Part one in our series on managing moisture - an introduction to Surface Temperature Factor, fRsi

fRsi, the surface temperature factor (also referred to as the Hygene Criterion), is a key metric in assessing the risk of condensation developing in a structure.

In simple terms, fRsi is a unit-less number between 0 - 1 that represents the likelihood of condensation at a junction. It is the ratio of the internal surface temperature to the difference in internal–external temperature, so the lower the number (ie closer to 0), the higher the risk of moisture forming.

The fRsi value for a junction is confirmed one of two ways:

1. A 2D or 3D heat flow model of your detail, or

2. From a pre-calculated value if your design uses a detail taken from a design library eg. PHINZ High Performance Construction Details Handbook.

   
   

You then compare the fRsi value against a benchmark value related to your climate zone - your fRsi value needs to be higher than the benchmark value (i.e. the fRsi/hygiene criteria) for your zone. 

If your detail does not meet the fRsi criteria, to avoid the risk of moisture developing in your structure, you need to improve the thermal performance of the detail. This could be by changing the geometry, materials or, if appropriate, introducing a thermal break.