Zero thermal bridging concepts
The concept of ‘zero thermal bridging’ can only be used successfully when using external dimensions. Due to geometry it is not possible to achieve zero thermal bridging using internal dimensions. The Passivhaus Standard considers a detail to be ‘thermal bridge free’ when the psi-value is <0.01 W/mK.
Rules of thumb
The use of external dimensions allows a Passivhaus Certifier to conduct a visual assessment of thermal bridging. If they are satisfi ed, then thermal bridging calculations may be avoided. A detail may be considered ‘thermal bridge free’ when the following rules are met:
Prevention: Try to avoid disruptions to the thermal envelope.
Penetration: Where an unavoidable disruption is made to the insulating layer, the thermal resistance in the insulation should be as high as possible.
Junction: At the junction between building elements insulating layers should join without interruption or misalignment.
Complexity: Minimise complexity – this reduces the chance of error and can assist buildability.
Geometry: Design edges and corners to have as obtuse angles as possible. Pay specifi c attention to corners, ground floor/wall interfaces, eaves, verges, gable ends, structural openings (head, jamb and sill).
Buildability: Ensure that simple robust details are developed with adequate construction tolerance.
Construction tolerance: Think of a reasonable dimension for a construction tolerance, and then double it.
Protection: Protect insulation materials from mechanical damage and the weather.
Workmanship: Avoid gaps and discontinuities and ensure a clean working environment.
Improvisation: Do not improvise, read the drawings and specification or ask the design team if unclear.
Inspection: Inspect details prior to closing any openings, to ensure that the there are no gaps and discontinuities.