The envelope of a Passive House is designed to have excellent levels of thermal insulation. To ensure that the insulated shell functions as intended, thermal bridges should be minimised. A thermal bridge is a weakness in the facade, or a path of least thermal resistance, and allows heat transfer across elements where insulation (resistance) is not provided to an adequate level.
When designing a Passive House, the thermal bridges in the construction can be the most difficult element to eliminate. The general guidance is that thermal bridges should be eliminated, or: Ψ ≤ 0.01 W/(mK). Thermal bridges occur everywhere in a building, and can be tough to track down.
Calculation of thermal bridges requires some technical nous. The most straightforward way of ensuring your proposed construction is optimised is to use a detail with established thermal properties. The Passivhaus-Bauteilkatalog or Details for Passive Houses is an example of one such resource, and provides data such as total U-value and thermal bridge coefficients for common constructions and junction details.