Insufficient subfloor ventilation

A fully-enclosed subfloor space created by a  continuous, concrete perimeter foundation wall may require ventilation. Although ventilation grilles were generally included in foundation walls, there were sometimes insufficient numbers by current standards to provide adequate subfloor ventilation. Original ventilation grilles may also have been covered by deck structures, raised garden beds or other landscaping.

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. But if the ground or subfloor space is damp, the first thing to do is identify the source of the moisture.

Dealing with moisture

If there is evidence of moisture, inspect the subfloor space to ensure that the source of the water is not 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:

  • installing additional vents in foundation walls
  • covering the ground with polythene sheeting at least 0.25 mm thick. Ensure the ground is shaped so no water accumulates on top of the polythene. Lap sheets a minimum of 150 mm to cover the whole ground and weigh them down with bricks or concrete blocks.

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Ventilation requirements

Current ventilation requirements are for a minimum of five air changes per hour – this figure should be doubled for wet sites. A clear opening area of 3500 mm2 (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 and in addition to a general dampness in the house, cause specific 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 finishes 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.