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, however, 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 an 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 presentation 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 loss through many types of details.
The proposed method involves modeling an assembly to find its heat loss, with and without the thermal anomalies, and attributing that difference to individual contributions of point or linear loads.
These transmittances only involve 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), so that no areas are needed in the calculations for these anomalies. In order to find the overall heat loss in a building assembly, all the linear and point loads can be simply added together with the clear field heat loss (the heat loss through the assembly without the thermal anomalies). With that total heat loss known, several other parameters, such as U-Value, can be easily calculated. Included in this presentation are examples showing where the area weighted average approach encounters drawbacks and how the linear transmittance method would be more appropriate.
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 and standards. 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.