Insufficient subfloor ventilation

Bungalows with perimeter foundation walls may not have sufficient subfloor ventilation

Where bungalows have fully piled foundations and subfloor cladding of weatherboards carried to ground level or base boards with gaps between, they are likely to have ample subfloor ventilation.

However, a bungalow with a fully-enclosed subfloor space created by a continuous concrete perimeter foundation wall may require additional ventilation. Although most foundation walls have ventilation grilles, there are not usually enough of them to provide adequate subfloor ventilation by current standards.

Dealing with moisture

If the ground under the subfloor space is dry and there is no evidence of moisture there is no need to increase the amount of ventilation. If the ground or subfloor space is damp, the first thing to do is identify the source of the moisture. 

Inspect the subfloor space to ensure that the water is not coming from leaking pipes, wastes or drains. Groundwater drainage may need to be addressed. If it is not possible to remove the source of subfloor damp, the options to address the problem include:

  • covering the ground with polythene 
  • installing additional vents in foundation walls. 

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Ventilation requirements

Current ventilation requirements are for a minimum of five changes of air per hour – this figure should be doubled for wet sites.

A clear opening area of 3500 mm 2 (100 x 35 mm) should be provided for each square metre of floor area. Vents should be located within 750 mm of corners and then evenly spaced around the building at 1.8 m centres maximum. No part of the subfloor should be further than 7.5 m from a ventilation opening.

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Problems caused by subfloor moisture

Insufficient subfloor ventilation can lead to higher moisture levels in the unprotected underside of flooring, general dampness in the house, and other problems such as ‘cupping’ of the top surface of floorboards due to a lower moisture content on the upper surface than the lower surface of the boards. This problem will be accentuated with linoleum or paint finish as the timber was unable to lose excess moisture through the upper surface.

If the subfloor framing has remained dry, it is likely to be in good condition but it should be checked. Ensure that there is sufficient ground clearance.