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A.2 Balcony Design Considerations
Part A: Understanding Balcony Thermal Bridging
The basic function of a balcony is to provide private or semi-private outdoor space, expanding the physical living space and range of activities possible in a dwelling unit. It is also a key part of the architect’s toolkit when developing the overall aesthetic and composition of the building.
Although the primary focus of this report is the thermal performance of the balcony connection detail, thermal performance is just one of several considerations of balcony design. There are other attributes in balcony design, both in the construction of the balcony itself and its impact on the overall building that need to be considered.
Structure
The structural integrity of the balcony and safety of its occupants is fundamental to the balcony design. The balcony generally cantilevers or is connected laterally back to the building and must resist dead, live, and lateral loading as well as meeting local seismic requirements.
The size of a balcony’s structural connection back to the building’s primary structure is directly related to its weight and loading requirements. By reducing the weight of a balcony, through its size, construction or materiality, the size of the structural connection back to the building can also be reduced. This in turn decreases the amount of thermal bridging through the building envelope. When optimizing the balcony structural connection, the stability or perceived “bounce” of cantilevered balconies must also be taken into consideration to ensure user comfort levels.
The durability of the connection materials must be considered when specifying a balcony connection detail. Concrete or steel balconies can be at risk of deterioration either through exposure to rainwater, seawater, or other incompatible materials. Thermal bridging can also lead to condensation buildup within the connection posing a threat to the durability of the structural material.
In addition to the balcony structure itself, it is important to understand the support requirements for balcony doors and glazing systems, and to ensure required support is provided; for example, in high performance building envelopes doors and glazing systems should be located in plane with the insulation in the opaque assembly, while other door and glazing systems may require continuous sill support. In balcony connection details where structural material is not provided directly below the wall, door and glazing system, additional structure may be required to support the sill of the door and glazing system (Figure 1).
Thermal Performance, Occupant Comfort and Resilience
Most of the focus in discussions around thermal bridging mitigation is to quantify and reduce the associated heat loss. While this is important in tackling the carbon footprint associated with buildings’ operation, of no less concern is the impact on both the building itself and its inhabitants due to the thermally compromised detail(s). These include: • Thermal resilience - the ability to provide shelter despite power outages or extreme weather events;
• Occupant comfort - eliminating discomfort due to radiant heat loss, removing the risk of mould growth and associated indoor air quality issues; • Building resilience - providing protection against rising energy and carbon costs, and increasing overall durability by reducing condensation risk.
Door/glazing system Structural door or glazing support angle Structural thermal break
Figure 1: Balcony detail demonstrating provision of an additional support angle to provide the required continuous sill support at base of balcony door and glazing system
Envelope Detailing
By its nature, a balcony or a balcony’s structural support inevitably interrupts the continuity of the building envelope as it is required to tie into the building’s primary structure. When detailing the balcony connection, the associated building enclosure control layers including air, thermal, water, and vapor control must be maintained.
Regardless of the balcony type, air barrier continuity must be maintained around balcony interfaces and penetrations. This can be more challenging with some connection details, particularly where the air barrier is penetrated by structural connections. These details require attention to proper detailing, material selection, and construction sequencing (e.g. use of air barrier membrane pre-stripping). Within the wall assembly, the vapour control layer must also be maintained through balcony interfaces.
A horizontal balcony surface must manage large amounts of rainwater which is important to prevent water ingress or damage to the balcony structure. Some balconies are sloped to allow water to drain off the balcony edge away from the building and some collect the water through a drain and direct it to an external or internal rainwater leader. It should be noted that routing a drain inside the building may add to the thermal bridging and associated heat loss.
Ventilation
In-slab exhaust ductwork for ventilation systems and clothes dryers are often run through the slab to vent into the balcony soffit. Conventional extended slab balcony’s allow for this strategy but it becomes challenging to maintain a duct penetration when attempting to break the thermal continuity between the interior floor slab and the balcony. Depending on the balcony connection detail alternative methods of ventilation may need to be considered.
Proper design and construction of the door leading to an exterior balcony (or patio) from a single-family dwelling unit or a house often presents significant challenges. A raised sill is required in order to achieve watertight exterior door opening. In addition, insulation or pavers installed above a balcony or a patio may result in elevation differences between exterior and internal surfaces. Accordingly, if not thoroughly planned and designed, an exterior balcony door may create a tripping, falling or slipping hazard.
It has been common practice in the City of Vancouver to construct a single “step-over” at the bottom of the balcony or patio door. To conform to headroom requirements in Part 9 of the VBBL, a minimum clearance of 1980 mm must be provided for all exterior balcony doors. It has been intended that a “stepover” would allow a person to step through the opening without stepping on the sill, thus maintaining the minimum head room clearance requirement of 1980 mm. This may be achieved where the curb height measured to the finished surface of the balcony deck or interior floor does not exceed the maximum rise of 200 mm permitted for a stair riser within a dwelling unit. The curb width must also not exceed 300 mm, including any baseboard heater along the inside of the curb, to ensure that people will instinctively “step over” rather than step on the curb, thereby maintaining the required headroom. Likewise, it is paramount that that the stepping surface will be on the same level on both sides of the door. See Figure 1.
In order to achieve level surfaces on the exterior and interior sides of the balcony door, it is important to take into account any finishes which may be installed, particularly on the balcony deck where insulation and/or pavers ← may be used. Pavers or insulation installed above the finished floor on the balcony side of the door with a 200 mm high “step-over” result in a higher than permitted “step-over” on the interior side. In order to alleviate the problem, designers or builders may add on a single step on the interior side of the “step-over” sill.
Part A: Understanding Balcony Thermal Bridging
Figure 1. Headroom Requirements for Residential Balcony Doors Policy / Code
Accessibility
Figure 2: Step over requirements from City of Vancouver’s Balcony and Patio Doors in Houses and Dwelling Units Bulletin
In order to encourage the provision of outdoor space, the City of Vancouver grants a floor space ratio (FSR) exemption of up to 12% of the maximum gross area available for new buildings that provide balconies. The City of Vancouver has also published design guidelines on ‘enclosed balconies’ to extend the yearround enjoyment of these outdoor spaces4 . Conventional approaches to constructing continuous slab balconies required little attention to Part 3 (Fire and Life Safety) of the British Columbia Building Code (BCBC). Until recently, accessibility to the balcony was not considered, however in recent years some jurisdictions have increasingly placed limits on the step over threshold dimensions to improve accessibility for persons with limited mobility, and in some cases require a percentage of units within a building to have wheelchair accessible balconies.
Section 3.8 of the BCBC differentiates between “accessible” dwelling units and “adaptable City Hall, 453 West 12th Avenue, Vancouver, BC V5Y 1V4 tel: 3-1- dwelling units.” In fully “accessible” dwelling 1, Outside Vancouver: 604.873.7000, website: vancouver.ca units, where people may be expected to be DOC/2016/154244
confined to a wheelchair, the Vancouver Building Bylaw (VBBL) and BCBC permit a maximum 13mm high threshold at the balcony door. Subsection 3.8.5 of the BCBC includes additional requirements to be incorporated into “adaptable dwelling units” which depending on the municipality may be required to be included in a building. "Adaptable dwelling units" are designed to allow occupants to age in place or entertain persons with mobility restrictions. In Vancouver, all dwelling units in new buildings are classified as “adaptable dwelling units.”
Figure 2. Minimum height to guard railing
A raised platform or a step on the exterior side of the balcony door located adjacent to a guard reduces guard height by the height of the raised platform. As a result, the guard becomes a hazard for children because the step or platform facilitates climbing. In addition, the guard becomes a falling hazard for an adult occupant.
In order to mitigate such risks without imposing undue burden to builders and owners, the City would permit measuring a minimum guard height diagonally. The minimum diagonal distance between the top of the guard and the top of the raised platform must be not ← less than 1,100 mm (see Figure 2). If this minimum safe distance cannot be maintained, the guard must raised.
Figure 3: Minimum height to guard railing from City of Vancouver’s Balcony and Patio Doors in Houses and Dwelling Units Bulletin
A recently published bulletin specifies the maximum step over threshold to be not more than 200mm high and 300mm wide. Alternatively, the VBBL will permit a “stepon threshold”; however, these may require interior landings and in some cases landings to the exterior with resulting guardrail height extensions to maintain the required minimum 1070mm height. See illustrations (Figure 2 and Figure 3) in Bulletin 2016-002-BU5. These threshold limitations and requirements should be considered in detailing the thermally broken balcony connection as well as the structural supports for the door and glazing systems described in the previous section. While not impacting the performance of the balcony, non-climbable guards of not less than 1070mm high are required by the building code on all balconies where the walking surface is more than 600mm above the adjacent surface.
Fire Protection
Part 3 of the BCBC mandates that for buildings required to be of non-combustible construction, the balcony supporting structure must also be of non-combustible construction.
However, the exterior balcony structure is not required to have a fire resistance rating. Accessibility Concerns This means that exposed steel or other noncombustible structural components of the balcony above exterior space would not require The City of Vancouver makes a continuous effort to create indoor and outdoor environments that are accessible to all minimum concrete cover or a fire protection . A balcony or a patio as described in this Bulletin is a portion of a dwelling unit that system as would otherwise be required on the needs to be accessible for occupants in wheelchairs. A typical 200 mm high “step-over” threshold does not exceed the height of interior floor slabs. a standard concrete curb within an accessible pathway outside of the building. Sentence 3.8.3.3.(5) in the City of Vancouver Building By-law permits curb cut ramps to be constructed The BCBC also requires additional fire without handrails even when their slope is more than 1 in 10. Accordingly, if required, a properly protection in rain screen wall assemblies incorporating a drainage cavity in excess of 25mm and containing combustible materials (cladding or insulation). To prevent the spread DOC/2016/154244 of fire within this cavity the code requires fire blocking at each floor level or at a maximum of 3m vertically and 20 horizontally. In continuous slab balcony construction—where the floor slab interrupts the envelope—this requirement is met by the slab. However, where the balcony has intermittent connection back to the base structure, this firestopping must be maintained by incorporating sheet metal flashing or non-combustible insulation within the drainage cavity.
Part A: Understanding Balcony Thermal Bridging
Constructability
Most Canadian high-rise buildings are built using conventionally reinforced cast-in-place concrete construction, where the internal floor plate extends through the building envelope to form the balcony slab. This typical construction methodology is coming under scrutiny due to its inherent thermal bridging. However, developers and contractors favour the simplicity and uninterrupted construction sequencing that this methodology allows. Any alternative construction methodologies will have to take this into account.
Some proprietary structural thermal break systems allow traditional construction methodology to be maintained with the addition of a thermally broken structural slab edge. Other balcony connection options required unique connection details and allow for off-site balcony prefabrication. Prefabrication provides efficiencies in balcony construction and construction sequencing by allowing the building envelope to be completed prior to the installation of the balconies.
Environmental
Balconies can be strategically designed to control environmental performance and improve the resiliency of a building. They can influence daylighting and shading, rainwater protection, wind, acoustics, and ventilation strategies, which all play a large role in building performance, occupant comfort, and a building’s marketability.
Daylighting and Shading
Through strategic placement of balconies, especially on the south face of a building, solar gains can be mitigated as the balconies serve as a shading device which reduces heating and cooling requirements. This also has an impact on the daylighting levels within the building.
Wind
The placement and location of balconies on a building can not only change the way wind interacts with a building but also airflow patterns around a building. High wind speeds around tall buildings can cause occupant discomfort while using balcony spaces and even lift unsecured items off of balconies. Wind tunnel and/or computer model studies are recommended to ensure safe conditions and occupant comfort.
Acoustics
The design of balconies can have a significant impact on noise penetration in residential units. Noise can be reduced through strategic balcony shaping, material choice, and the presence of a solid parapet at the edge of the balcony6 .
Rainwater Protection
Depending on their configuration, balconies can be used to protect spaces and building envelope systems below from rainwater and rainwater damage.
Mock-up of a thermally broken balcony for Delta Land Development Ltd.'s multistorey timber Canada’s Earth Tower.
