Ground Moisture Barrier - Doing it Right

15 June 2022

Complying with Ground Moisture Barrier for rentals involves more than just rolling out a sheet of plastic!

Installing a moisture barrier (thick polythene sheeting) on the ground under your house will keep the moisture in the ground and stop it evaporating up into your home making it damp and unhealthy.

Moisture Barrier is a requirement in the Healthy Home Standards. Any properties with suspended timber floors must have a ground moisture barrier if the subfloor is more than 50% enclosed*.

For the moisture barrier to be effective and compliant with the Healthy Homes Standards if it has been installed after 1 July 2019 then:

  • The barrier must either meet the specifications in NZS 4246:2016 or achieve a vapour flow resistance of at least 50 MNs/g.
  • It must be installed by an "appropriate professional installer".
  • Any leaks from water or waste pipes, wet areas, cladding and so on must be dealt with before the material is laid.
  • There must be no surface water flowing under the building – rain should drain away from the house.
  • The ground under the house should be shaped so no surface water accumulates on top of the vapour barrier.
  • The vapour barrier should completely cover the soil & be butted up to surrounding foundation walls and piles.
  • The sheets should be weighted down to avoid being moved by the ventilating air movement.

* Unless it's not possible to do so for reasons like having no access.

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