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

A ground moisture barrier is a sheet of polythene laid over the soil under a suspended floor, so moisture in the ground cannot evaporate into the subfloor and rise into the house. Under the Healthy Homes moisture ingress and drainage standard, a rental needs one if it has a suspended floor and the subfloor space is enclosed, unless installing one is not reasonably practicable. A house built on a concrete slab has no subfloor space and does not need one.

Why such a cheap thing is in the law

Air under a suspended floor does not stay there. It rises through gaps in the floor and wall cavities, carrying moisture from the ground with it; the installation standard notes that winter condensation in the roof space of older houses often starts this way. For the cost of materials and a few unpleasant hours under the house, a barrier cuts that supply off, which makes it one of the best value jobs in the whole regime.

What counts as an enclosed subfloor

The regulations use a specific test. A subfloor is enclosed if airflow into and out of the space is significantly obstructed along at least 50% of its perimeter by one or more of these:

• A masonry foundation wall.
• Cement boards, timber skirting or other cladding.
• Other parts of the building, or an adjoining structure.
• Any other permanent or semi-permanent structure that significantly obstructs airflow.
• Rock, soil or similar material, which is common on sloping sections where the ground rises to meet the floor.

Two points in the official guidance catch people out. Vents in a perimeter wall usually do not stop the space counting as enclosed, so a vented brick foundation generally still needs a barrier. Trellis, on the other hand, does not significantly obstruct airflow, and the guidance expects baseboard cladding to have a gap of at least 20 mm between boards along their full length if it is to let air through. Even where a subfloor is not enclosed, a barrier is recommended if the ground is naturally damp.

What the barrier has to be

A barrier installed on or after 1 July 2019 must either be polythene that meets, and is installed in line with, section 8 of New Zealand Standard NZS 4246:2016, or an alternative material with a vapour flow resistance of at least 50 MN s/g that was installed by an appropriate professional installer. Section 8 calls for polythene film at least 0.25 mm thick, which is sold as 250 micron polythene. Tenancy Services hosts NZS 4246:2016 free to read.

A barrier installed before 1 July 2019 does not have to meet the NZS 4246 rules, but it must not have significant holes or tears that let moisture through. If it is badly damaged, its replacement has to meet the current specification.

How to install one yourself

Tenancy Services lists the ground moisture barrier among the jobs a landlord may be able to do themselves. The full method is the ten-step guide in section 8 of NZS 4246:2016, where it is called an on-ground vapour barrier. In outline:

1. Make it safe. Turn the power off at the main switchboard before going under the house, especially if there is foil insulation, which can be live. Stay out altogether if there is asbestos, damaged wiring or sewage contamination, and get that sorted first.
2. Fix the water before you cover it. Look for leaking pipes, downpipes discharging under the house and surface water running in. A barrier laid over an active leak only hides it.
3. Clear stones, timber offcuts and anything else that could puncture the polythene.
4. Roll out runs of polythene at least 0.25 mm thick and turn them up the inside of the foundation walls by at least 50 mm. On a sloping site, stop the lowest edge at least 50 mm short of the wall so any water that gets on top can drain away.
5. Slit the sheet to fit around each pile, fold it at least 50 mm up the pile and tape it.
6. Overlap joins between runs by at least 150 mm, and tape joins and cuts so the tape covers at least 50 mm on each side.
7. Pin or weight the sheets along the overlaps and edges at no more than 1 metre intervals.
8. Keep going until the whole accessible area is covered, then photograph it before the hatch goes back on.

One caution from the standard: damp, swollen tongue-and-groove floorboards can shrink once the subfloor dries, leaving gaps or creaks. If you would rather not crawl under the house, insulation installers and licensed building practitioners install barriers too.

When it is not reasonably practicable

There is a specific exemption where there is no existing barrier and it is not reasonably practicable to install a polythene one that meets NZS 4246. The regulations define not reasonably practicable narrowly: a professional installer cannot get to the area without substantial building work or substantial damage, cannot do the work without greater health and safety risk than is normal for that kind of job, or it is otherwise not reasonably practicable for a professional installer. In practice:

• The test is an experienced professional installer, not you. A space too tight for a landlord may be workable for someone who does this every week.
• Partial access means partial coverage. Areas of subfloor too low to reach safely can be left, but the accessible areas must still be covered.
• No hatch is not an excuse. If the subfloor is otherwise accessible, the guidance expects you to create access, for example by removing a cladding sheet or fitting a hatch, as long as that is not substantial building work.
• The exemption ends when it becomes practicable, such as during repiling or when floorboards are replaced.

If you rely on it, write down why, ideally backed by an installer who has actually looked, because the compliance statement must say the subfloor is exempt and briefly explain why. The general exemptions, such as a partial exemption where you do not own the whole building, are covered in our guide to Healthy Homes exemptions.

Keeping it working, and proving it

Anything installed to meet the standards must be kept in good working order. A barrier generally does not need regular inspection, but tears that let water pool on top need repairing, and water sitting on the surface usually means storm water or a plumbing leak to fix.

The evidence is simple: a dated photo of the installed barrier, the receipt for the polythene or the installer's invoice, and a note of the date. The compliance statement then needs to say either that the property has no enclosed subfloor or that each enclosed subfloor has a barrier. In Landy, those photos and invoices sit in the property's moisture and drainage section, where the next statement will need them. The full standard is on our moisture and drainage page.