Whole Building Energy Performance
Data on Linear Transmission and Categories of Details
The concept of using linear and point transmission as a way of calculating effective thermal resistance has been common in Europe, in part because it is incorporated in energy use standards of some countries and programs such as Passivhaus. Because heat flow and linear transition is so geometry- and material-dependent, the most accurate estimated Ψ values come from specific 2D or 3D modeling, see also explanations in chapter 2.5 heat flow calculations. 2D modeling services with programs such as THERM are widely available and should be considered. There are, however, sources of generic data that allow anyone with a calculator or spreadsheet program to assess the impact of thermal bridges using "catalog data".
A good source of data, in the absence of more specific information is ISO 14683:2007, Thermal bridges in building construction -- Linear thermal transmittance -- Simplified methods and default values, which is available for download on the internet. This standard outlines the methods of calculating linear transmission used in the European standards and provides an Annex with default Ψ values for many generic intersections of building enclosure assemblies. The default values, some shown on the below, are based on 2D analysis using:
- assumed depths of building assemblies, and
- assumed values of insulation in the building assemblies
The assumptions used were select to provide conservative Ψ values (i.e. maximum expected values). The standard generally provides three different Ψ values, based on:
- external dimensions, measured between the external faces of the enclosure assemblies;
- internal dimensions, measured between the finished internal faces of each room (thus excluding the thickness of internal partitions); and
- overall internal dimensions, measured between the finished internal faces the enclosure assemblies (thus excluding thickness of internal partitions)
Any of these dimensional system can be used in the Q = [Uo · A + Σ(Ψi · Li) + Σ(χj · nj) ] ΔT calculation procedure, provided it is used consistently throughout the procedure. Most energy modelers calculate their area inputs from plans and elevation that show external dimensions so they would use Ψe.
Figure 26A: examples (like balconies, intermediate floors, parapets) used in the International Standard ISO 14683, they give a certain number for the typical energy loss for a constuction
Figure 26B: examples (like balconies, intermediate floors, parapets) used in the International Standard ISO 14683, they give a certain number for the typical energy loss for a constuction
In the ASHRAE-sponsored research program, ASHRAE 1365 RP Thermal Performance of Building Envelope Details for Mid- and High-Rise Buildings, 3D thermal modeling was used to assess the heat transfer and surface temperatures of 40 construction details common in noncombustible construction. Data is provided on the specific of construction effective U-values, linear and point transmission at junctions and temperature indexes at critical surfaces.
The authors of ASHRAE 1365, Morrison Hershfield Limited, have subsequently undertaken work for a variety of clients extending this catalog of information. They have summarized their observations in the table below categorizing the range of Ψ values they have come to expect. Morrison Hershfield recently completed the Building Envelope Thermal Bridging Guide – Analysis, Applications, and Insights in 2014 and is available for download. This Guide expands on the 1365-RP methodology, adds the results of many more details, and provides insights to various strategies to mitigate thermal bridging in the context of energy savings and cost effectiveness.
Table 7: Summary of results Morrison Hershfield Limited has observed, categorizing the range of Ψ values they have come to expect.