By Smita Chandra Thomas
Imagine a man standing outdoors on a blustery cold day, wrapped in nothing but a flimsy sheet of plastic, trying to warm himself up with cup after cup of hot coffee. The same principle is at work in a building being pumped with hot air while clad in walls made of glass and steel, chugging away to keep the inside warm. Buildings encased in glass and steel are poor insulators, yet they are ubiquitous.
Why this matters
Poor thermal performance in buildings leads to higher loads on fossil fuel-fired heating and cooling systems, as well as larger renewable systems. The COP21 climate conference in Paris reiterated the urgency of reducing fossil-fuel energy use worldwide. The largest opportunity to meet this goal lies in buildings, globally the largest consumer of fossil fuels.
Top U.S. architectural firms have risen to the occasion and signed up for the Architecture 2030 Challenge to design new buildings with ‘zero’ fossil-fuel consumption by 2030. Zero-energy buildings are no futuristic dream either; while the exact definition of zero-energy is being hashed out, thousands of zero-energy units exist in the U.S. today.
The first opportunity to reduce energy use in buildings lies in the building skin, which can impact the total energy use of a building by 10 to 50 percent. And yet we continue to see buildings being encased in glass. Here are five myths surrounding glass skins that explain why this is cause for concern.
Myth: Glass is energy-efficient
Yes, there is energy-efficient glass, but only compared to other glass. For a building material, energy-efficiency chiefly refers to its resistance to heat flow (indicated by R-value), higher resistance being better. The table below shows some rounded R-values for windows versus insulation used in solid walls.
|Single-pane window||Less than R-1 (close to metal)|
|‘Efficient’ Double-pane and triple-pane window||R-2 to R-3|
|‘Top-of-the-line’ Electrochromic glass window||R-6 at center; R-5 or less including frame|
|Fiberglass batt insulation 3.5” thick||R-11|
|Foam insulation- expanded polystyrene 3” thick||R-10|
|Fiberglass batt insulation 6” thick||R-19|
|Foam insulation – extruded polystyrene 6” thick||R-30|
The table above illustrates that solid walls have much higher resistance compared to glass. Solid walls gain additional R-value (R-1 to R-2) from other material layers and air gaps, but may lose 10 to 30 percent from thermal bypasses such as floor slabs and wall ties. All things considered, a well-constructed solid wall resists the transfer of heat flow several times better than the best glass window.
Glass is also at a disadvantage because it allows more radiant heat gain by allowing sunlight to pass through – a significant problem in hot climates – even with Low-E glass. It is mildly helpful in cold climates during the short daytime hours with direct sunlight exposure, but not significant enough to make up for the heat loss through low resistance.
Some creative and high-tech solutions to combat heat transfer in glass walls include double-walled systems (which take up valuable urban space) and double-glazed ventilated wall systems whose thermal performance is only marginally better.
Myth: Buildings need lot of glass for natural light
Glass does serve the important function of providing daylight. It is worth noting, however, that it takes a fraction of full daylight to provide sufficient lighting inside a building for normal use. In fact, glare is a real problem in most all-glass buildings — making the use of interior shading devices a necessity. Daylight can be introduced using other advanced technologies instead such as translucent insulation.
Smart design strategies include smaller windows with features that help daylight penetrate deeper into the space.
Myth: Buildings need a lot of glass to provide views
Windows are important for views, such as a grand view from an open space. But all glass walls on every floor are an overkill, impractical for interior furnishings, and can induce vertigo — necessitating waste of space near walls in tall buildings.
Besides, views to the inside of an all-glass office building from an opposite building can be messy! Optimal window sizing and positioning, on the other hand, can provide strategically-framed views.
Myth: High-rise buildings need glass walls because they are lightweight
Glass-and-steel building skins are relatively low-weight, which makes them a practical choice in tall buildings. A typical insulated brick wall construction is about five times the weight of a glazed wall – not a good alternative. But other light-weight insulated options are available, such as exterior-grade metal-clad insulated panels – both thermally superior and aesthetically pleasing. The thermal performance of the metal framing— required for structural integrity – can also be improved by replacing smaller metal elements such as window frames with viable alternatives such as insulated fiberglass which is lighter and has better thermal properties.
Myth: Glass buildings are ‘green’
This is the mother of all myths. Confusion about the relationship between ‘green’ buildings and glass walls exists because many famous green buildings are clad in glass walls. The fact is, those buildings are efficient in spite of the glass walls, not because of them. As described above, glass walls have poor thermal performance. Green buildings encased in glass rely on other strategies to reduce energy use, and notable efficiency targets are indeed achieved. But starting with optimized exterior walls would take those buildings a whole lot further.
Time for a new international style of architecture
In short, glass building skins do not jibe with energy efficiency goals. They may appear seductive and iconic, but they disregard the environment.
Building designers who understand the basics of the path to ‘zero’ energy – designing passive design solutions before leaning on mechanical systems and renewable energy – can lead us away from all-glass to more sophisticated solutions. Will you do your part to spread awareness about this?
Smita Chandra Thomas is a U.S.-based sustainability consultant and a LEED AP with an undergraduate degree in Architecture, and a Master’s in Building Science from the University of California (LA and Berkeley). For nearly two decades, Ms. Thomas has been focused on enabling energy efficiency in buildings through building science for lesser waste, greater economy, better health, and a sustainable environment for future generations. Ms. Thomas runs her private consulting practice Energy Shrink in Washington DC. She can be reached at <email@example.com>.