Thermal bridging and energy performance · Lesson 1 of 4

Thermal bridging and energy performance: principles and terminology

Why heat takes the easy path through metal, the three kinds of transmittance (Uo, psi and chi), how psi is found, what AFS tools leave out, and where the designer of record's responsibility begins.

You will be able to: Define a thermal bridge, tell clear-field, linear and point transmittance apart with their symbols and units, and state who owns the thermal numbers on a project.

10 min · 10 slides · 7 questions

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Slide 1 of 10

Heat takes the easy path

BC Hydro's guide says thermal bridging is caused by highly conductive elements that penetrate the thermal insulation, or by misaligned planes of insulation.

These paths let heat bypass the insulating layer and reduce how well the insulation works.

If major bridges are not addressed, adding insulation may not cut overall heat flow much, because heat takes the path of least resistance.

Sources

Building Envelope Thermal Bridging Guide, version 1.3 (2019) - BC Hydro Power Smart (prepared by Morrison Hershfield; BC Housing among the co-sponsors)

Videos for this lesson

What this video shows: BC Housing's introduction to the BC Hydro guide: an overview of the U-value calculation method and a summary of what the guide contains. Uploaded in 2018, so check figures against the current guide. Checked against the video's description.

Video: Video 1 – Introduction to the Building Envelope Thermal Bridging Guide by BC Housing, on YouTube. Chosen from the video's published description.

Glossary

Thermal bridge: BC Hydro's guide: a localized area of high heat flow through an insulated envelope, caused by highly conductive elements that penetrate the insulation or by misaligned planes of insulation.
Clear-field transmittance (Uo): BC Hydro: the heat flow per area of the plain wall, roof or floor assembly, including uniformly distributed bridges such as brick ties, girts and studs.
Linear transmittance (psi): BC Hydro: the extra heat flow caused by a linear detail such as a shelf angle, slab edge, parapet or window interface, expressed per length: W/(m·K) or Btu/(h·ft·°F).
Point transmittance (chi): BC Hydro: the extra heat flow of a single countable thermal bridge, such as a structural beam penetration, expressed per point: W/K or Btu/(h·°F).
Designer of record: In this course, the architect or engineer who owns the thermal numbers of a project (psi values, U-factors and code compliance); AFS fabricates the metal the approved drawing shows.