Case Model: How a Thermal Bridge Changes Internal Surface Temperature

This model shows the relationship between temperature factor and surface temperature. It is an illustrative sensitivity study, not a ψ calculation or a measured junction.

Objective and assumptions

Assume indoor temperature 20 °C, outdoor temperature −10 °C and three illustrative temperature factors f = 0.90, 0.75 and 0.60. Factors are inputs, not results from materials or geometry.

Reproducible method

Use f = (θsi − θe)/(θi − θe), rearranged as θsi = θe + f × (θi − θe). With a 30 K difference, surface temperatures are respectively 17, 12.5 and 8 °C.

What the result shows

At matching temperatures, a lower factor corresponds to a colder internal surface. This explains why junction assessment needs surface temperature rather than only average wall U-value.

Sensitivity

Changing outdoor temperature to 0 °C at the same factors gives 18, 15 and 12 °C. Constant factors are an assumption of this simplified check.

Limitations

For a real bridge, determine the factor from geometry, materials and suitable model boundaries. Without humidity and an applicable criterion, this example establishes neither condensation risk nor compliance.

ZeroHouse assessment diagram; not a construction detail.

Project review matrix
Item	Parameter	Check / result
f = 0.90	θe = −10 °C	θsi = 17 °C
f = 0.75	θe = −10 °C	θsi = 12.5 °C
f = 0.60	θe = −10 °C	θsi = 8 °C

What this means for your project

Use the model to understand sensitivity, not to select a detail. A project needs calculated factors, humidity and the relevant acceptance criterion.

Sources and scope of review

Checked: 2026-09-09. Editorial guidance; not evidence of compliance for an individual building.