SR344 Vapour control in New Zealand walls (2016)

This study aims to provide updated guidance on the role of vapour control layers in New Zealand walls.

A series of wall specimens were installed into a BRANZ test building that was humidified periodically over a 2-year period. Measurements within the walls showed that the humidity at the sheathing/underlay reached 100% in almost all of the walls, but this only manifested itself as liquid droplets in a minority of walls. The only walls that did not reach 100% humidity were those that had a smart vapour retarder between the interior lining and the insulation in the stud space.

The hygrothermal simulation software WUFI was used to simulate the wall specimens. It was originally expected that, for the simulation and measurements to agree, airflows through/in the walls would have to be accounted for. In general, the WUFI models agreed well with the experiment without this addition, with the main exception being the moisture level in the cavity. However, it is felt the airflow processes in walls still warrant further investigation.

The study suggests that it is not necessary to add a specific vapour control layer to prevent accumulation of liquid moisture. This is in line with previous advice from BRANZ. It is also shown that this practice is in line with current international guidance for similar climate zones, with acrylic paint systems and internal linings providing enough vapour resistance to prevent condensation damage further into the wall while still providing good drying capacity.