Thermal Bridging Effective U-Factor Calculator
Slab edges, window perimeters, balconies and fasteners let heat bypass insulation. Add them to the wall’s clear-field value to see its real R-value.
Method reviewed October 26, 2026. Version 1.0.0. How it works
Result
- Effective R-value
- 13.33h·ft²·°F/BTU
- Effective U-factor
- 0.075BTU/h·ft²·°F
Effective R-13.3 versus R-20 clear-field: bridges carry 33 % of the heat.
- Thermal bridges carry 33 % of the heat through this wall — improving the details may matter more than adding insulation.
- Clear-field U-factor
- 0.05BTU/h·ft²·°F
- Share of heat flow through the bridges
- 33.3%
- Heat flow through the wall
- 3,750BTU/h
Show the working
- Clear-field U₀ = 1 ÷ R1 ÷ 20 = 0.05
- Linear bridges Σ(ψ × L)20 BTU/h·°F
- Point bridges Σχ0.5 × 10 = 5 BTU/h·°F
- U = U₀ + (Σψ·L + Σχ) ÷ area0.05 + 25 ÷ 1,000 = 0.075
The linear and point transmittance method
Researchers at the National Research Council of Canada describe the overall U-factor of a building component as:
- U = clear-field U + (Σ psi × length + Σ chi) ÷ area
The clear-field U-factor covers the wall away from the details, including evenly repeated framing. Psi (ψ) is the extra heat flow per foot of a linear detail per degree, such as a slab edge or window perimeter. Chi (χ) is the extra heat flow at a point, such as a beam or anchor.
Where the values come from
Psi and chi values depend on the exact detail and come from two- or three-dimensional heat-flow modelling, published thermal-bridge catalogues, or manufacturers of thermal-break products. No values are built in — the defaults are examples only. Enter psi in BTU/h·ft·°F (or W/m·K in metric mode) and chi in BTU/h·°F; 1 W/K equals 1.8956 BTU/h·°F.
Worked example
These are the example values the calculator opens with, and the result the engine calculates for them.
Inputs
- Wall area
- 1,000 ft²
- Clear-field R-value (away from bridges, incl. air films)
- 20 h·ft²·°F/BTU
- Linear bridge 1: psi value
- 0.2 BTU/h·ft·°F
- Linear bridge 1: length
- 100 ft
- Linear bridge 2: psi value
- 0 BTU/h·ft·°F
- Linear bridge 2: length
- 0 ft
- Linear bridge 3: psi value
- 0 BTU/h·ft·°F
- Linear bridge 3: length
- 0 ft
- Point bridge: chi value (BTU/h·°F each)
- 0.5
- Number of point bridges
- 10
- Inside − outside temperature
- 50 °F
Result
- Effective R-value
- 13.33 h·ft²·°F/BTU
- Effective U-factor
- 0.075 BTU/h·ft²·°F
- Clear-field U-factor
- 0.05 BTU/h·ft²·°F
- Share of heat flow through the bridges
- 33.3 %
- Heat flow through the wall
- 3,750 BTU/h
Effective R-13.3 versus R-20 clear-field: bridges carry 33 % of the heat.
Assumptions
- Effective U = clear-field U + (Σ ψ × length + Σ χ) ÷ area, the linear- and point-transmittance method; ψ and χ come from a catalogue or 2-D/3-D modelling for each detail.
Limitations
- No ψ or χ values are built in — they depend on the exact detail and must come from a published catalogue, the manufacturer or modelling.
- Steady-state; the clear-field value must already include evenly repeated framing.
Frequently asked questions
How is this different from the Wall Assembly R-Value Calculator?
That tool handles evenly repeated framing with the parallel-path method. Use its result as the clear-field value here, then add the linear and point details.
Why can thermal bridges matter so much?
A continuous concrete slab edge or balcony can carry as much heat as a large area of well-insulated wall. The tool shows the share of heat flowing through the bridges.
Can I enter metric psi values?
Yes. Switch the site to metric units and psi is entered in W/m·K; lengths in metres.
Sources
- Evaluating Suggested Lengths used for Cut-off Planes using the Effective Length Calculation Procedure (Hayes, Ghobadi & Moore, NRC Canada; COMSOL Conference 2020) — National Research Council Canada authors, hosted by COMSOL. Equation 1: U = (Σ(ψL) + Σχ) ÷ Area + U₀, with ψ the linear thermal transmittance (W/m·K) times its length and χ the point thermal transmittance (W/K); values come from modelling or catalogues.
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