The Concept of Linear and Point Transmittance and its Value in Dealing with Thermal Bridges in Building Enclosures

Thermal bridging through insulating layers can greatly reduce the thermal performance of building assemblies. As such, determining the effects of thermal bridging is often of immense importance to building engineers, energy modelers and architects in accurately designing a building. This can be very difficult to accomplish, and as a result many building codes and standards do not comprehensively address this problem.

In North America, the common approach in calculating the area effects of thermal bridging is to use an area weighted average of U-values. For assemblies with easily definable geometry and U-values, such as walls with windows, the area weighted process is straightforward and has been well established. However, for many assemblies, identifying the effective area of a thermal anomaly used in calculations can become either too arbitrary or too complex, especially when dealing with three dimensional heat flow paths. The purpose of this paper is to introduce a simple methodology, incorporating the concepts of linear transmittance, in order to assist practitioners in overcoming these complexities for determining the heat flow through many types of details.

The proposed method involves modeling an assembly to find its heat flow, with and without the thermal anomalies, and attributing that difference to individual contributions of point or linear loads. Determining the area of influence of a thermal anomaly is not needed in the heat transfer calculations and only involves the number of point occurrences (i.e. # of steel beam penetrations through exterior insulation) or linear distances (i.e. slab length across a building face). In order to find the overall heat flow in a building assembly, all the linear and point loads can be simply added together with the clear field heat flow (the heat flow through the assembly without the thermal anomalies). With that total heat flow known, several other parameters, such as an overall U-Value, can be easily calculated. Included in this paper are examples showing where the area weighted average approach encounters drawbacks and why the linear transmittance method is better suited to quantify the effect of thermal bridging of opaque building envelope assemblies.

The idea of linear transmittance has been widely used in practice in various forms across Europe. This concept, however, has yet to gain wide acceptance in North American codes, standards and practices. This paper is intended to bridge the two continental approaches, and incorporate linear transmittance into current methods of assessing building enclosure performance in North America.