Today's highly energy efficient windows are "proxy" green building materials, meaning that unless the sash or frame is wood, the materials generally aren't reclaimed, recycled, or repurposed. But in the sense that they conserve energy, it means that they reduce energy consumption which in turn lowers carbon emissions by burning coal and fossil fuels.
When first considering a window upgrade, it's important to do the research and homework. The rules for qualifying for an energy tax credit in the US for windows and skylights can be tricky. It depends on several factors, including geographical location of the home, type of window frame, U-factor, and solar heat gain coefficient (SHGC).
Choose Window Frames
There's a wide selection of frames on the market. The choice depends on climate, ease of maintenance, and personal preference. Here are some to consider:
- Wood. This is the most traditional material to use. One of the greatest advantages is that wood insulates naturally. The biggest disadvantage is maintenance. It must be painted periodically to keep it from warping, cracking, and rotting. It's possible to repair window frame wet rot, but it's better to prevent it. This alone makes it a better choice for a 1-story home than multi-story.
- Vinyl. Vinyl is very popular today. This material is very affordable and durable but does better where the climate doesn't go to great extremes.
- Vinyl-clad. A much better alternative because it's more weather-tolerant, offers better insulating qualities, and can provide the beauty of wood indoors and low maintenance outdoors.
- Aluminum. This metal is lightweight and is resistant to warping and splitting and requires no painting. On the downside, contractor-grade aluminum windows are notorious for transferring heat and cold. In northern climates they sometimes have condensation issues.
Focus on Insulating Values
The U-factor is a key property to consider; it measures thermal conductivity. The lower the number, the better the insulating quality. For example, an aluminum window frame without a thermal break that contains a single glass pane has a U-factor of about 1.30. Go to double glass (air-filled) and the number drops to 0.81. Make it low-e filled with argon rather than air and the value drops to 0.64.
Why is argon-filled better than air? Simply because it insulates better because of a lower thermal transfer. Krypton insulates a bit better but is much more expensive. Low-emittance coatings, referred to in the trade as low-e, is actually a microscopic metal oxide layer on the glass surface.
The solar heat gain coefficient is represented as a number between 0 and 1. It measures the fraction of incident solar radiation that the window will allow into the home. With this property, a lower value also means better energy-efficiency. Even solar window film can help lower the number.
It's easy to see that frame materials, the number of glass panes, and the type of gas fill can be combined in many ways. Choosing replacement windows can seem overwhelming but it's important to be armed with the facts because window salesmen enjoy a reputation for being pushy (let's close this deal now!), and some might try to sell options that raise the bill but are not needed in a particular situation.
Kelly Smith - Kelly has over 30 years experience as a journeyman carpenter and 20 as a freelance writer and photographer.
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