What is a Thermal Bridge?
Thermal bridges are localized areas with higher thermal conductivity than their neighbouring areas. The rate of heat flow though a thermal bridge depends on a number of factors:
- The temperature difference between the heat source and heat "sink"
- The thermal conductivity of the materials passing through the insulation layer
- The cross sectional area of the thermal bridge
- How easily heat can get into and out of the thermal bridge, which, in turn, depends on:
- The relative area and surface conductivity of the surfaces of the thermal bridge facing the source of heat and those facing the heat sink
- The lateral heat flow paths in the assembly that can bring heat to and from the thermal bridge
It is simple to say that "heat flow takes the easiest path," but it is sometimes very difficult to analyze what those three-dimensional paths are, how much heat flows through them, and what actually happens when you block one path. In fact, this analysis was almost impossible before the availability of 2D and 3D computer models. The recognition of how significant thermal bridges can
be – and what the best ways to mitigate them are – has grown in direct relation to the availability of such tools. Still, one needs to understand the basic principles of heat flow through thermal bridges in order to effectively mitigate them.
Material Thermal Bridges
The most obvious kind of thermal bridge occurs when a thermally conductive element passes through an insulating layer. A typical example would be anchors penetrating a layer of insulation, see Figure 3. These metallic anchors allow more heat to flow than the surrounding insulation.
Figure 3: Cross-section through three materials (dark gray with high conductivity (steel), gray with medium conductivity (concrete), and light gray with low conductivity (insulation)); the direction of heat flow is shown by the arrows. The heat flows from the warm room (bottom edge of the image) to the colder area (top edge of the image) through the materials.
Figure 4: Cross-section of a building corner. The streamlines show the direction of heat flow from the warm to the cold area. The linear heat flow from an undisturbed wall is significantly affected by the geometric conditions.
Geometric Thermal Bridges
There is another kind of thermal bridge that depends on geometry, rather than on materials with different conductivities. Geometric thermal bridges can occur when the heat-emitting surface is smaller than the heat absorbing surface. Building corners are a typical example, see Figure 4. Interior surfaces in the corner can be colder than other interior surfaces because more heat can flow due to the larger emitting surfaces.