Overview

The mechanisms for implementing thermal bridging in the VE are:

 

Clear field transmittances are thermal bridges from uniformly distributed secondary structural components in construction assemblies such as brick, ties, framing or studs.

 

They are defined in the ApCDB editor in the surfaces tab by setting the conductivity property of a continuous layer (typically ties, fasteners that typically have a small effect) or by inserting a composite layer (typically framing, studs, joists, rafters, mortar, battens, plaster dabs, mullions in curtain walling).

 

 

This is a simple allowance for non-repeating thermal bridging that is defined in the ApCDB editor on the thermal bridging tab. This is the only option for internal thermal bridging (internal partitions, internal floors/ceilings).

 

This option has historically been used for compliance purposes or can be utilised in early design when details and Psi values are not known. This option cannot be used in conjunction with external linear non-repeating transmittances (Psi).

 

 

Linear non-repeating transmittances occur in the building envelope such as:

 

- Junctions/transitions between external elements - corners of walls, wall to roof, wall to floor, roof to roof.

- Junctions/transitions between internal walls and external walls & roofs.

- Junctions/transitions of intermediate & ground floors and external walls.

- Around windows and doors (sill, lintel, jamb).

- Around rooflights (kerb).

 

Linear non-repeating transmittances are defined by Psi values in the ApCDB editor on the thermal bridging tab. Psi values for standard details for a number of standards are provided, or the user can enter custom values. This option cannot be used in conjunction with linear non-repeating transmittances (coefficient). The VE automatically ‘takes-off’ the linear dimensional information from the model geometry.

 

Random transmittances (area, linear, point):

 

Random transmittances occur in the building envelope as a result of specific features such as:

 

- Structural components for elements such as balconies, fire escapes etc.

- Penetrations such as flues, pipes & ducts.

 

Random transmittances can be defined as area, linear or point types to reflect the design detail, for example a balcony might be a slab (area) or bracket (linear) or bolted steel (point).

 

Random transmittances are defined by geometry in Model-iT and assigned types created in the ApCDB via the Random thermal bridging tool. The VE automatically ‘takes-off’ the linear dimensional and count information from the model geometry.

 

External thermal bridging locations definable in the VE follow those defined in ISO 14683: 2017, with the following differences:

 

Fig: Thermal bridge locations (extract from ISO 14683)

 

Specifically for location GFn users need to take care when setting up the model to avoid double counting, for example if a bracket at the base of the wall exists or a simple slab U-value approach is adopted GFn maybe non-zero. If an exposed floor slab exists, such as the details in ISO 14683 table C2 GF1 to GF8 a KIVA definition might be utilised and GFn set to zero.

 

The detailed thermal bridging approach utilises the internal and external surface temperatures to determine the heat flow. For the external surface this includes incident and long wave radiation i.e. the same external boundary state as the parent clear field / opaque surface. In contrast the simplified approach is derived from a steady state method that utilises outside air temperature.

 

Finally, it should be noted that thermal bridging will utilise the external condition set by the surface adjacency, so for example an external wall set-up as a basement wall with an adjacency condition set to a profile.