Home improvement projects can serve several purposes. You might want to fix something that has been a problem, or make part of your house more comfortable, or reduce the time and money you spend on maintenance, or increase the resale value of your house, or save on heating and cooling expenses over the long run. Energy-efficient windows help you achieve all those goals at once.
Seeing the Benefits
No matter how large your house is, or how small, windows account for an important part of your heating and cooling costs. Because most window surfaces have lower insulating values than the surrounding walls, and because the frames around windows are often poorly sealed, you can lose as much energy from all your closed windows as if one was left wide open.
When only a few companies made windows with multiple panes, low-emissivity coatings, exterior cladding, and so on, shipping could make up half the price of a window. They are large and heavy, and many have to be custom-built to fit a particular opening. But because
energy-efficient windows benefit homeowners in every climate, they have become very popular for both new construction and remodeling projects. Manufacturers and distributors now offer them across the country.
Few other home improvements yield as many benefits as energyefficient replacement windows. In addition to savings on heating and cooling, and noticeably greater indoor comfort, new windows add to the resale value of a house. In fact, the increase in resale price is often greater than the price of installing the new windows.
Common Features
Every major window manufacturer offers energy-efficient options. Each has its own set of design characteristics, but most models share the following properties:
- Insulating frame materials: Newer wood composites and synthetic lumber provide strong frames with better insulating qualities than traditional wood frames.
- Multiple panes: Two layers of glass will reduce heat transfer, and three can provide both added thermal insulation and soundproofing.
- Thermal spacing: A gap between the panes further improves insulation. The material used as a spacer can also help reduce heat transfer and condensation.
- Gas fills: Inert gases like argon or krypton between the panes will provide better R-values than regular air.
- Low-emissivity glass: Outer surfaces treated with lowemissivity coatings allow light to pass through, but reflect infrared and ultraviolet light. This keeps heat out in the summer and during winter, and protects your furniture and artwork from sunlight.
Estimating the Costs
With many projects intended to reduce energy consumption, the costbenefit ratio is relatively small. Weather stripping and caulk may be inexpensive, but they take lots of time and labor to install. Because the gains are hard to see, the effort is harder to make. Replacing an entire window requires much more work than sealing the edges of an existing one, but the benefits are immediate and obvious.
According to research figure from Lawrence Berkeley National Laboratory, if you install energy-efficient windows in San Diego, which has a warm and stable climate, you can save nearly 4 million Btu per year, which translates to $65 in current heating costs. That makes the payback time fairly long—unless energy prices go up. In Great Falls, Montana, where the temperature and humidity change dramatically over the course of a year, an energy-efficient window can bring savings of roughly 62 million Btu, or $695 per year. If energy prices rise, that figure will go higher as well. The payback time would be almost immediate.
Comparing Products
Building supply centers, lumberyards, and specialty stores can all stock energy-efficient windows, but comparison shopping can be difficult. Few retailers carry more than one brand, which means you have to visit several places to check one set of windows against
another. And while the common features listed above apply to most replacement windows, manufacturers try to set themselves apart by giving similar products different names, and emphasizing the qualities their windows offer while minimizing any weak points.
To see how different windows compare, use the current standard for technical information—the Energy Star rating system from the U.S. Environmental Protection Agency (EPA). It will help you check the measurable differences in performance among all products that qualify for Energy Star labeling. These tests and measurements are set by a nonprofit industry group called the National Fenestration Rating Council (NFRC).
Measuring Performance
Any window that qualifies for the Energy Star designation must include information on the following characteristics:
- U-Factor: This tells you the rate of heat transfer, which indicates the level of insulation provided. Unlike R-values, where higher numbers mean better performance, you will
want to find windows with the lowest U-Factor ratings. - Solar heat gain coefficient: This is a measure of how much heat from sunlight gets through the window and into your house. Values range between zero and one; again, lower numbers mean better performance.
- Visible transmittance: Similar to solar heat gain coefficient, this is a measure of the light each window allows to pass through. These values also range between zero and one. The difference is that you generally want less heat gain but more light transmittance.
- Condensation resistance: In locations where differences in humidity cause excessive condensation, this rating becomes very important. Ranging from 0 to 100, this figure indicates a window’s resistance to condensation; higher ratings mean greater resistance to water buildup.
- Air leakage(AL): This figure shows how much air gets past the window in a set period: cubic feet of air per square foot of window per minute (cfm/ft2). Lower values mean less air leakage. Most building codes require an AL value of 0.3 cfm/ft2.
