High-Performance Windows: Looking Through the Options: Page 2 of 6

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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

Conduction is the flow of heat through a material by contact. It is why the handle of a cast-iron skillet that’s been on the stove is hot. While we usually think of heat conduction occurring through solid materials, it also occurs through liquids and gases. Differences between the thermal conductivities of air, argon, and krypton (used in the spaces between panes)  affect the speed at which heat conducts through multipane windows.

Convection is the flow of heat that occurs when warm molecules (gas or liquid) physically move. Warm air rises and cool air sinks; this process happens in an insulating glass unit (IGU; a sealed glazing assembly with at least two layers of glass separated by an air- or gas-filled space). Inside an IGU that’s warm on one side and cold on the other, convection loops may form, increasing the convective heat transfer.

Advances in Energy Performance

Layers of glass. Among the oldest approaches for boosting window energy performance is adding a second layer of glazing. Thomas Jefferson added storm windows to Monticello and doubled the R-value of those windows (though the term “R-value” hadn’t been introduced yet). Most glass energy performance comes from the air films on both sides of it. When there are multiple layers of glass, there is a relatively still layer of air that lies between each glass layer. The earliest IGUs relied on welded-glass edges, rather than modern butyl rubber and silicone sealants, so they sealed extremely well—better than today’s IGUs.

Additional layers of glazing further boost the R-value. Nearly all windows in Sweden have been triple-glazed since the mid-1970s, when the country was very seriously affected by oil embargoes. Most of the largest U.S. manufacturers are finally offering triple glazing, including Andersen, Marvin, and Pella.

More glazing means more weight, which means the hardware has to be sturdier (and is more expensive). To reduce the overall weight with triple-glazed windows and open up the option to add even more layers, a few manufacturers are instead using suspended plastic (polyester) films between the layers of glass. Alpen High Performance Products is the technology leader among window manufacturers using suspended films, and the company made the quad-glazed windows that I just installed. Several other window manufacturers, including Hurd and Marvin, offer suspended films.

Spacing between layers. Increasing the airspace in an IGU offers a significant energy performance boost. In the 1980s, the standard was about 1/4 inch. Increasing the airspace to 1/2 inch boosts the center-of-glass R-value in an IGU without low-emissivity coatings (see “Thermal Effects of IGU Spacing” graph) from 1.75 to 2.04—a 17% improvement. Window performance is often reported as U-factor (the inverse of R-value). In this case, the U-factor drops from 0.57 to 0.49. Optimizing that air space thickness is an easy, inexpensive way to improve energy performance, and most window manufacturers have done that.

Low-e coatings. The most important advance in window performance in recent decades has been the development of low-emissivity (low-e) coatings, microscopically thin, transparent metallic coatings that transmit short-wavelength light very well, but absorb and slow the transmission of long-wavelength heat radiation.

There have been two types of low-e coatings. Soft-coat, “sputtered” low-e has the lowest emissivity, but this type of coating is delicate and must be protected (facing the airspace) within the IGU, because weather or abrasion can damage it. Sputtered coatings are usually layers of silver, each layer just a few angstroms thick (there are 245 million angstroms in 1 inch). There can be one, or even two or three, layers of silver—leading to the common designation low-e2 (low-e squared) or low-e3 (low-e cubed).

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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