We ask a lot from windows: energy efficiency, aesthetics, durability, affordability, and more. Which window frame materials and low-e glazing options balance these choices best? This article explores all the options and decodes the performance labels we see when buying windows.
When we poke a hole in the wall and stick a window in it, we strike a high-stakes bargain. We want a visual connection to the outdoors that lets in daylight and that is itself pleasant to look at, both from the inside and the outside. We expect windows to provide fresh air and cooling breezes at times, but at other times we expect them to be completely airtight and provide good thermal insulation. Insects should be kept out; children and pets in. In heating climates, we want to get solar heat gain from windows, but not too much, and in all climates we don’t want glare.
Along with these key functions, we need windows to be durable in every way: resistant to condensation, wind, driving rain and ice, as well as the occasional baseball from over the neighbor’s fence or hurricane-driven debris. Windows must operate easily and accommodate attachments like curtains, awnings, and other devices. We want windows that are quick to install, that integrate with the rest of the building envelope, and that won’t break the bank. Given that they are a big investment, they should last a long time—several decades at least. We want windows to not cause undue environmental harm during their life cycle, whether from material extraction, manufacture, disposal, or as a hazard to birds.
In this article we’ll try to untangle some of the many threads that go into choosing windows for our homes, offices, and other buildings. This article will be most relevant to light construction, but many of the concepts and technologies extend to commercial glazing and curtainwall systems. EBN will also cover window attachments and retrofit options later in 2011 as we learn from research we’re doing as part of a Lawrence Berkeley National Laboratory (LBNL) project.
The good news for anyone buying windows today and in the next ten years is the ongoing technical and market progress. Speaking about overall trends in the window market, John Carmody of the University of Minnesota told EBN, “Over the last 20 years or so, we’ve been through a generation of double-glazed low-e [low-emissivity] windows penetrating the market.” Now, said Carmody, the Energy Star program, codes, and other factors are moving us “toward the next generation of higher performance” with innovations in glazing, coatings, frames, and more.
The window frame material and appearance often get the most attention from occupants, so we’ll start there. Once that foundation is in place, we’ll look at glazing options.