Thermal bridging and energy performance · Lesson 3 of 4

Thermal bridging and energy performance: methods and worked application

The equation for effective U, the seven steps and the takeoff rules, two worked examples with every catalogue input sourced, how the result goes into an energy model, and the record a design team keeps.

You will be able to: Apply the effective U-value equation and the seven-step method to a parapet and a shelf angle, taking lengths along the plane of heat transfer and recording every input with its units and source.

11 min · 11 slides · 8 questions

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Slide 1 of 11

The effective U-value equation

The effective U-value is the sum of all linear heat flows (psi x length) and point heat flows (chi), divided by the total opaque area, plus the clear-field Uo.

In symbols, U_T = (sum of psi x L + sum of chi) / A_total + Uo, in W/(m²·K) or Btu/(h·ft²·°F).

BC Hydro describes it as a simple process of addition and multiplication.

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 on how to bring U-value calculations and the BC Hydro guide into the overall building design process. Uploaded in 2018. Checked against the video's description.

Video: Video 2 – Workflow by BC Housing, on YouTube. Chosen from the video's published description.

What this video shows: A U-value calculation and workflow worked on an example six-storey multi-unit residential building. Uploaded in 2018. Checked against the video's description.

Video: Video 3 – Example Calculation by BC Housing, on YouTube. Chosen from the video's published description.

What this video shows: The finer points of specific details and assemblies that may need extra attention when their effects are included in the calculation. Uploaded in 2018. Checked against the video's description.

Video: Video 4 – Special Considerations by BC Housing, on YouTube. Chosen from the video's published description.

Glossary

Effective U-value (U_T): BC Hydro: the total effective thermal transmittance of an assembly, found by adding the clear-field Uo to the sum of every linear heat flow (psi x length) and point heat flow (chi) divided by the total opaque area.
Plane of heat transfer: BC Hydro: the plane through which heat crosses the envelope; takeoff lengths are taken along it, so a balcony counts where it meets the wall and a parapet counts along the wall, not around the parapet.
De-rating an insulation layer: BC Hydro's method for energy models built from layers: lower only the insulating layer's R-value until the model's overall U-value matches the calculated one, leaving the other material properties alone.
Takeoff: Measuring the areas of clear-field assemblies and the lengths or counts of linear and point details from plans, elevations and sections, to multiply by their transmittances.