We are starting construction of a new house on the lake and our contractor has asked us to start selecting our “window and door package”. I had no idea there were so many choices in window frames and glazing. Can you provide a little background on modern glazing choices, hopefully, in simple terms, so I can get a basic understanding of our options before we start selecting products?

The range of products available is daunting, but I think that window technology is an important one to understand because windows account for approximately 35% to 50% of the heat loss in residential buildings. They are holes in your wall through which heat passes from the interior to the exterior and vise versa. A good window will reduce heat loss to the exterior and capitalize on solar gain in the winter.

However, it is a little more complex than that. Let’s start with modern window technology. Modern windows are built as sealed unit thermo pane (double glass) windows. Each pane of glass has two surfaces. You will have an option to have one of the four glazed surfaces coated with a low emissivity (loE) coating. The location of this coating on one of the four glass surfaces significantly affects the window’s performance characteristics. For instance, when you want to pull solar heat into the building, the best location for this loE coating is on the third glass surface from the exterior. In other words, windows with this feature take best advantage of the sun’s ability to reduce your heating requirements by drawing solar energy into your home. 

 However, there will be locations in your home, perhaps along the south wall where you may want to reduce the heat of the sun so your home will be cooler during summer months. The loE coating on these windows should be installed on the second glazing surface from the exterior. If you are planning to install a heat pump / air conditioner, this window glazing choice will reduce energy consumption for summer cooling or simply help keep your unconditioned house cooler in warm weather.

The loE coating can contribute about 10% to 27% of the total energy requirements of your home. As well, you have an option to fill the vacuum between the two panes of glass with inert argon gas. This gas significantly increases the thermal performance of the window (its R value). Some European countries mandate triple glazing, because windows are the single biggest contributor to heat loss in a building. This fact may also affect your decision to install skylights which are essentially holes through your ceiling and roof, the last and most important heat loss membrane in your house.