Linear Thermal Bridges at Junctions

Construction tab in model data under Construction header


You can model linear thermal bridging at junction such as wall to wall corners, wall to floor edges etc using the settings under the Linear Thermal Bridges at Junctions header on the Construction tab. These settings are used at zone level and apply to all surface junctions in the zone. Defaults are provided at building and block levels.


The default Psi values in DesignBuilder are the based on BRE IP 1/06 values degraded by the greater of 0.04 W/mK or 50%. More details on this and how the data relates to the UK NCM can be found in the NCM modelling guide.


Note: The linear bridging defined by these Psi values is used in Simulations, SBEM and DSM calculations as well as in Heating design and Cooling design calculations.

Specify Psi Values

If you would like to include the effect of thermal bridging in the zone/block/building then check this option and enter the Psi values in the cells below.

Note: Negative psi values can be entered, however if the total zone linear heat conduction rate is calculated as negative it is not included in the model (since EnergyPlus has no concept of negative conduction). In this case a corresponding reduction in surface area must be included manually elsewhere in the zone. The required negative conductance is written out in W/K to the IDF file as a comment.


Note that while the effect of the heat transfer through linear thermal bridges is included in Heating and Cooling design calculations and in Simulations when the option is activated, the bridging component of the heat transfer is not displayed explicitly in the Heat Balance results displayed on the DesignBuilder results screens. However, you can obtain a report of the sum of the linear thermal bridging heat conduction for each zone by viewing results using the ResultsViewer. To do this look for Surface Inside Face Conduction Heat Transfer Rate <IDFZoneName >_LINEAR BRIDGING" reports in the ResultsView report list panel.

Psi Values Including Metal Cladding

The Psi values under this header are linear transmittances for junctions where there is metal cladding in a construction on one or more sides of the junction. Metal cladding is important for thermal bridging calculations as junctions involving it tend to create a higher heat flow than those without.


Units are W/m-K or Btu-in/h-ft2-F, i.e. heat flux per length of junction per unit difference in inside to outside temperature.



You can define whether a construction includes metal cladding on the Constructions dialog.


Junction type Description  
Roof-Wall The average Psi value for junctions between external walls and roofs  
Wall-Ground floor The average Psi value for junctions between external walls and ground floors  
Wall-Wall (corner) The average Psi value for external wall corners  
Wall-Floor (not ground floor) The average Psi value for junctions between external walls and internal floors  
Lintel above window or door The linear transmittance created by lintels above windows and doors  
Sill below window The linear transmittance created by sills below windows  
Jamb at window or door The linear transmittance created by window and door jambs  

Psi Values NOT Including Metal Cladding

Definitions for Psi values not including metal cladding are as above.