What is a ground moisture barrier, and does my rental need one?

A ground moisture barrier is a heavy polythene sheet laid over the soil under a house, weighted or pinned down, so ground moisture stops evaporating up into the subfloor air. If your rental has an enclosed subfloor, walls or cladding closing the underfloor space in, the moisture and drainage standard requires one wherever installing it is reasonably practicable.

Why such a cheap thing is in the law

The air under a suspended timber floor does not stay under it: it moves up through gaps and into the rooms above, carrying whatever moisture it holds. Damp subfloor, damp house, and no amount of heating fixes a supply problem. The barrier cuts the supply at its source for the cost of materials and a few unpleasant hours of crawling, which makes it one of the highest-value jobs in the whole regime.

The practical points

• "Enclosed subfloor" is the trigger; a fully open pile foundation with free airflow is a different situation.
• If access genuinely does not permit installation, do what is practicable and keep evidence, ideally a professional's written view, of why the rest is not.
• Photograph the installed barrier before the access hatch closes. It is invisible afterwards, and that dated photo is the entire evidence question answered forever.

Requirements and specifics are maintained at tenancy.govt.nz. If the job is not done and the subfloor is accessible, it is usually the best value item left on the list.