When it comes to improvements in building products, no change has been more dramatic than with window technologies.
Prior to the 1970s energy crisis, most windows were a single layer of glass (single-glazed), with energy performance (a measly R-1 or so) mostly derived from the air film next to the glass (a layer of relatively still air forms along any material). Some windows were double-glazed, roughly doubling the performance (to R-2), but that was about it.
It is far different today. I recently installed windows in our Vermont home with an R-12.2 at the center of the glass. These fiberglass-framed windows have four layers of glazing (two panes of glass and two suspended films), three low-emissivity coatings, and krypton gas between the panes—and they admit enough solar energy to help passive solar heating in our cold climate.
As windows have improved, they have gotten a lot more complicated. Choosing windows is no longer a matter of deciding on double-hung versus casement and whether to get divided lites. This article examines the technologies that have revolutionized window performance over the past several decades and provides guidance on how to select windows that are best for your particular application.
To understand why certain windows and window technologies make sense for particular applications, it’s important to understand the very different—and important—roles that windows play:
In some parts of the country, blocking solar heat gain is more important than bringing that solar heat into the home. While a really high R-value may make sense in cold and snowy Vermont, it may not in mild, temperate coastal California. These differences may even apply within the same house—installing windows with different capabilities on the east and west sides of the house than on the south, for example. Balancing these needs can be challenging, but is a lot easier if we understand how windows work.
A window’s energy performance comes down to physics. There are three modes of energy transfer—radiation, conduction, and convection—and all three play an important role with windows.
Radiation is the transfer of energy via electromagnetic waves or charged particles that travel in waves. Sunlight, a form of short-wavelength radiation, is readily transmitted through most glazing; while most glazing blocks long-wavelength heat radiation. As sunlight travels through a window and is absorbed by surfaces in a room, those surfaces warm up and begin emitting longer-wavelength heat radiation.