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, Heating design and Cooling design calculations.
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.
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.
Definition of Metal cladding
Constructions involving metal cladding are roof or wall systems where metal forms an integral part of the construction, such as metal twin skin systems where the insulation is located between the metal skins and where the metal skins are typically in the range 0.4 mm to 1.2 mm. Cladding with z-spacers would come into this category as would composite metal panel systems.
If the metal is simply used as an external shield against weather, such as a rainscreen, this is not classed, for the purposes of calculations as "metal cladding".
Metal cladding systems are divided into two broad categories, these being:
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 |
Definitions for Psi values not including metal cladding are as above.
How it Works
DesignBuilder calculates a total linear bridging transmittance for each zone by summing the length for each of the 14 bridging categories and multiplying each by the Psi value entered for that category. The total bridging length is calculated based on the zone outer dimensions.
You will need to confirm that you have set the Psi values accordingly with the convention set.
The total bridging transmittance to outside for the zone is included in the EnergyPlus model using a single standard WallExterior surface type per zone with no film resistance applied. These surfaces are located below the building to avoid interfering with shading calculations.
The area of these fictitious surfaces is calculated such that the total conductance is the same as that of the sum of the linear bridges based on the known conductance of the predefined construction called LinearBridgingConstruction.