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Ground Vapour Barrier


Around 1.2 million New Zealand houses have suspended timber ground floors.


A surprising amount of moisture is released from the ground, even where the ground doesn’t seem especially wet.

On average, 0.4 litres of water can evaporate from 1 m² of ground in 24 hours – that’s 60 litres per day on average for a 150 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 most older houses don’t meet current requirements for subfloor ventilation. NZS 3604:2011 Timber-framed buildings requires subfloor spaces to be ventilated.


Adequate ventilation is achieved with a minimum of five air changes per hour (10 for wet sites).


This can be achieved by:

• not less than 3500 mm² of clear open area for every 1 m² of floor area

• ventilation openings high in the subfloor space, near to the underside of bearers

• no part of the subfloor more than 7.5 m from a ventilation opening. If ventilation holes are blocked – which often happens with plants or banked-up gardens or items stored under the house – they should be cleared.


A good way to reduce moisture levels is to install polythene sheeting that is at least 0.25 mm thick over the ground under a house. This vapour barrier will restrict the evaporation of moisture from the ground into the subfloor space.

For the vapour barrier to be effective:

• any leaks from water or waste pipes, wet areas,
cladding and so on must be dealt with before the
material is laid
• check there is no surface water flowing under the
building – rain should drain away from the house
(see drawing)
• the ground under the house should be shaped so
no surface water accumulates on top of the vapour
• the vapour barrier should completely cover the soil
• adjacent sheets should be lapped a minimum of
75 mm
• sheets should be butted up to surrounding
foundation walls and piles
• the sheets should be weighted down to avoid being
moved by the ventilating air movement.
Fix any plumbing leaks before laying polythene.

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