HALIFAX, NOVA SCOTIA – Maple Apartments is one of the largest mixed-use residential high-rises in downtown Halifax. Developer Southwest Properties aimed to create a “contemporary, iconic addition to the urban landscape” while minimizing the building’s energy use. Structural thermal breaks helped achieve these goals by limiting heat loss through the floor slab by thermally isolating the buildings’ cold exterior balconies.
Morrison Hershfield, a global leader in building science and energy performance, was contracted to ensure the total building performance meets the developers’ goals. David Kayll, P. Eng, MH director, building science east, commented, “Our client, Southwest Properties, wanted Maple Apartments to be as energy-efficient as possible. They asked us to look at all options available, so we created a whole-building energy model and ran a parametric analysis of multiple variables. As soon as we knew how the HVAC and lighting were to be handled, we considered various energy-saving envelope elements. We ran thousands of scenarios and then selected the best combinations. One of these included thermal breaks at the concrete balconies.”
“There is a point at which putting more insulation in the roof and walls is not helpful unless you do something about thermal bridging at the balconies,” he continues.
The inclusion of structural thermal breaks between the interior floor slabs and exterior balconies greatly reduces heat loss through the penetration, while increasing the temperature of interior floors by up to 19°C (34°F) compared to non-thermally broken slabs during cold weather periods.
In addition to limiting heat loss through the balcony penetrations, inclusion of structural thermal breaks significantly reduces the risk of condensation and potential mould growth within the interior of the building. Prior to modern advancements in building envelope construction and the inclusion of low permeability vapor barriers, profuse air leakage through building envelopes allowed interior humidity to dissipate and equalize with relatively low winter exterior humidity levels. In most instances this limited the humidity level of interior spaces to the point that even chilled interior surfaces would not reach the dew point.
By contrast, in modern building envelope conditions, which incorporate low permeability air barriers and flashings, a much more airtight assembly is achieved. This results in a typical interior humidity level of 30-60 percent. With an interior air temp of 21°C (70°F) and relative humidity of 45 percent, the dew point would be at 9°C (48°F) meaning condensation would occur on material surfaces that are at or below 9°C (48°F). Condensation leads to durability concerns with interior finishes and can support and propagate mould growth on adjacent surfaces and in stagnant cavities. Both concerns can expose developers to potential long-term liability and remediation costs.
Maple Apartments has 22 stories with retail on the lower floors and 300 residential units above. Balconies run continuously along the front and back of the building, and intermittently along the sides.
Kayll points out that the design team specified Isokorb® concrete-to-concrete structural thermal breaks from Schöck North America to eliminate thermal bridging concerns at the balcony conditions.
Concrete structural thermal breaks consist of an insulation body that runs the full height of the slab. Stainless steel reinforcing bars are positioned at the top of the insulation body to accommodate tension forces imparted by the cantilevered slab. Compression concrete modules placed at the bottom of the assembly transfer compressive forces from the cantilevered slab, which in conjunction with the tension bars accommodate the moment at the connection. Additional stainless steel reinforcing bars run diagonal through the insulation body to address shear loads at the connection. By minimizing the cross-sectional area of the structural elements that penetrate the insulation body, as well as using less thermally efficient materials (stainless steel transfers 70% less energy than mild carbon steel), structural thermal breaks can reduce heat transfer by up to 90% at the connection.
In addition to improved thermal performance, utilizing stainless steel through the insulation provides protection from potential corrosion within the cold joint between the concrete and the thermal break.
The reinforcing bars extend on both sides of the thermal break and are sized to fully develop as required. They are simply wire tied to the reinforcing cages of the balcony and floor slab as specified by the Engineer of Record.
Wes Campbell of Campbell Comeau, the structural engineer for Maple, says, “We had no problem with the thermal breaks…the construction crew poured both the floor and the balcony at the same time. After the first two floors, the crew was pouring just as fast as if there were no thermal breaks to work with.”
Kayll adds, “The crews readily accommodated themselves to the product. The thermal breaks were pretty much ‘plug-and-play.”
Justin Elkin, P.Eng., project manager of Southwest, reports “There was no problem working with the product in rain, cold or heat,” and that Southwest is pleased with both the energy savings and the appeal to tenant comfort. “We don’t build to sell,” says Elkin, “but rather to own for the long term. We want high quality, both to attract and satisfy tenants and to benefit from long-term energy savings.”
Completed in 2017, Maple Apartments won the 2018 Canadian Rental Development of the Year award, presented by the Canadian Federation of Apartment Associations, and the Excellence in Green Building – New Construction 2018 award, presented by the Canada Green Building Council, Atlantic Chapter.
thermal breaks for balconies
Campbell Comeau Engineering