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.
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).