Curtain walls have long been a choice exterior facade for building developers and architects, with glass and aluminum curtain walls a common selection for high-rise buildings in Toronto.
With buildings striving to reach higher energy and environmental targets, developers are looking at ways to increase the performance of their curtain wall systems.
One way to increase a curtain walls performance is by evaluating the system as a whole, rather than looking at the performance of each building component. Dave Andres, a building science specialist with Morrison Hershfield, spoke about this during the Construction Canada conference in Toronto this past December.
Hershfield says that when evaluating how to meet necessary R-values with curtain walls, “the best practice is to look at different technologies to assume the efficiency. Different configurations will affect the efficiency.”
R-value is the measure of thermal resistance for a building material. A material with a higher R-value provides more insulation.
Andres says that building developers will need to find out what the maximum R-value they can achieve from walls by looking at the system as a whole rather than each individual component. Features such as the vision area — the parts of a façade that allow vision from inside to outside — have a large impact on the R-value. Reducing the vision area can improve the performance of a building’s cladding, as opaque walls have a much smaller impact.
By using 3-dimensional analysis software to create the assembly of a curtain wall, Andres says that developers can identify how each individual detail works within the big picture.
An ASHRAE research project has identified issues with how thermal performance affects a building envelope. ASHRAE 1365: Thermal Performance of Building Envelope Details for Mid- and High-Rise Buildings shows that it is necessary to evaluate a whole system, as high levels of heat loss can occur even in systems using insulation with the highest R-values.
Developers previously used steel to frame curtain walls. But this was problematic, as steels acts as a conductor and creates thermal bridges that contribute to heat loss in a system.
Today, glass and aluminum curtain wall combinations are more popular for use as cladding and for exterior walls. The benefits of this combination include limited air leakage, prevention of rain penetration and limited excessive heat loss (or gain). In Canada, building codes allow only a limited air leakage rate — three times more stringent than in the United States — a result of the colder climate. Preventing air leakage reduces ice build-up on the exterior walls of a building.
Heat loss and gain can be prevented through the use of double- or triple-glazed glass, a popular choice among architects in Canada. The thickness of glass used is dependent on factors such as the wind load and solar radiation of a building. Contemporary building facades depend on high-performance glazing systems to meet a building’s energy targets.
“The temperature index (of a building) is important for cold climates or temperatures with high humidity indoors,” says Steve Murray, a principal with engineering and management firm Morrison Hershfield.
Different types of glass can be used to construct curtain walls to meet necessary energy requirements. For vision areas, the glass can be fabricated from float glass, and there are options for heat-treating the material. This increases the resistance to thermal and mechanical stresses.
A major benefit of glass curtain walls is daylight harvesting. By maximizing the amount of light that penetrates a building during the day it reduces the need for artificial lighting. There are many ways to maximize daylight within a structure. Murray uses Frank Lloyd Wright’s iconic Fallingwater house as a prime example, the building’s orientation and its windows maximized the amount of natural light in the house.
Efficient curtain wall systems will let light in, without letting the heat out. By evaluating the system as a whole, developers can harness the benefits of curtain walls, while ensuring the system meets (or exceeds) energy and environmental regulations.
Leah Wong is the online editor of Building Strategies & Sustainability and Canadian Property Management magazines.