“We concluded structural thermal breaks would be a more constructible solution,” recounts the project’s engineer of record. “The owner and contractors overcame initial cost concerns once they understood how structural thermal breaks fit into the design strategy, which emphasized sustainability and human comfort.”
While relatively new to North America, structural thermal breaks have been used in Europe for more than 35 years.
“I have known about structural thermal breaks since 2007, when they first started appearing in North America,” the engineer explains. “We installed them on smaller projects before using them on 3 Civic Plaza, which is the largest project in British Columbia to date to incorporate structural thermal break technology.”
Design work for the project began in 2012 and ground was broken two years later by ITC Construction Group.
How structural thermal breaks function
The structural thermal breaks used in this project consist of a fabricated module of graphite-enhanced expanded polystyrene (EPS) insulation along with stainless steel rebar running through the material for tension and shear strength. The insulating block is approximately 98 per cent less conductive than concrete, and the stainless steel rebar is roughly one-third as conductive as carbon steel rebar, effectively reducing heat loss at the penetration.
The thermal break modules are positioned in line with other building envelope insulation, and tied into the rebar of the interior floor and exterior balcony slabs prior to casting in concrete. Thus the thermal break insulates the interior floor slab from the exterior balcony, significantly reducing heat loss, while transferring the loads imposed on the exposed slab back to the interior.
Depending on the type of construction, structural thermal breaks may also be designed for steel-to-steel connections.
The rate of heat flow through a thermal bridge depends largely on the thermal conductivity of the material penetrating the envelope, with carbon steel exhibiting the greatest conductivity of the most common structural building materials, and stainless steel at one-third the thermal conductivity of carbon steel.
Structural thermal breaks for steel construction consist of an 80-mm (3-in.) thick proprietary insulating material, placed between stainless steel plates on each face with a stainless steel tube welded between them. The plates and tube impart the module with the requisite stiffness to transfer axial, shear, and bending forces, while minimizing or eliminating the risk of mould and corrosion by preventing interior surfaces from cooling and forming condensation.
The placement of the engineered unit between the endplates of steel beams minimizes the surface area where the loadbearing components of the thermal break cross the insulating layer and attach to the structural steel beams, satisfying both thermal and structural requirements. The use of stainless steel loadbearing components contributes to the insulating performance of the module, as a material with a conductivity of approximately two-thirds less than that of structural carbon steel.
As a loadbearing element, this thermal break is engineered to handle normal forces in addition to bending and vertical shear forces transferred by the steel beam. At minimum, two modules are stacked one atop the other, unless only low shear force is being transferred. Additional thermal breaks can be incorporated vertically and/or horizontally to handle higher loads at the connection points.
Team specifies four types of thermal breaks
The residential units in 3 Civic Plaza begin on the 15th floor, with balconies ranging in size from 4.7 to 9.5 m2 (51 to 102 sf).
To insulate and support them, the design team specified 1755 structural thermal break modules consisting of:
- modules providing flexural strength for cantilevered balconies (Figure 1);
- modules providing shear strength only for supported balconies (Figure 2); and
- insulation filler modules (Figure 3).