A Comfortable and Energy Efficient Home through Science – Part 1

I am a big believer in using principles of science and an understanding of the sun’s movement through the sky to enhance your comfort while also making it a more energy efficient home. I am going to devote the next few blogs to giving readers some simple tips whether they are remodeling or building a new home. Today I will discuss window selection.

Energy Efficient Window Glass

Select your windows individually for each orientation of your home. While the window frame material and design, i.e. modern, traditional, plantation style etc. can be the same on all sides, don’t choose the same glass. The window glass or “glazing” needs to be fine-tuned for each cardinal direction. This is because the sun’s rays vary in angle and intensity with the orientation and time of day.

Energy Efficeint Home – Efficient Window Selection

Here in the San Francisco Bay Area where we have cool mornings, east facing windows that let in the sun’s warmth will be a welcome feature. The west facing windows on the other hand should have glazing that limits heat gain and glare as much as possible. These characteristics can be achieved by checking the “U” value and the “SHGC” (solar heat gain coefficient) of the glazing.

The lower the U value, the better insulating the window. This is achieved by the low-e coating on the surface of the glass. Low-e glass is a good idea for all orientations and all climates in the US. Look for the glazing with the lowest possible U Value for all sides – N, S, E or W. When checking SHGC on the other hand, select the lowest possible SHGC for the West side. The East can have higher SHGC values to help with the cold mornings. Ultimately you will have to balance U value & SHGC with the VLT (visible light transmittance). The higher the VLT, the clearer the window but also the greater the heat gain. Limit high VLT glass (i.e. 70 and up) for a limited few “picture windows”. You can safely go as low as a VLT in the range of 50-60 without hurting your view.

Energy Star Criteria for windows

Window Frame

Keep in mind the energy and maintenance consequences of the window frame material. Aluminum frames offer the benefit of sleek profiles but due to the high thermal conductivity of Aluminum, they are really bad for the energy efficiency and comfort of your home. Vinyl is great for durability and energy efficiency, but PVC production has serious human health and environmental impacts. All wood windows are beautiful but need lots of maintenance leaving aluminum clad wood windows as the best alternative for an energy efficient home.

Thermally Broken Window Frame

Always insist on a “warm edge” spacer. Glazing spacers are the weather seal between the panes of glass in a dual pane window. Warm edge spacers are “thermally broken” to retard heat transmission and are a little more expensive than conventional aluminum spacers.

Finally, choose the windows appropriate for the climate. European windows are designed for much colder environments and hence have high SHGC values. Selecting such a window for our climate here in the SF Bay Area will significantly increase your air conditioning load. Dual pane windows with an argon gas fill will be a cost effective upgrade from air-filled windows and casements are the most energy efficient of all types (i.e. single hung, double hung, sliders etc.)

Energy Efficient Windows – Insulated or Thermally Broken Frame