High-Performance Windows: Looking Through the Options

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High-performance windows
High-performance windows are multipurpose—including providing an inside view of the outside world, while contributing to a home’s overall energy efficiency.
riple-glazed, low-e window from Unilux
This triple-glazed, low-e window from German company Unilux offers excellent insulation performance, yet transmits enough sunlight to be effective for passive solar heating.
Well-placed windows
Well-placed windows can provide ample daylighting and ventilation to interior spaces.
Old single-pane window
Old single-pane windows are a constant household energy drain and compromise a home’s overall thermal comfort.
Alpen HPP windows
Alpen HPP can manufacture windows that have up to five glazing layers (three suspended films between two layers of glass), resulting in lightweight high-performance units that weigh only a little more than standard double-pane windows.
Gas-filled window
Some gas-filled windows, such as this one from Alpen HPP, are shipped with bladders to equalize pressure fluctuations in transit. They are removed during installation.
wood with aluminum cladding
Frame options include wood with aluminum cladding.
vinyl with polyurethane foam fill
Frame options include vinyl with polyurethane foam fill.
uninsulated vinyl
Frame options include uninsulated vinyl.
NFRC window label
NFRC window labels include U-factor, solar gain, transmittance, and optionally, air leakage and condensation resistance.
High-performance windows
riple-glazed, low-e window from Unilux
Well-placed windows
Old single-pane window
Alpen HPP windows
Gas-filled window
wood with aluminum cladding
vinyl with polyurethane foam fill
uninsulated vinyl
NFRC window label

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.

What We Ask of Windows

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:

  • Delivering natural light into a home
  • Providing views to the outdoors
  • Adding aesthetic appeal to a home
  • Slowing heat loss
  • Slowing unwanted solar heat gain
  • Admitting passive solar heat—sometimes even serving as a home’s primary heating system
  • Preventing condensation
  • Contributing to home security

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.

Understanding Heat Flow

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.

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Comments (1)

Jim and Elaine Stack's picture

another very ECO choice is to modify the windows already in place. Instead of ripping out the old and adding to the landfill you can add Inflectors on the inside of the window opening. They have a reflective side for summer and dark side to absorb IR heat in winter. They are also air tight and take makes a big difference.
Since they are held in place with magnetic strips you can flip them over for heat rejection or gain in a minute. You can even take them off and enjoy a fresh breeze when it's temperate out.
I put them on all my windows and it makes a big difference. Our little 4 Kw system runs our 100% electric home and vehicle with energy to spare. .

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