As much as one-half of the energy lost through the building envelope occurs through its windows. Researchers in Norway are developing silica aerogel for use in insulated glass units due to the material's low density and low thermal conductivity.

As much as one-half of the energy lost through the building envelope occurs through its windows. Researchers in Norway are developing silica aerogel for use in insulated glass units due to the material's low density and low thermal conductivity.

Credit: JoshuaDavisPhotography/Flickr

One positive outcome from the intensified scrutiny of buildings’ energy performance is the development of highly insulated glazing technology. Three of the most notable approaches are multilayered-, vacuum-, and aerogel windows—all of which aim to proffer U-factors of 0.50 to 0.70 W/(m²K) to meet the latest energy-efficient building standards.

A photograph of the large (a) and small aerogel granules (b).

A photograph of the large (a) and small aerogel granules (b).

Credit: Tao Gao, NTNU

While multilayered and vacuum windows are prevalent, researchers are only beginning to explore the possibilities associated with silica aerogel glazing. Its low density and conductivity could improve glass envelopes' thermal performance without the need to accommodate the weight of an additional glass lite or to mend thermal bridges in edge seals—considerations when specifying multilayered and vacuum units, respectively. Still, interest in aerogel glazed units (AGUs) has been limited so far due to their novelty and cost relative to the alternatives. That could change soon. Researchers from the Norwegian University of Science and Technology (NTNU) in Trondheim, Norway, have found a link between aerogel granules' size and their thermal performance: Smaller particles lead to lower unit U-factors at the expense of visible light transmission. As a result, AGUs are well suited for applications where privacy and thermal performance are concerns.

The team incorporated silica aerogel granules of varying sizes in the cavities of double-glazed units. They found that units filled with granules that ranged in diameter from 3 millimeters to 5 millimeters experienced a 58-percent heat-loss reduction when compared with a standard double-glazed insulated glass unit (IGU). Units filled with granules smaller than 0.5 millimeters in diameter experienced a 63-percent heat-loss reduction when compared with the double-glazed IGU. While the larger particle size resulted in a visible light transmittance of 50 percent, the smaller granules let in only 15 percent of visible light. That's compared to a visible light transmittance of 81 percent for the standard double-glazed IGU.

Although the results do not show a significant difference in heat loss, they reveal the need for further study of aerogel glazing units. AGUs can be beneficial in cases where some transmitted light is necessary but where clear views are undesirable either for privacy or because transparent IGUs do not meet the performance levels of AGUs. And although the AGU prototypes tested do not yet meet the optimal range of U-factors established by the researchers, the technology is still in its early stages.

Blaine Brownell, AIA, is a regularly featured columnist whose stories appear on this website each week. His views and conclusions are not necessarily those of ARCHITECT magazine nor of the American Institute of Architects.