Approximately 1/3 of the heat loss or gain in a home
is through the windows.
LEED for Homes Pilot Rating System, U.S. Green Building Council
Remember when all we needed to know about windows was whether or not to choose wood or aluminum, and whether to spend the extra money for dual glazing? Well times have changed and window and patio door design is now a sophisticated science with its own set of specialized jargon. Choosing the greenest and most energy efficient window is now a much more complicated process and depends in large part on your climate, whether or not your home is going to be passively heated using the sun, and even on what direction the window is facing.
Unfortunately, in order to make an intelligent choice and compare products, you’ll need learn some of the industry jargon used to rate today’s high performance windows.
U-value – In the winter we want our windows to keep heat in the house, but in the summer we expect them to do their best to keep heat out. A window’s ability to do this via the conduction mode of heat transfer is based something called the U-value. In the U.S. the U-value is a measurement of how quickly heat conducts through the entire window assembly including the frame. The lower the U-factor, the more resistant the window is to heat transfer. For example, a single-glazed aluminum window does a great job of losing heat because it’s U-factor is pretty crummy, about 1.30. A typical, good quality, double-glazed wood window in the U.S. today does a much better job with U-values in the range of 0.30, and some of the best windows have U-values less than 0.20.
Solar Heat Gain Coefficient – The solar heat gain coefficient (SHGC) is a measure of how much solar “radiant” energy can pass from the outside through the window. If a window has a rating of 0.50, that means that the window admits 50 percent of the heat energy that strikes the window. This measurement is typically for the entire window, so the amount of solar energy or heat that gets through depends on the type of glass, low-e coatings, and the area of the frame. A typical value for a double-glazed window would be about 0.25 on the low side and over 0.50 on the high side. A 0.25 SHGC window would admit half as much solar heat energy as a 0.50 SHGC window. For passive solar designed homes, you’ll want to specify high SHGC windows on the southern exposure.
Low-e Coatings – A low-e (the “e” stands for emissivity) coating is an extremely thin, metallic layer applied by vapor deposition to the surface of the glass. The coating is thin enough to see through, and has the ability to reflect energy that would otherwise escape from the home . In a cold climates these coatings help keep the house warm by preventing the escape of infra red energy from the inside of the home. In a hot climate their role is to keep solar energy from entering the home by reflecting it back outside at the window surface. A wide variety of low-e coatings are available. Spectrally selective coatings are especially good for cooling climates where they reduce solar heat gain without blocking an excessive amount of visible light. Low-e coatings typically lower SGHC ratings, but some specialty coatings are designed for passive solar applications in colder climates and will let beneficial solar radiation in but still block or reflect longer wave infra red energy from escaping from the home.
Now that you know the basic jargon, what makes for a good high performance green window? Every window is made up of six basic elements:
- the frame
- glass panes (glazing)
- the gas between the glass panes
- the spacers that separate the glass panes
- and seals and hardware for operable window elements
High performance window frames are typically made from either rigid extruded vinyl (PVC), aluminum clad wood, or pultruded fiberglass. Vinyl affords the lowest cost but has some environmental issues with the release of VOC’s such as phthalates, the leaching of lead and cadmium fillers, and the release of dangerous toxins such as dioxin when burned. Some would argue that the cost benefits outweigh the risks, but from a “green purist” point of view its hard to make a strong argument for vinyl.
Aluminum clad wood is relatively sustainable, is a mediocre to fair insulator and esthetically pleasing, but the most expensive of the three options. Because the wood used in quality windows must be highly stable, it tends to be vertical grain, all heart wood which adds pressure on our old growth forests. Fiberglass is priced somewhere in between wood and vinyl and according to at least one independent Canadian study done for the Waterloo Region Green Home Assessment, foam filled fiberglass windows are the greenest of the three choices.
Glass or glazing is basically the same for any high performance window, but the type and quality of the low-e coating can vary with manufacturer and greatly effect both the U-value and SHGC.
The gas fill between the window panes can be air, argon, or krypton. Argon and krypton improve performance because they have lower conductivity than air. Argon is less expensive than krypton and more commonly used.
Spacers are used to separate the panes of glass and can be made from aluminum, stainless steel, or foam silicone. All spacers have a desiccant included to absorb any moisture introduced during manufacture that may condense on the inside after installation. Because metal spacers dominant the market, and metal is a great conductor, spacers are one of the weak links in a window assembly.
For operable windows like sliders, casements, and awnings, some type of rubber or other seal is used to prevent air leakage. In general, casement and awning windows perform better than sliders, because the hardware allows the window to be “cam-locked” against the seal for a more air tight fit.
So what’s best? What is the greenest window? Weighing all the factors, for my money its a foam filled fiberglass frame, dual or triple paned with one or two low-e panes, argon filled, silicone (non-metallic) spacer window in an turn & tilt, awning, or casement configuration. If you’re looking for U-values less than 0.20, the best performing windows I’ve found in North America are from a small company in Canada called ThermoTech.