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.
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.
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.
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.
|The average Psi value for junctions between external walls and roofs
|The average Psi value for junctions between external walls and ground floors
|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.
DesignBuilder calculates a total linear bridging transmittance for each zone by summing the bridging length for each of the 14 categories and multiplying each by the user-defined Psi value for that category. The total bridging length is calculated based on the zone outer dimensions.
You must check that you have set the Psi values according to the convention set.
The total zone linear bridging transmittance is included in the EnergyPlus model using a single WallExterior surface per zone with fixed interior and exterior convection coefficients of 3 and 25 W/m2K for the interior and exterior sides of the surface respectively. Visible, solar and thermal heat transfer is effectively zero through use of zero of absorptance values. The material layer used in the wall has a resistance of 0.6267 m2K/W. Note that the overall resistance of the wall construction (LinearBridgingConstruction) including the layer and the film convective resistances is approximately 1 m2K/W, i.e. a conductance of 1 W/m2K.
The area of each WallExterior surface is calculated so that its total conductance (W/K) is the same as that of the sum of the zone linear bridges. To achieve that the height of each surface is set to 1m and the total zone linear bridging conductance is written to the Length field. This results in the user-specified linear bridging conductance being applied in the simulation at all timesteps.
The linear bridging WallExterior surfaces are located below the building to avoid interfering with shading calculations.
Note: When checking the "Envelope Summary, Opaque Exterior" tabular data in the Summary report generated by EnergyPlus, please bear in mind that these fictitious surface areas are included.