Thermal bridge allowance only applies to commercial buildings under NCC2019 and not to residential builsings. The following are copied from NCC 2019 page 664:

Total System U-Value (W/m².K), for the purposes of Volume One, means the thermal transmittance of the composite element allowing for the effect of any airspaces, thermal bridging and associated surface resistances.
Total System U-Value (W/m².K), for the purposes of Volume Two, means the thermal transmittance of the composite element allowing for the effect of any airspaces and associated surface resistances.

At Energy Compliance, we allow a negative effect of thermal bridging (or thermal break) in our wall cross section thermal analysis. For instance, in a wall with timber studs, the thermal bridge reduction is at least R0.3 and it can be as much as R1.0. This is significant effect which adversely affects the building thermal performance if unaccounted.

For instance, the wall construction shown below has 90% covered by insulation and 10% covered by timber studs at 600mm and 3 noggins.

The total resistance of the bridged layer is calculated as per equations 5 and 6 of NZS 4214:

Considering timber stud wall thermal bridge effect reduced the total insulation value of the wall from R2.2 to R1.93 (12% reduction).

If timber frame is replaced with metal frame + 12mm EPS insulation (R0.2), then the thermal bridge effect can reduce the total insulation value of the wall from R2.2 to R1.1 (50% reduction). Note that a better thermal break material such as 12mm PIR (k=0.024 W/mK) that provide R0.5 thermal break insulation effect can significantly reduce the thermal bridge allowance.

Following are further examples of thermal bridge effect in reducing the overall R value of the wall or roof:

 

The logic behind thermal bridge

to better understand thermal bridge effect, consider it as a physical barrier or an air pocket that stops you from adding insulation to say 10% of the roof or wall. This small areas with reduced or no insulation creates a massive heat leakage. See the following examples. The effect can be best described as having a highly insulated house and leaving the door open. Drop in insulation ussually occurs where the roof rafters and wall studs are.

If you add R4 insulation to 90% of a wall or roof surface and add R0.1 to the other 10% then the overall insulation of the wall or roof becomes: R0.82

If you add R4 insulation to 90% of a wall or roof surface and add R0.2 to the other 10% then the overall insulation of the wall or roof becomes: R1.38

If you add R4 insulation to 90% of a wall or roof surface and add R0.4 to the other 10% then the overall insulation of the wall or roof becomes: R2.1

If you add R4 insulation to 90% of a wall or roof surface and add R0.8 to the other 10% then the overall insulation of the wall or roof becomes: R2.85

If you add R4 insulation to 90% of a wall or roof surface and add R3.0 to the other 10% then the overall insulation of the wall or roof becomes: R3.47

If you add R4 insulation to 90% of a wall or roof surface and add R4.0 to the other 10% then the overall insulation of the wall or roof becomes: R4.0

As demonstrated above, adding a small insulation to the areas with drop in insulation (rafters or studs) radically improves the loss in insulation. For this reason, the small added insulation over the roof rafters and wall studs are called thermal break. You can download the excel file we created for the above figure from here. Overall R value calculator considering thermal Bridge

As thermal bridge occures in the location of stud, rafters and purlins, in following this construction components are illustrated:

Purlin vs Rafter

Purlins are along the wall and rafters are perpendicular to the wall. If you increase the spacing between the rafters, purlin depth will increase. Purlins center to center spacing is around 1.2m.

Studs are vertically installed columns (every 600mm) or insulation gap makers in the wall.

 

Important notes:

A bridge layer parallel with insulation layer should never be bounded by air layer (NZS4214). It should be bound by solid layer like plaster board and concrete walls. In calculating the parallel insulation and thermal bridge material R value, it is critical to consider the air layer R value for each of the parallel layers separately and then assess the combined overall R value.