Insufficient ventilation

The subfloor of villas was typically constructed:

• as a fully clad wall with weatherboards
• using horizontal base boards with gaps between
• using vertical tongue and groove boarding
• using vertical baseboards.

BRANZ House Condition Surveys have repeatedly found insufficient ventilation under timber-framed floors.

Where a weatherboard or solid boarded cladding is fixed to subfloor framing, it was usually taken right down to ground level with little or no provision of ventilation openings. It is common for the ground levels around villas, whether paved or in gardens, to have been built up over time. Often this has blocked ventilation or has resulted in water running under the house because the ground outside is higher than that under the house.

Horizontal base boards, which were more common on villas built close to the ground, may provide sufficient ventilation openings but this will depend on the dampness of the soil and the subfloor space. 

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Options to reduce subfloor moisture

Options to reduce subfloor moisture are:

• install modern vents into clad basement walls
• cover the ground with polythene. 

For ventilation, a minimum of five changes of air per hour are needed – double that figure for wet sites. A 3500 mm2 (100 x 35 mm) clear opening should be provided for each square metre of floor area, with the vents located within 750 mm of corners and then evenly around the building at maximum 1.8 m centres. No part of the subfloor should be further than 7.5 m from a ventilation opening.

Lay polythene sheeting at least 0.25 mm thick to completely cover the ground under the house. Ensure that the ground is shaped so no water accumulates on top of the polythene. Polythene sheets should be lapped a minimum of 150 mm, preferably taped, and tightly butted up to foundation walls and piles. Weigh down the sheets with bricks or concrete to avoid them being displaced by air movement.

The subfloor space should also be inspected to make sure that water is not leaking from pipes, wastes or drains and accumulating under the house.