The July 2010 issue of Environmental Building News asks whether we should end our love affair with all-glass buildings. The short answer is "yes." With most large commercial buildings there is an energy penalty associated with increasing the glazing area beyond 20 to 30 percent. Given today's glazing technologies, it does not make sense to create highly glazed buildings. That is especially the case with "green" buildings, where extra effort is being made to reduce energy consumption and environmental impact.

The energy penalty of more glazing

In researching and writing this article, "Rethinking the All-Glass Building," I got energy modeling support from Fiona Cousins, P.E., Scott Bondi, P.E., Ph.D., and Cameron Talbot from the New York City office of Arup, one of the world's leading engineering firms. I used that data to make the case that highly glazed façades don't make sense from an energy standpoint. As covered in detail in the full feature article, the Arup engineers modeled a ten-story, 100,000 square-foot commercial building in three cities, New York, Miami, and San Francisco with four different glazing types (single-glazed clear, double-glazed clear, double-glazed low-e, and triple-glazed low-e), three different building footprints (square, moderately elongated, and highly elongated), and four different glazing fractions (20%, 40%, 60%, and 80%), measuring the impact of these variables on annual energy consumption as well as peak heat and cooling demand.

Summary results of that modeling are shown in the graphs below. Neither exterior nor interior shading is assumed in this study, but lighting energy use and plug loads are factored in (i.e., highly glazed facades reduce lighting energy use).

As can be seen in the graphs, as the glazing area is increased, energy consumption and peak energy demand increases in all climates and with all glazing configurations. In colder climates, the energy penalty of increasing the glazing area is greater, and with higher-performance glazings the energy penalty of increasing the glazing area is lessened.

The increase in peak heating and cooling demand or load (a measure of how large the heating and cooling equipment has to be to maintain comfort) with increasing glazing area is even greater than the increase in annual energy consumption as the glazing area in increased. With a square building footprint and standard double glazing in New York, for example, going from 40% glazing to 80% glazing increases the peak heating demand by 48% and the peak cooling demand by 39%, while the same change increases the annual energy consumption by only 26%.

Arup also examined the impact of building shape (footprint) and orientation on energy consumption and peak loads as the glazing area is increased. While those graphs aren't shown here, they show that increasing the glazing fraction of an elongated building (with greater facade area) has a greater impact on energy consumption than it has with a square building. While the difference between a north-south orientation and an east-west orientation was not as great as I had expected, the north-south orientation (with the long sides facing east and west) results in greater annual energy consumption.

Making all-glass buildings work

The article devotes considerable attention to how we can minimize the energy penalty associated with increased glazing area. These strategies include:

• Substitute insulated spandrel panels for glazing (this can retain the all-glass look).

• Use the best spectrally selective, low-e glazings, and (where possible) specify different glazings for different orientations of the building.

• Increase the number of layers of glazing.

• Provide fixed exterior shading to control solar heat gain and reduce cooling energy use.

• Provide exterior roller blinds or shades to control both solar heat gain and heat loss.

• Provide automated, interior blinds to control solar heat gain (this is not as effective as blocking that solar gain on the outside of the glazing).

• Use lightshelves and other features to bring daylighting deeper into buildings and keep that solar heat gain further from the façade zone.

• Install dynamic glazing whose properties can change to regulate energy flow.

Changing aesthetic preferences

There are a lot of good reasons we like all-glass buildings: from the speed of construction of curtainwall assemblies to the fact that curtainwall manufacturers end up bearing responsibility (and liability) for those façades. But design and aesthetics are likely the dominant reasons. A woman architect I interviewed for the article even suggested that architects are drawn toward all-glass buildings for the same reason they are attracted to women's lingerie--the façades being "sleek, smooth, sexy, shimmering, simple--and simultaneously transparent and mirroring."

In my conclusions to the article I quote architect Henry Siegel, FAIA, who calls the insistence on transparency "a real failure in leadership and vision in the design community." He calls for "broadening the definition of design excellence to include values other than aesthetics."

I agree, and I'd like to see the green design community champion a shift away from the all-glass building aesthetic--at least until heavily glazed facades can be created without any increase in energy consumption or peak heating and cooling loads.

I invite you to share comments on this blog. Do you agree with the general arguments in this article and, if so, how do we convince clients--especially green clients--that all-glass isn't the best way to go?

Alex Wilson is the executive editor of Environmental Building News and founder of BuildingGreen, LLC. In addition to occasional blogs relating to EBN articles, Alex writes two weekly blogs: Energy Solutions, and Alex's Cool Product of the Week. You can sign up to receive e-mails about BuildingGreen blogs, by filling in your e-mail address in the upper right corner of any blog page. You can also follow Alex's latest articles and musings by signing up for his Twitter feeds.