The amount of glass in the facades of buildings has been the subject of fascination for generations and is now the subject of fierce debate in energy efficiency circles.  Expansive glass provides an unmitigated and undeniably powerful visual connection to the outside.  The sense of being protected within an interior space while having a broad vista before you can, when done well, have an almost elemental appeal.  But cladding buildings entirely in glass has serious consequences for energy performance.  In the residential sector, the extremely limited insulating value of glass (even double-glazing) can contribute to high heating loads in colder seasons, and solar gain in summer can lead to high cooling loads, and glare and other comfort issues can be a problem year round.  In the commercial sector, where cooling is typically the dominant mode, the low insulating value of glass can, technically, help your interior loads but also causes a variety of glare, direct solar gain and other comfort issues.  The material properties of glazing are such that, even with triple-glazed units, it is basically not feasible to achieve Passive House certification with an envelope that is more than about 40% glazing.

I have to admit to a personal bias, in high-design terms I prefer early Botta to late Meier (look it up, kids.) In 2009 at Urban Green Council I was involved in the development of a program called “Glass vs. Mass” on the implications of all glass buildings. One of the architects on the panel admitted, in the manner of someone opening an AA meeting, “My name is Doug, I’m an Architect and I LOVE glass buildings.”  This got a good laugh, and you can’t deny that the views provided by mostly glass facades are enticing.  And its not just architects that like glass.  The market has spoken.  Glass facades are virtually synonymous with Class A commercial and luxury residential buildings.

The other factor in favor of glazing is cost.  Glass facades are, relatively speaking, inexpensive.  An aluminum and glass curtainwall (or window wall) is typically less expensive and speedier to erect than a built up wall of block, insulation, masonry or other cladding.  Curtainwalls are also lighter, which breeds it own efficiencies in speed and, sometimes, structural framing. But in the coming decades the cost of glass may be changing.

You’ve heard of peak oil, the point at which we’ve effectively identified all available oil reserves and our extraction of it slows, the amount each year declining as we use up known reserves.  Most of you probably know that glass is made from sand, but personally I was not aware that glass is made from a particularly high-purity sand- a limited resource that, similar to oil, we take for granted but are depleting rapidly.  Some estimate we have only 20 years of this glass-sand left, and well before we reach that date extraction will become more costly.  Extraction is also likely to become more damaging to surrounding ecosystems- which is already a significant impact.

The folks at Dezeen have an interesting piece on this subject, here.  My favorite anecdote is that you cannot make concrete from desert sand, so places like Dubai have to import sand from Australia for their big infrastructure projects.

We want glass facades for a variety of reasons, and we can have them because they are not expensive.  Many advocates for high performance buildings hope that codes will preclude all-glass facades in the future.  But maybe cost will drive this change before regulations.  It will be interesting to see if we continue to favor glassy facades when they become more expensive.

– Yetsuh Frank, BE-Ex


  • Passive House / High Performance

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