If you have old windows, they are likely losing large amounts of energy through the frames and glazing. By upgrading old windows, you can reduce heating and cooling costs in your home.
Windows come in a number of different frame and glazing types. (The frame is the perimeter assembly of the window unit, that is attached to the building in installation, and holds the sash and glazing in place.) By combining an energy-efficient frame choice with a glazing type tailored to your climate and application, you can customize each of your home’s windows.
Types of Window Frames
When windows are designed and made, 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, but vinyl, wood, fiberglass, metals with designed-in thermal breaks and some composite frame materials provide good to excellent thermal resistance.
Wood window frames insulate relatively well, but they also expand and contract in response to weather conditions. Wood frames also require regular maintenance.
Composite window frames consist of composite wood products, such as particleboard and laminated strand lumber. These composites are very stable, they have the same or better structural and thermal properties as conventional wood, and they have better moisture and decay resistance.
Vinyl window frames are usually made of polyvinyl chloride (PVC) with ultraviolet light (UV) stabilizers to keep sunlight from breaking down the material. Vinyl window frames do not require painting and have good moisture resistance. The hollow cavities of some vinyl frames can be filled with insulation, which makes them thermally superior to standard vinyl-over-wood frames. Vinyl millwork products have been around a fairly long time, at least since the postwar rebuilding period in Europe.
Alumunum window frames are very strong, light, and almost maintenance-free. If a frame is all-metal, the window frames conduct heat very rapidly, which makes metal by itself a very poor insulating material. To reduce heat flow and the U-factor, metal frames should have a thermal break — an insulating plastic strip placed between the inside and outside of the frame and sash. Most windows for residential uses that are referred to as aluminum are actually aluminum cladding over wood. Many aluminum windows from the major makers are of very high quality.
Fiberglass window frames have been making large gains in the marketplace recently. They are dimensionally stable and have air cavities that can be filled with insulation, giving them superior thermal performance compared to wood or uninsulated vinyl. Fiberglass, which is a composite, is stronger than vinyl, and is often more dimensionally stable. They share the advantage of vinyl and aluminum windows that they are low-maintenance; they may be painted but do not have to be; and manufacturers offer a variety of colors.
Types of Window Glazing
In addition to choosing a frame type, you will need to consider what type of glazing or glass 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 types of glazing for different windows throughout your home.
Insulated window glazing refers to glass with two or more panes of glass within a window sash. To insulate the window, the glass layers are spaced apart and hermetically sealed, leaving an insulating space filled with air or other gases. Insulated window glazing primarily lowers the U-factor, but it also lowers the SHGC. The thickness of the insulated window glass panel, as well as the way in which it is bonded or sealed at the perimeter, should also be considered. Typically the space in insulated glass is filled with air.
To improve the thermal performance of windows with insulated glazing, some manufacturers fill the space between the panes with inert gas — commonly argon or krypton — that has a higher resistance to heat flow than air.
Low-emissivity (low-e) coatings on glazing or glass control transfer of radiant heat through windows with insulated glazing. Windows manufactured with low-e coatings typically cost about 10% to 15% more than regular windows, but they reduce energy loss by as much as 30% to 50%.
A low-e coating is a microscopically thin, virtually invisible, metal or metallic oxide layer deposited directly on the surface of one or more of the panes of glass. The low-e coating lowers the U-factor of the window, and different types of low-e coatings have been designed to allow for high solar gain, moderate solar gain, or low solar gain. A low-e coating can also reduce a window’s VT unless you use one that’s spectrally selective. VT is…
Low-e coatings are almost always applied during manufacturing. If your windows do not have a coating providing effects you want, if you are a ‘do-it-yourselfer’ you can look into applying window films. These films while not cheap are inexpensive compared to total window replacements, last 10 to 15 years without peeling, save energy, reduce fabric fading, and increase comfort. Consider other conventional window treatments as well.
Even the most energy-efficient window must be properly installed to ensure energy efficiency. Therefore, it’s best to have a professional install your windows.
Window installation varies depending on the type of window, the construction of the house (wood, masonry, etc.), the exterior cladding (wood siding, stucco, brick, etc.), and the type (if any) of weather-restrictive barrier. Some of these factors affect the exterior details of the window that is best suited for your building as well.
Windows should be installed according to the manufacturer’s recommendations and be properly air sealed during installation to perform correctly. To air-seal the window, caulk the frame and weatherstrip the operable components.
Learn more in Part 3 of our Energy-Efficient Windows series in our June 15th Issue of Green Energy Times.