superior thermal performance compared to wood or un-insulated vinyl. Composite frames are very stable, they have the same or better structural and thermal properties as conventional wood, and they have better moisture and decay resistance.
In addition to choosing the frame type, you will need to consider what type of glazing you should use to improve your home’s energy efficiency. Based on various window design factors such as window orientation, climate, building design, etc., you may even want to choose different glazing for different windows throughout your home.
In colder climates, south-facing windows could use high-solar-gain glazing, while east and west-facing windows that are hard to shade would use low-solar-gain units. North facing windows can be either. During the summer, when solar heat gain is less desirable, a properly sized roof overhang will shade south-facing windows during the hottest hours of the day, while allowing the lower winter suns solar heat to enter the home. In climates with significant air conditioning loads, specify windows with low SHGC values (< 0.40) and in general, high (> 70%) Glass Visible Transmittance.
Single pane glazing has very poor performance, and is generally not used in modern construction. The more popular insulated window refers to windows with two or more panes of glass. The glass panes are spaced apart and hermetically sealed, leaving an insulating air space, which primarily lowers the U-factor. Because heat conduction across an air space still contributes some heat loss, performance can be increased by replacing the air with a lower-conductivity gas such as Argon or Krypton. However, most double-glazed, sealed and insulated glazing units have air between the panes. For Prescott’s high altitude conditions, windows should support pressure relief options that prevent seal and glass damage.
Low-emissivity (low-e) coatings control heat transfer through windows. Such coatings are used to reflect the infrared (heat) and the ultra violet portion of the solar spectrum while passing the visible light. A low-e coating is a microscopically thin, metallic oxide layer deposited directly on the surface of one or more of the panes of glass through a vacuum deposition process. A low-e coating is spectrally selective, filtering out 40% to 70% of the heat normally transmitted through a window.
The diagram shows the green line passing the suns complete spectrum including the Ultra Violet and Infrared heat bands. The red and blue lines show the effect of low emissive coatings, and the black line shows an ideal, but unachievable spectrum of visual light only.
The higher a windows visible light transmittance (VT), the better. Windows with a low VT look gray and depressing and any window with a VT below 0.40 “would be almost unethical to sell as clear glass. Another important factor is the interface between a window and wall, which plays a critical role in air tightness. To provide air-barrier continuity a wet sealant between the window frame and the rough opening flashings is recommended.
Finally, when selecting windows, it’s always wise to compare warranties. The bigger the manufacturer and the better the warranty, the greater the chance that a manufacturer will stand behind its products and resolve disputes. Most big window manufacturers warrant window glass for 10 years and the frame hardware for 20.
Next time we will look at the advantages and disadvantages of high performance triple glazing.