Thermal Bridging Can Degrade Wall Performance 70%
Designers routinely underestimate total heat flow through their wall assemblies by 20% to 70%, according to a new report prepared by the engineering firm Morrison Hershfield and sponsored by several other Canada-based partners. The good news? Tweaking envelope details may be more effective than adding another inch of insulation.
That’s because adding insulation doesn’t do a lot of good if thermal bridging allows heat to flow right through the assembly. “Building Envelope Thermal Bridging Guide – Analysis, Applications, and Insights” aims to help practitioners reduce thermal bridging with guidance on how to calculate the R-value of an entire assembly, accounting for the thermal performance of each part.
Details like small, cross-sectional areas of shelf angles, or flashing around windows, not only degrade assembly performance but also increase the risk of condensation (see The Hidden Science of High-Performance Building Assemblies). The guide catalogues the thermal performance of some common wall, floor, and balcony slab assemblies, glazing transitions, and parapets for easy comparison, and provides the material data used to calculate the performance of those details.
Construction cost estimates are also included, showing, for example, that a thermally broken concrete balcony can cost $266 more per meter, or $81 more per foot, compared to a continuous concrete balcony. However, “Even some expensive options look attractive when compared to the cost-effectiveness of adding insulation,” argues the report. A case study included in the appendix finds that, although upgrading to thermally broken balconies and parapets for a high-rise multi-unit residential building with 40% glazing would initially cost more than increasing the effective wall assembly R-value from R-15 to R-20, the payback period would be more than seven times shorter.
Published September 23, 2014