A.1 Overview

Part A: Understanding Balcony Thermal Bridging

As a basic principle, heat travels through the path of least resistance. In the context of balconies, these paths typically occur where highly conductive materials penetrate through the thermal barrier to provide structural support for a balcony.

Mid and high-rise concrete buildings typically have balconies that are constructed as extensions of the building’s castin-place concrete floor slab. The effective R-value for such penetrations is approximately R-1 (0.2 RSI) where insulated opaque wall assemblies typically have effective R-values of R-5 to R-20 (0.9-3.5 RSI). The result is a path of least resistance for heat to escape through to the balcony slab.

Illustration of heat loss path through cantilevered balcony slab. The majority of heat loss is through the balcony slab and fenestration.

“After accounting for windows and doors, exposed concrete slab edges and balconies can account for the second greatest source of thermal bridging in a multi-storey building.” 3

The impact of thermal bridging at balcony slabs is well understood and has been studied for many years using two and three dimensional heat flow simulation modelling2 ,3 .

Thermal bridging through cantilevered concrete floor structures increases heat loss and results in low interior surface temperatures which have the potential to cause a number of undesirable effects, including:

• Increased space heating and cooling requirements for the building;

• Condensation and mould growth due to colder interior surfaces; and,

• Occupant discomfort due to radiant heat loss to colder interior surfaces and convective air drafts.

To mitigate these undesirable impacts and allow buildings to achieve more stringent building performance requirements, alternative balcony connection details must be considered. These details attempt to lessen heat loss by reducing the extent and conductivity of the materials penetrating the thermal barrier.

Thermal imaging of a building with cantilevered concrete balconies. Red and white horizontal stripes on the right are indicative of thermal bridging and heat loss through the slabs at balcony locations.

A thermal model of a cantilevered concrete balcony. Plume of green and yellow radiating inwards at balcony connection is indicative of thermal bridging occurring.