As a highly transparent material that stands up to all but the most extreme of weather conditions, is easily formed into any size or shape, and, once formed, will last for thousands of years, glass is still one of the most innovative and crucial materials used in architecture. Although contemporary building practices allow us to form huge, glittering skyscrapers of glass that rise hundreds of meters into the air, the ancient material’s original purpose – to welcome light into weathertight and secure interiors – remains its most important more than a thousand years on.
As important as glass is to almost every typology of architecture in the form of windows, when it comes to the roof of a building, the use of glass is not so simple. We’ve understood the power and danger of combining light and glass ever since we saw a magnifying glass used to concentrate the heat of sunlight into incredibly high temperatures in children’s cartoons. Under a glass roof, the solar gain can make for uncomfortable internal environments without the correct protective precautions.
Meanwhile, the transition of heat the other way, escaping through the roof of a property, can also be problematic under a glass roof. Although the U-value (thermal transmittance) of double-glazing is around the same as a standard roof (2.8 W/m2K compared to 2.5), the metal frames used to support the glass panels can reduce the overall value to 3.5. Add to this, the fact that glass roofs can’t receive additional insulation, and with it the possibility to reduce the U-value of a standard roof to around 0.2, and the thermal properties of glass roofs in winter take quite a hit.
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Used to bounce heat back where it came from, specialist solar control glass (facing out) and low-emissivity glass (facing in) can keep the heat where it’s needed and keep temperatures level – low-E glazing can contribute to a U-value of 0.8 W/m2K. Despite this extra effort to improve the thermal properties of glass, along with other ventilation control techniques for passive cooling, if it is done right, the benefits of using glass as a ceiling material can far outweigh the disadvantages. The interiors of homes and buildings, filled with natural light, can use one of the Earth’s natural heating sources to keep warm, use innovative ventilation methods to stay cool and fresh and use the natural patterns of the sky and the stars to decorate them from above, all combining to make them feel like outdoor living.
Let There Be Light: Outdoor Environments Reproduced Under Interior/Exterior Glass
The positive effect of natural light in interior spaces can be crucial to businesses, refreshing employees and clients not only during their desk hours but especially so in energizing spaces to recharge between long and arduous meetings of attrition. At the Chongqing Lijia Smart Hall in Chongqing, China, for example, architects at Gensler wanted to create an architectural geometry that coexists harmoniously with its environment. With a Y-bar structural design system, skylights run around the edge of the building to form a ‘well-lit pre-function zone that serves as a pleasant relief for visitors between sessions,’ as the architects explain.
Keeping visitors clear-headed and refreshed is also crucial in retail spaces, too, where expansive interiors can mean there’s often little natural light. In replacing an outdoor farmers market that was covered with dark netting with an indoor market underneath glass skylights, MINAX Architects has given the Bloom Liutan Market the fresh, open, and light feel of an outdoor farmers market, while protecting its produce and customers from the elements.
Projects using glass skylights to fill interior environments with natural light:
Chongqing Lijia Smart Hall / Gensler
Bloom Liutan Market / MINAX Architects
Beijing Fengtai Station / gmp Architects
The Greenhouse Effect: Warming the Interior with Controlled Solar Gain
While a glass roof creates an interior environment with as much natural light as an exterior space, the heat of the sun can be magnified when it passes through glass – unless it is mitigated and controlled. With a 3,800-square-meter glass roof, the Albert Einstein Education and Research Center in Sao Paulo, Brazil, controls the effect in various ways. The outer layer’s glass panels are ‘coated with triple-silver solar protection to reduce heat gain and printed with translucent ceramic dots to shade sunlight,’ explain Safdie Architects. By using a mechanical system to deliver cooling to the atrium only when and where it is required, the environment’s comfort, humidity, and energy usage can all be controlled, and even used to balance humidity in the rest of the building.
Projects using glass roofs to create a greenhouse-style atrium filled with greenery:
Albert Einstein Education and Research Center / Safdie Architects
Qingdao Future City / CLOU Architects
Ventilation: Keeping Cool, Fresh and Breezy
Along with natural light, the importance of fresh air should never be underestimated when designing a large space. With a total length of 220 meters and a depth of 38 meters, the International Institute for Geo-Information Sciences includes a considerable stretch of interior space across its entire structure. To fill the interior with light, fresh air, and biophilic greenery, four large atria were carved out of the structure to ‘serve as the building’s green lungs,’ describe the architects. Connected to the outside and filled with habitats for both flora and fauna, the glass-topped atria are small ecosystems with ‘plants rooted in over a meter of soil, creating a stress-free workplace with clean air.’
As a more architectural method of providing a building with clean air, the Shenzhen Hongshan Middle School features a glass roof above its entrance hall/courtyard that is split into six angled sections in a diagonal arrangement. ‘Each basic unit,’ explains the architects CAPOL, ‘is joined with the upper and lower edges of two hollow trusses, which are convenient for ventilation, as well as the structural integrity of the roof structure.’
Projects using natural and technical ventilation systems underneath glass roofs to fill interiors with fresh air:
International Institute for Geo-Information Sciences / Civic Architects + VDNDP
Shenzhen Hongshan Middle School / CAPOL
Shading, Water, and Other Innovative Ways of Keeping Cool
As well as the scientific use of glass with solar protection and translucent ceramic dots to control solar gain – as done at the Albert Einstein Education and Research Center – there are many other possible methods of shading and cooling a glass roof to choose from. A system of adjustable shades and embedded blinds provides Mendel’s Greenhouse in Brno-Střed, Czech Republic, with natural cooling and ventilation in the heat of the summer months, for example.
At Infinitus China’s global GQ, Infinitus Plaza, a network of labs, research facilities, and conference and exhibition spaces wrap around two glass-topped atria. Set within Guangzhou’s humid sub-tropical climate, the atria are cooled by sprinklers that spray atomized particles of collected rainwater onto the roof to dissipate heat by evaporative cooling. Meanwhile, insulating low-emissivity glazing provides the building with effective heat insulation.
Projects that use shading and evaporative cooling to keep comfortable interior climates underneath glass roofs:
Mendel’s Greenhouse / CHYBIK + KRISTOF
MOUT Venlo Coffee / Buro Moon
Infinitus Plaza / Zaha Hadid Architects
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