Higher-performing Balconies: Integrating manufactured structural thermal breaks

by Elaina Adams | January 1, 2013 12:03 pm

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By Dieter Hardock, Dipl.-Ing., and Dave André, P.Eng., LEED GA
In Canada, it is widely stated buildings account for approximately 35 per cent of the total primary energy use, and roughly 30 per cent of the country’s total greenhouse gas (GHG) emissions[2]. In the continuous quest to achieve better building energy performance, the  design/construction community must develop and use innovative tactics to minimize operational energy use.

One significant aspect of energy loss involves conductive heat transfer through the building envelope, meaning the heat flow through solid elements due to temperature differences between interior space and exterior space. Common strategies to minimize this process by increasing the envelope’s overall R-value include:

• specifying high-performance glazing, such as improved double and triple-glazing with higher performance framing systems to reduce U-values;
• improving the façade insulation with increased R-values by using better and thicker insulation; and
• reducing the window-to-wall ratio (WWR), as windows are usually the thermally weakest areas of the wall.

These assemblies are largely responsible for the overall thermal performance of an exterior wall. Traditionally, not a lot of attention has been paid to the various thermal bridges that are integral to these larger envelopes because they were thought to represent a relatively small percentage of the overall energy loss.

As an example, one can consider the exposed concrete slab edges of a typical 1970s, lightly insulated, high-rise apartment building. The heat loss at the slab edge would be a relatively small percentage of the whole building’s losses. As the thermal performance of overall wall systems is improved, however, the heat loss through thermal bridges becomes a much greater percentage of total building energy loss, and thus more important to consider and control.

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Although there are manufactured structural thermal breaks available for numerous connections, this article focuses on concrete balconies. Balconies remain a popular design feature for residential construction in Canada. Often formed by extending the concrete structural slab through the building envelope, these penetrations can represent a significant energy loss that can result in reduced thermal comfort and possible condensation.

Recent three-dimensional heat transfer analysis—published in American Society of Heating, Refrigerating, and Air-conditioning Engineers (ASHRAE) 1365-RP, Thermal Performance of Building Envelope Details for Mid- and High-rise Buildings—indicates there can be as much as five times the amount of heat loss through an exposed concrete slab compared to an insulated one. The result is an increasing demand for, and interest in, thermal bridging solutions that will reduce these effects.

The popularity of off-the-shelf manufactured structural thermal breaks has steadily increased in Europe, thanks to this type of product’s performance, as well as the material and system testing being completed by those manufacturers. Now considered standard building practice across the Atlantic, these products have recently come to the Canadian market.

Thermal bridging
Thermal bridges are localized assemblies that penetrate insulated portions of the building envelope with thermally conductive materials. The associated heat loss results in a reduction of the indoor surface temperatures, which may create conditions for condensation and mould growth.

Generally speaking, there are many different structural elements that penetrate the building envelope and may form thermal bridges, such as balconies, canopies, slab edges, parapets, or corbels. These are common architectural features or essential structural elements in residential buildings as well as in hotels, schools, museums, or gyms.

In the case of uninsulated balcony slab connections, the interaction of the physical geometry of the balcony slab, the ‘cooling fin’ effect (i.e. increasing the exterior surface area leads to increased heat flow), and the material properties (i.e. the reinforced slab’s thermal conductivity) can result in significant heat loss.

Uninsulated balcony connections can be critical thermal bridges in a building envelope. Buildings relying on them have significant incentives for adoption of structural thermal break technology to improve thermal comfort, energy efficiency, and possibly indoor air quality (IAQ), by reducing mould growth potential.

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Thermal break performance
Structural thermal breaks reduce heat flow between the inside to the outside, while also conserving structural integrity. With uninsulated balconies, for example, the reinforced concrete at the connection is replaced with an insulating material while continuous reinforcement bars are used to transfer moment and shear loads.

In some instances, these bars may be replaced by stainless steel where they penetrate the insulating material as this metal is much less thermally conductive than conventional reinforcing steel. The use of stainless steel not only reduces thermal conductivity, but also ensures longevity through its inherent corrosion resistance. Other materials are also used in some proprietary systems with the aim of lowering thermal conductivity,  such as including concrete modules to transfer compression loads.

The combination of all these aspects means structural thermal breaks can average an equivalent thermal conductivity as low as 0.2 Watts per metre Kelvin (W/m·K), instead of typical values of 2.3 W/m·K for reinforced concrete at an untreated balcony connection. This reduces the thermal conductivity at the connection by up to 90 per cent, which significantly reduces the heat flow and also substantially improves the indoor surface temperature in the living area.

Typically, a range of structural thermal breaks are available from manufacturers, depending on the load requirements and deflection criteria. This selection allows for customizable solutions between structural and thermal performance to be found.

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Integration in Canadian design
The National Building Code of Canada (NBC) serves as the model building code for provincial jurisdictions. Part 4, “Structural Design,” includes provisions applicable to manufactured structural breaks as part of the overall structural design. Compliance to these code provisions is generally objectively based. Conversely, sections like Part 3, “Fire Protection, Occupant Safety, and Accessibility,” contain provisions that may be deemed applicable to structural breaks, depending on the Authority Having Jurisdiction’s (AHJ’s) interpretation of the code and the material used. It is therefore recommended this be discussed at early design stage.

As manufactured structural thermal breaks become more widely used in Canada, it seems likely future iterations of building codes will adopt specific language to address these components’ use. Additionally, the manufacturers can be helpful in satisfying certain current code provisions by providing fire and thermal ratings or code-compliance reports for materials.

To ensure the requirements of a project are met, an integrated design process between the project architect, relevant specialty consultants, construction team, and the manufacturer’s technical staff is recommended. For example, an appropriate design solution for a high-rise residential building with cantilevered concrete balconies will vary based on regional construction practices and cladding assemblies. Brick veneer, architectural precast concrete, exterior insulation finish systems (EIFS), and painted concrete will all yield different outcomes depending on thicknesses and other variables. Therefore, the project designers will be required to determine and illustrate the location/placement of the thermal breaks, taking into account considerations from the integrated design team.

This team includes:

• the code consultant to comply with requirements for fire resistance/protection specific to the project details;
• the building envelope consultant and architect to maintain continuity of the critical barriers (i.e. air, moisture, vapour, and thermal); and
• the structural engineer to avoid interference with the structural attachment of other elements (e.g. glazing and framing).

Additionally, the structural engineer should take into account:

• slab rotation at the balcony (primarily due to the elongation of the unbounded bars in the break) can result in a possible camber of the balcony, depending on the drainage requirements;
• expansion joints in the exterior structure due to thermal elongation of the exterior element, which are required by using structural thermal breaks, and are recommended by comprehensive manufacturer testing; and
• lap reinforcement to ensure the transmission of the loads into the slab—the normally used slab reinforcement for balconies should consider the overlapping with the manufactured thermal break’s reinforcement to ensure load transmission from the balcony to the slab.

The manufacturer’s technical staff may also be able to offer support with these factors, based on project experience and internal research/testing. Some companies provide recommendations for these considerations in their technical manuals.

To summarize, the following key points should be discussed among the design team before the drawings are finalized:

• minimum clearance needed for structural attachment of
  other elements;
• fire protection requirements at selected details;
• continuity of the building envelope critical barriers;
• waterproofing transition and connection details; and
• installation procedure and construction sequencing.

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Construction installation
For cantilevered concrete balconies, co-ordination issues can be simplified by making the acquisition and installation of the structural thermal break part of the concrete contractor’s scope. Also, an ‘early’ submittal can ensure satisfactory lead time with the component supplier’s engineering, manufacturing, and delivery, along with onsite technical assistance schedules.

A main advantage of manufactured structural thermal breaks is they come readily assembled to reduce additional installation time. The products can therefore be installed with relative ease and speed between the other onsite reinforcement.

Conclusion
As energy efficiency requirements in the Canadian building construction market continue to become more stringent, greater emphasis and attention will be necessary to reduce thermal bridging during design. In addition to the energy use benefits of thermal breaks, there are also other benefits such as reduced risk of condensation and mould occurrence, and improved user thermal comfort.

Manufactured structural thermal breaks provide an attractive option because the product/system testing has already been completed to facilitate ease of adoption in design and construction. The integration and installation of thermal breaks has various considerations, necessitating collaboration among the project stakeholders to ensure the requirements are met. This is no different from any quality construction project.

Dieter Hardock, Dipl.-Ing., is a product manager at Schöck Bauteile GmbH (Baden-Baden, Germany), which develops and manufactures products related to the prevention of thermal bridges and impact noise in buildings. He can be contacted via e-mail at dieter.hardock@schoeck.de[7].

Dave André, P.Eng., LEED GA, is a project manager/building science consultant at Morrison Hershfield’s Toronto office. He has more than eight years of experience in building science ranging from envelope failure investigation and rehabilitation on existing buildings to field review for new construction projects. André can be reached at dandre@morrisonhershfield.com[8].

Source URL: https://www.constructioncanada.net/higher-performing-balconies-integrating-manufactured-structural-thermal-breaks/

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