Thursday, June 3, 2010

green building 101: windows

{This is the second installment of a feature called Green Building 101 where I cover a topic in green building in more depth. Since a lot of my time lately has been spent thinking about, talking about, and working with windows, that's the topic of this article.}

Windows serve two main functions in a building: they let in light and they ventilate. In addition, windows can be an important part of the solar heat gain in a passive solar house, and can also be a beautiful addition to the architectural design of a building.

In general, the functions of letting in light and ventilating are at odds with the primary function of the building as a whole — to keep the occupants comfortable, dry, and secure, protected from the elements of the outdoors that defines the reason why any of us live in buildings. Windows are at odds, because at their very essence, they are essentially holes in the walls of the building.

Historically, windows literally were holes in the walls. They might be covered with a curtain of fabric, glazed with paper, or covered with wooden latticework. But, rain, snow, cold, wind, critters, and bandits could still get in. Wooden shutters were used to lock out unwanted intruders, but had to be opened when light and fresh air was wanted in or smoke out.

It wasn't until the Middle Ages that the technology became more widely available to make sheets of glass to cover windows, and not until much later that this technology became widely available to regular homeowners. Glass was a major technological breakthrough in windows because it kept out many of the unwanted elements while still letting in light, and operable sashes could be made to hold the glass that allowed the window to open for ventilation and lock closed as desired.

In modern times, window technology has advanced dramatically from the earliest days of glazing, allowing windows to be not just the holes in the walls they once were, but insulating, solar heat gaining, ventilating contributors to the heating and cooling systems of the building.

Structurally, windows are made up of two main components: a frame and an insulated glass unit (IGU). The IGU is composed of the panes of glass, the air in between the glass panes, and the spacer that holds the whole thing together. Commonly available IGU configurations are 2-pane, 3-pane, and 2-pane with a heat mirror film, which functionally acts as a 3-pane or 4-pane window. The IGU is the part of the window that lets in light and heat, and contributes to the insulating qualities of the window. Well-made IGUs have the following characteristics:

  • \Relatively large air space
  • Inert insulating gas filling the air space
  • A well-sealed, non-thermally-conductive spacer
  • Special coatings on the glass that enhance the insulating and solar heat gain properties of the window
The air space between panes of glass is crucial for optimal functioning of the IGU. The air space needs to be large enough to provide insulation and ideally filled with inert gas that reduces thermal convection of outside air temperatures to inside space. Inert gases are the far right of the periodic table: Helium, Neon, Argon, Krypton, and Xenon. The heavier the inert gas, the more insulating it is in the IGU. Argon is commonly used in IGUs, because it is relatively inexpensive. Krypton and Xenon increase the U-value (or insulating value) of a window substantially over Argon, but are less commonly used.

At the high elevations in our area, it's nearly impossible to get inert gas filled IGUs, as the IGUs are manufactured at a lower elevation and transported up. Pressure release valves must be built into these IGUs in order to prevent the bulging of the IGU that would happen if a sealed window were transported to a higher elevation with lower air pressure. The gas inside the window expands at the higher air pressure and the gas must move somewhere. Because all the inert gas winds up escaping through the pressure release valve, inert gas is not used in our market except by one known manufacturer with a special balloon compensator system for keeping the inert gas inside the IGU.

The spacer that is used to construct the IGU is often made of steel, a relatively good thermal conductor, which is not so good for the window as the spacer then carries outside temperatures in, all around the perimeter of the glass. Some manufacturers use composite materials that are more insulating which makes for a better performing window. In any case, it's important that these IGUs are sealed properly to prevent condensation between the panes, a problem in poorly constructed windows.

Special transparent coatings — "Low-E" being the most well known of them — can be applied to any of the surfaces of the glass unit to increase the insulating factor or U-value of the window, and increase or decrease the solar heat gain of the glass, depending on the energy design of the building.

Window frames are the structural component that bears the engineering load of the window. It also is used to attach the window to the building, holds the hardware for operable units and security, and contributes to the insulating properties of the window. Frames are made of all kinds of materials including wood, aluminum, vinyl, and fiberglass. For an energy efficient house, well-made frames have the following characteristics:
  • They are well sealed to the IGU to prevent moisture and air infiltration
  • They are insulated and made of insulating materials
  • They are durable and beautiful for a long time, support the sash and IGU, and operate smoothly
Good window frames feel solid and durable and operate well. The best ones have insulation through the middle and the glass is wet-glazed in the sash.

If you're building a new house, you've got a huge number of choices for windows and it's worth shopping around. Perhaps even do an energy model of your home to see how well various windows will perform and what the return on investment will be on higher end, more energy efficient windows.

If you have an existing home and a budget for replacing old, leaky windows, an energy model is a good idea again to make sure you're buying the best window for your dollar and that it matches the aesthetic and performance characteristics of the rest of the house.

In any case, new or old, make sure that windows are installed properly. Use a bituthane tape around the window opening to prevent water from penetrating, caulk around the entire window frame before install, and use a sealant like low expansion foam or EPDM gasket to fill any cracks around the window. If the windows in your existing home are drafty and sweating, sometimes repairing the install job is a good first step to improving their performance even without replacing them altogether.

Most of all, make your windows work for you and enjoy the views and the sunshine they bring into your home. 

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