Blocked ventilation openings in foundation walls

Cause

The soil level has been built up, covering ventilation openings

Repair

  • lower or remove raised garden beds until the subfloor vents are completely clear. Ensure there is no planting that will block the vents as it grows

  • ensure good subfloor ventilation:

    • there should be not less than 3500 mm² of clear open subfloor venting (grilles or gaps between baseboards) for every 1 m² of floor area

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

    • vents should be spread evenly around the building perimeter to allow cross-ventilation

    • no part of the subfloor should be more than 7.5 m from a ventilation opening.

Cause

Paving covers or partly covers the ventilation openings

Repair

  • break up and remove concrete or paving. Reform the ground surface so
    rainwater will flow away from the building

  • where clearing existing vents is impractical, create new ventilation openings so there is at least 3500 mm² of clear open subfloor venting for every 1 m² of floor area

  • where new ventilation holes are necessary in concrete or concrete masonry foundation walls, take care that openings are not formed in the top course of the blockwork and that reinforcing is not cut. Install surface-mounted proprietary grilles.

Cause

Planting and foliage restrict air flow through ventilation openings

Repair

  • prune back plants and lower or remove raised garden beds until the subfloor vents are completely clear. Ensure there is no planting that will block the vents as it grows.

  • ensure good subfloor ventilation:

    • there should be not less than 3500 mm² of clear open subfloor venting (grilles or gaps between baseboards) for every 1 m² of floor area

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

    • vents should be spread evenly around the building perimeter to allow cross-ventilation

    • no part of the subfloor should be more than 7.5 m from a ventilation opening.

Cause

There is an insufficient number or size of ventilation openings for the subfloor area

Repair

  • for good subfloor ventilation:

    • there should be not less than 3500 mm² of clear open subfloor venting (grilles or gaps between baseboards) for every 1 m² of floor area

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

    • vents should be spread evenly around the building perimeter to allow cross-ventilation

    • no part of the subfloor should be more than 7.5 m from a ventilation opening

  • where necessary, create new ventilation by:

    • replacing or refixing the baseboards with space for air flow between them

    • installing new ventilation grilles in sheet cladding/bracing on sloping sites

    • cutting additional ventilation openings in concrete or concrete masonry foundation walls and installing surface-mounted proprietary grilles (see drawing). Take care that openings are not formed in the top course of the blockwork and that reinforcing is not cut.

Cause

There is a build up of paint on ventilation grilles reducing air flow

Repair

  • for painted grilles or vents, completely remove paint build-up that may reduce the air path space

  • ensure good subfloor ventilation:

    • there should be not less than 3500 mm² of clear open subfloor venting (grilles or gaps between baseboards) for every 1 m² of floor area

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

    • vents should be spread evenly around the building perimeter to allow cross-ventilation

    • no part of the subfloor should be more than 7.5 m from a ventilation opening.