How thick should my thermal break be?

This is one of those ‘how long is a piece of string’ questions and depends on a range of considerations such as what (if any) performance criteria you are targeting, specific geometry of a detail as well as structural limitations. 

There is an inherent conflict between thermal and structural performance - the thicker the better from a thermal perspective but thinner the better from a structural one! Typically, we see thicknesses of 25mm of HiLo Break X specified by designers in a horizontal connection. If you are 'just' looking to optimise performance without spending too much time/money on this question, this is definitely the thickness we often see specified as a general rule of thumb.

Unfortunately the basic calc of R = 1/U that is tempting to use to demonstrate compliance with NZBC E3 1.1.3 tends to be inaccurate and gives an overly conservative required thickness. This is because heat flows in 3D so the specific geometry of the connection is not accounted for in the simple calculation. You can undertake 2D or, for an even higher degree of accuracy, 3D heat flow/thermal analysis if the project is targeting a certain performance criteria or has the budget to cover the modelling costs. Alternatively, a more general understanding of the performance can be found in published handbooks (such as BETB) with comparison to similar details. 

If you’re struggling with the detail of your structural thermal breaks, reach out and we can see if we can help.