Breaking the Thermal Bridge: A Look at Vienna House’s Innovative Prefab Balconies

BC Housing’s Vienna House project will be the first British Columbia building to use a new type of suspended balcony that will provide residents with semi-private outdoor space without undermining the project’s ambitious energy-efficiency goals.

Sapphire Balconies, a company based in Reading, England, is supplying the project’s 42 balconies. It is manufacturing them in Europe, shipping them to a Vancouver-area warehouse for final assembly, then delivering them to the Vienna House site via flatbed truck.

*This diagram highlights the structural elements of the suspended balconies that will be attached outside the Vienna House apartments facing Stainsbury Avenue.* ***Source****: Sapphire Balconies.*

From there, a Kindred Construction crane will lift them into position, and workers will guide them onto hooks and brackets pre-installed on the sides of the building. It will take one worker two to three hours to install the two brackets. A second worker then attaches the stabilizer bars when hoisting the balcony. The actual installation on the brackets takes less than half an hour.

“The Sapphire balconies check a lot of boxes on the goals in the Vienna House project charter,” explains John Wall, Principal and co-founder of Public Architecture.

“They are durable, they support resilience by providing passive shading, and they improve quality of life for residents.”

More than just a balcony

As John Wall explains, Sapphire’s suspended, prefab approach offers a number of advantages for a project like Vienna House:

Speed: Sapphire reports that the developer of a high-rise housing project in Hamilton, Ontario, says his construction team is shaving a full month off its timeline by using the product.
Safety: The installation approach all-but eliminates fall risk; safety-harness equipped workers can fasten them to the building from inside the patio door openings.
Quality: Factory fabrication ensures a high level of precision and finish, eliminating the inconsistencies and imperfections that can be associated with on-site construction.
Weight: The aluminum balconies are significantly lighter than concrete, which reduces the load on the building’s foundation, and allows for additional floors without needing as much concrete.
Accessibility: Rain flows through the decking to a catchment area underneath, eliminating the need for a traditional step-over sill that would be a barrier for wheelchair users.
Energy performance: Compared with the conventional approach, Vienna House’s balconies will permit just a fraction of heat to escape the building.

That last point is most interesting from a sustainability perspective, because conventional apartment balconies come with a hidden downside.

How heat sneaks out at night

To understand the energy-efficiency performance of Sapphire’s balconies, we need a quick primer on the challenges associated with the conventional approach.

Most mid- and high-rise concrete residential buildings are built using a decades-old construction technique known as “slab and column.” Workers lift buckets of concrete via crane, and empty them into forms to create suspended slabs, then add columns to support the next floor. To add balconies, builders cantilever the floor slab out past the exterior walls, then add railings.

Balconies help developers market their units for a relatively modest capital cost premium. They offer “curb appeal in the sky,” and residents love them. But they also increase energy use and greenhouse gas emissions by squandering heat—the result of a phenomenon called thermal bridging.

A thermal bridge is a structural component of a building that conducts interior heat to the outdoors. As the infrared image included here shows, cantilevered concrete slab balconies do this particularly well. Because they transfer so much heat, the building’s heating system consumes extra energy to keep its residents warm. Typically, this is natural gas burned in boilers, which produces greenhouse gases.

This infrared camera image of a typical high-rise apartment building—captured on a cold Canadian night—reveals the otherwise-invisible impact of thermal bridging. The red and white horizontal stripes on the right reveal where the building is shedding its heat through its cantilevered concrete slab balconies. **Source**: Thermally Broken Balconies: Alternative Strategies for Low Carbon Buildings, March 2022, Perkins & Will, RDH Building Science, and BC Housing, used with permission.

In contrast, the Sapphire Balconies that will be installed at Vienna House are “thermally broken.” Because they attach to the building via hooks, slide-on brackets, and suspension rods, they transfer very little heat to the outdoors.

While thermally-broken balconies offer numerous benefits, they typically cost a bit more than traditional cantilevered balconies. As is often the case with many green building technologies and components, they incur a modest up-front cost premium. This cost is typically paid off over the long term via reduced energy use—and in other ways, such as improved accessibility, that cannot yet easily be attached to a dollar value. The Vienna House porches will lower the building’s maintenance costs and overall energy costs, and will be crucial to help the building meet its Passive House objective.

Vienna House apartments will be heated with small electric baseboard heaters but, thanks to the Sapphire balconies and the building’s very thick insulation, they will rarely need to come on. That means permanently low utility bills for residents.

Crews will begin hanging the balconies in approximately February 2025, once the building’s cladding has been attached.