Windows, Part 1: Low-E Coatings

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Feeling a draft through those old windows? If caulking and sealing around your existing windows doesn’t do the trick, or if you have inefficient or uninsulated windows that are not well-suited for your climate, it may be time for replacements. Whether you are looking for windows for new construction or replacements for an existing house, one important thing to consider is energy performance — windows that help keep heat from the sun outside during the summer, and heated air inside during the winter. Not only will the right windows keep your home more comfortable, but they will also reduce heating and cooling bills.

First, a few words about sunlight… Sunlight is comprised of 38% visible light, 3% ultra-violet (UV), and 59% infared light (IR). Visible light enables us to see things; UV light fades fabrics and furnishings; and IR light is basically heat.

At the very least, you’ll want to select a double-pane window, but other options such as low-E; argon/krypton gas filled; and triple-glazed, are available as well, for even better thermal performance.

Low-E (low emissivity) coatings are microscopically thin, virtually invisible layers of metallic oxide bonded to the surface of a window’s glass, or glazing. They prevent heat (IR rays) and UV rays from passing through the glass which helps to keep your home cooler in the summer and warmer during the winter. Low-E coatings also help to reduce condensation on the glass because the inside surface temperature of the glass is warmer.

The 3 major types of low-E coatings are:

  1. Hard Coat allows for high solar gain, making them ideal for cold climates and passive solar design projects because they perform better in winter.
  2. Soft Coat, or spectrally selective, allows for low solar gain, making them ideal for hot climates because they perform better in summer.
  3. Soft Coat with moderate solar gain reduces heat loss, lets in a reasonable amount of solar gain, and is suitable for climates with both heating and cooling concerns.

To further insulate windows, sometimes the gases argon or krypton are used to fill the cavity between the panes of glass. Argon is inexpensive, nontoxic, nonreactive, clear, and odorless, and performs better than just air between the panes of glass. Krypton is also nontoxic, nonreactive, clear, and odorless, but has better thermal performance than argon. Krypton is more expensive than argon though. Sometimes a mixture of the two is used as a compromise between thermal performance and cost.

Look for the Energy Star label for the most efficient windows and read the attached NFRC (National Fenestration Rating Council) label. The NFRC label provides ratings for U-factor, Solar Heat Gain Coefficient (SHGC), Visible Light Transmittance (VT), and Air Leakage (AL). Soon labels will include a Condensation Resistance (CR) rating as well. This ‘window selection’ tool is helpful to compare energy usage costs with different window options.

Sources: The Efficient Windows Collaborative and

Tag(s): Energy, Greenhouse Gas Emissions / Global Warming, Greening the Home, The Great Green Outdoors


One Response to “Windows, Part 1: Low-E Coatings”

  1. john on April 29th, 2009 3:02 pm

    Although some windows are called vinyl windows with gas filled it doesn’t make them any better.

    any window that allows noise to penetrate it is considered pure trash based on how thin the window is.
    I’ve never had old windows freeze up when it was cold outside and I found Old windows were better based on its thickness and would perfer all windows that way with ultraviolet tint

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