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.
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.
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 above 14 bridging categories and multiplies each by the Psi value entered for that category. The total transmittance for the zone is modelled in EnergyPlus 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 linear bridging defined by these Psi values is used in Simulations, Heating design and Cooling design calculations,