EDMONTON, ALBERTA —The landmark of Edmonton, Alberta's ICE District, the Stantec Tower is Western Canada's tallest skyscraper and the city's first mixed-use high rise. The 251 m (823 ft) building's 66 storeys provide office space on lower floors, residential condominiums on upper floors, and a challenge to engineers and architects: how to keep interior floors opposite exterior balconies warm while minimizing energy use in one of Canada's coldest cities.
As the architectural and engineering firm designing the structure, Stantec solved the problem by positioning load-bearing structural thermal breaks at the building envelope between balconies and the floor slabs supporting them. "The thermal breaks create an insulated gap between the concrete on the outside of the building and the slab inside without impacting structural integrity,” explains Steven Weinbeer, Project Engineer from the Stantec Edmonton office.
The goal is to prevent thermal bridging where building envelope penetrations conduct heat from the interior floor slab and dissipate it into the environment. In addition to wasting heat energy, uninsulated balcony slabs chill interior slabs, promoting condensation and mold formation on adjacent surfaces.
The Stantec Tower is part of a 25-acre mixed-use entertainment, shopping and sports complex in downtown Edmonton, housing Rogers Place (a venue for hockey and concerts), offices, retail outlets, high-end residences, hotel space and restaurants.
The office portion of the Stantec Tower houses the headquarters of Stantec and workspace for other firms. Floors 30 to 66 will house 483 "Sky Residence" condominiums of varying sizes and prices.
To insulate the building envelope, the designers utilized a high performance, thermally broken double-glazed curtain wall system. Sealed double-glazed units have thermally broken spacers with high performance low e-coating and argon filled gas. System thermal performance is about 1.45 W/m2K,” he continues.
Curtain wall glass allows natural light in, while keeping moisture and air out.
To prevent 200 balcony slabs from conducting heat energy through the insulated envelope and into the environment, the architectural and mechanical team installed Isokorb® load-bearing structural thermal breaks.
Terrance Wong, project architect and principal in charge of architecture from Stantec Vancouver, says, “The team installed the thermal breaks in Edmonton for thermal comfort because of the extreme cold. The owner was convinced that omitting them would incur huge additional mechanical costs.”
While Wong had not used them previously, he says, “It’s a product I’ve known about for five years.”
Supplied by Schöck North America, the structural thermal breaks consist of graphite-enhanced expanded polystyrene insulation module and high-strength stainless steel tension and shear reinforcement for structural integrity. The rebar extending from both sides of the module is tied into the rebar of the balcony and floor slabs.
Weinbeer concedes that construction firms are sometimes wary about the extra step of installing structural thermal break modules, but says the cost benefit outweighs any installation concerns.
Weinbeer says it’s too early to determine actual energy savings, but that thermal modelling performed by Stantec in 2015 indicates the interior temperature of slab edges equipped with Isokorb™ thermal breaks would be “six to seven degrees higher” than interior slab edges lacking them. Wong adds that installing them eliminated the need for baseboard heating at balcony doors and windows.
The findings are consistent with a July 2013 report by the building envelope consulting firm Morrison Hershfield, which used EnergyPlus simulation software to analyze the potential performance of a 32 floor, 422-unit residential building constructed using Isokorb™ thermal breaks at balcony penetrations. The study determined that the thermal breaks would reduce heat loss through the balconies by 75 percent while “significantly reducing the risk of condensation and mold growth.” Morrison Hershfield also concluded “the thermal breaks would reduce the overall heating energy consumption by 7.3 percent compared to a building with conventional balcony slabs.”
“Because this was a LEED Building, we had to use a higher standard than the Alberta Building Code 2006. Instead we referred to ASHRAE 90.1 and NECB (National Energy Code for Buildings)” says Wong.
As formulated by the American Society of Heating, Refrigerating and Air-Conditioning Engineers, ASHRAE 90.1 sets baseline standards for energy-efficient building design. NECB is a Canadian standard, published by Natural Resources Canada (NRC, a government ministry) which also sets criteria for energy efficiency in new buildings.
The latest version of the National Energy Code of Canada for Buildings “…is an important step toward Canada’s goal for new buildings, as presented in the Pan-Canadian Framework, of achieving ‘Net Zero Energy Ready (NZER)’ buildings by 2030. The NECB supports this goal by reducing the overall thermal transmittance of roofs, fenestration and doors; reducing losses through thermal bridging in building assemblies...”
Structural thermal breaks can play a major role in meeting standards set in codes and in lowering heating costs. At the Stantec Tower, they serve the basic function of keeping high-rise condominiums warm in a northern region with frigid winters.
“It’s really about thermal comfort … That was the driver behind deciding to put this product in the building,” concludes Wong.
thermal breaks for balconies
Stantec Architecture Vancouver
WAM Development Group (ONE Properties)
RDH Building Science