Ground vapour kits and underfloor moisture

Published 8 November 2019

Damp home? Get a ground vapour barrier

Around 1.2 million New Zealand houses have suspended timber underfloors. A surprising amount of moisture is released from exposed soil, even where the ground doesn’t seem especially wet.

On average, 0.4 litres of water can evaporate from 1m² of ground in 24 hours – that’s 40 litres per day on average for a 100 m² house. If this moisture accumulates under a house, it can cause damage outside and inside the building and affect the health of the occupants. BRANZ house condition surveys have found that the majority of older houses don’t meet current requirements for subfloor ventilation.

Ventilation is recommended every 1.8 metres on an underfloor cavity but thick black polythene is recommended to use as a ground vapour barrier on any fully enclosed underfloor.

Additionally, about 75% of existing houses with suspended floors don’t have enough ventilation when compared to the Building Code.  That moisture will rise up into your house.

Considerations about installing polythene:

• Check to make sure no water can enter your subfloor area through perimeter vents and make sure vents are clear of shrubs and plants to allow airflow under the house.

• If you have drainage issues consult an expert.

• Also, check there are no pipe leaks under the house as this will pool on top of the polythene.

• Polythene is also easiest installed on top of flat soil, so clear out all rubbish or rake rubble to the sides of the house.

Polythene can be installed professionally for $8.35 per m² (incl GST) for low-income homes* or $11.50 per m² for everyone else.

Alternatively, you can buy supply kits (100m2, 5x rolls of tape and a box of pins) for $170 if you are a keen DIY’er.

Here are some good tips on how to install your Ground Vapour Barrier.

* Low-income refers to those who are eligible for a Warmer Kiwi Home subsidy.