Published on December 17th, 2007 | by Stephanie Evans2
Energy Efficient Windows for Big Energy Savings
If windows are the eyes of your home, and eyes are the windows to a soul, shouldn’t your home’s windows be clean and green? This blend of old adages highlights the benefits of installing energy efficient windows from a design perspective, though resulting energy savings for the environment and cost savings for the owner are probably reason enough!
The most widely discussed benefit of green, energy efficient windows is the minimization of heat loss in the winter and heat gain in the summer, though this is certainly not the only aspect to consider:
- Several quantifiable energy efficiency aspects to windows include: the frame, insulated glass material, glass spacers, and correct installation.
- Also in play are aspects that are not so easily quantifiable, such as the window size and solar orientation (which direction the window face).
- Long-term maintenance and aesthetics are two features frequently lost in the blur of manufacturers’ technical specifications and government code requirements, though they contribute significantly to a window’s actual sustainability factor.
Let’s take a window apart, piece by piece, for some "eye of the home" soul-searching.
Your home’s windows have tremendous visual and physical impact, both on the inside and the outside of your home, and . . . . if windows are indeed the eyes of your home, they can, through the use of sustainable and eco-friendly options, look out on a greener, cleaner, more energy efficient world.
Wood, fiberglass, aluminum, or vinyl? Each one of these options can be called "green" and that title can be defended with evidence depending on which manufacturer’s website you read. However, research indicates there are equally compelling arguments against the eco-friendliness of each. Here’s a simple breakdown:
- Wood frames are energy efficient, though they will cost you quite a few pennies at the outset and in regular maintenance costs. They are relatively easy to repair if broken, and you can ensure a sustainable product by purchasing from a certified supplier.
- Fiberglass frames are energy efficient. The initial cost is high but the maintenance is little to none. The sustainability factor is somewhat in question, as these can be difficult to source.
- Aluminum frames do not rank especially high as an energy efficient option because they are heat conductors—the flow of energy is not as regulated as with other framing products. They are quite cost effective and it doesn’t take much to maintain them.
- Vinyl (also called PVC, uPVC and PVCu), while popular for its low cost, low maintenance, resistance to moisture, and energy efficiency, is now raising concerns about the release of highly poisonous chemicals, such as cadmium, during the production process and potential off-gassing.
As more manufacturers claim that their products result from green production processes, a life cycle assessment (LCA) is a good way to test the merits of those claims. An LCA is an objective evaluation of the environmental benefits and burdens associated with a product, process, or activity. LCAs identify:
- the energy and materials used in creating a product
- the wastes released to the environment during production, transit, use, and ultimate disposal of a product
As you consider a product for purchase, try to put it in LCA perspective. For instance, all framing materials (with the exception of wood) tout low-maintenance as a significant factor of greenness. Consider this as we evaluate part of a window frame’s life cycle:
- Wood frame windows do typically require painting or sealing approximately every five years, which potentially releases detrimental volatile organic compounds (VOCs) from the paint, stain or sealant into the surroundings.
- However, properly maintained, high-quality wood windows have a life expectancy four times greater than vinyl (60-120 years according to some sources).
- Despite the "never needs painting" claims of vinyl and aluminum window manufacturers, their colors do degrade over time (currently 25 years is the longest and most expensive guarantee). Worst of all, they cannot be repaired if needed—the entire window must be replaced.
As anyone with an old home knows, heat is transferred from the inside to the outside during the winter. During the summer, heat is transferred from the outside to the inside. With windows, this happens either by conduction, as heat is transferred through materials touching one another, or by convection, as gases or liquids circulate to transfer thermal energy.
Double pane, or insulated, windows are the most energy efficient options. They consist of panes of glass separated by spacer bars with a sealed air gap in between. Since air is not the best heat conductor, the gap acts as a buffer that slows the transfer of heat and provides sound insulation. Previously, the air gap was filled with air or dry nitrogen but in recently made models, it is filled with argon or krypton gases, which have low conductivity values that increase thermal performance.
Triple and even quadruple pane windows were popular for a time but due to high cost and hefty weight issues, double panes are now the standard, especially with the introduction of low-emittance coating technology, or low-e. Low-e is a simply a microscopic layer of reflective metallic coating applied to the panes so that heat is reflected either into or out of a building.
Low-e retains the energy savings that come with tripling or quadrupling the number of panes while conserving cost and weight—it is said to weigh 50% less than triple pane options.
Spacers are used in between the layers of glass in a window to hold them together at the required distance. Spacers were formerly comprised of aluminum but are now made from more efficient materials such as reinforced thermoplastic, vinyl, or fiberglass.
Orienting Your Home’s "Eyes to the World"
Although states differ in their standards, California’s Title 24 of the Building Code requires that an analysis of heat transfer (based on the amount and type of glazing and solar orientation) be performed as part of the application for a building permit. While the state claims to have saved more than $56 billion in electricity and natural gas costs since the program’s implementation in 1978, it is nearly impossible to accurately declare either the real cost or savings to homeowners.
Depending upon who performs the analysis, estimates may include calculations that do not accurately state solar orientation, such as calculating a north-west facing window as due west, or calculations that do not include reduction for nearby shade trees, which increases the stated potential solar gain significantly. For more information about orientating windows for optimal efficiency, check out Daylighting for Green Homes and Spaces.
If you live in a neighborhood governed by a Homeowner’s Association (HOA), energy-efficient windows that look different from the rest of the neighborhood will likely require HOA permission that may or may not be granted. While the United States Green Building Council‘s Leadership in Energy and Environmental Design (LEED) program promotes and operates the nationally accepted Green Building Rating System, many HOAs are slow to catch on. The green housing movement is slowly making more friends in regulatory bodies at all levels, but it is not likely that an HOA, unless very recently established, includes aesthetic allowances for LEED building materials. It is important to check with your HOA before making any costly changes to your home, even if changes are made with good intent for the best of reasons.
Your home’s windows have tremendous visual and physical impact, both on the inside and the outside of your home, and are most certainly a significant economic investment. Anthropomorphized or not, the spirit of your home is reflected by the decisions you make as a consumer. If windows are indeed the eyes of your home, they can, through the use of sustainable and eco-friendly options, look out on a greener, cleaner, more energy efficient world.