Part 3.4.1 Subfloor Ventilation
Appropriate Performance Requirements
Part 3.4.1 applies to the subfloor space of all suspended floors of a building or deck, including but not limited to, timber and steel framed subfloors and suspended concrete slabs.
Acceptable construction practice
Compliance with this acceptable construction practice satisfies Performance Requirement for subfloor ventilation.
18.104.22.168 Subfloor ventilation
Subfloor spaces must—
Table 22.214.171.124 SUBFLOOR OPENINGS AND GROUND CLEARANCE
|CLIMATIC ZONE (see Figure 3.4.1)||Minimum aggregate subfloor ventilation openings (mm2/m of wall)||Minimum ground clearance height (mm)|
|No membrane||Ground sealed with impervious membrane||Termite inspection or management system not required||Termite inspection requiredNote 1|
CLIMATIC ZONES BASED ON RELATIVE HUMIDITY
In addition to (a), a subfloor space must—
be cleared of all building debris and vegetation; and
have the ground beneath the suspended floor graded in accordance with 126.96.36.199; and
contain no dead air spaces; and
have openings evenly spaced as far as practicable (see Figure 3.4.2); and
have openings placed not more than 600 mm in from corners.
the ground within the subfloor space must be sealed with an impervious membrane; or
subfloor framing must be—
durability Class 1 or 2 timbers or H3 preservative treated timbers in accordance with AS 1684 Parts 2, 3 or 4; or
steel in accordance with NASH Standard 'Residential and Low-Rise Steel Framing' Part 2.
188.8.131.52(e) specifies additional requirements for preventing deterioration of subfloor members where the ground or subfloor space is excessively damp, as would occur in areas with high water tables, poor drainage or in areas frequently affected by flooding or water inundation.
TYPICAL SUBFLOOR VENTILATION DETAILS
Typical Cross Ventilation of Subfloor Area
SUBFLOOR CLEARANCE REQUIREMENTS
(see Notes to Table 184.108.40.206)
Subfloor ventilation is cross ventilation of the subfloor space between the underside of the subfloor and the ground surface under a building.
Ground moisture rising into or entering the subfloor space can create a damp environment which encourages timber rot, fungus growth and the potential for termite activity. Subfloor ventilation increases air flow, reducing any damaging water vapour in the subfloor space.
Factors that can affect achieving satisfactory levels of subfloor ventilation include height above ground, prevailing breezes (air transfer), differential temperature and humidity between the subfloor and the external environment and good building practice.
The amount of subfloor ventilation required for a building is related to the relative humidity likely to be encountered in that location.
shows three broad climatic zones based on the prevailing relative humidity and includes a description of the relative humidity conditions which define each zone. If reliable weather data is available, these descriptions may be useful in determining which zone a particular location is in.
The zones shown in Figure 3.4.1 were determined by analysis of the average relative humidity at 9 am and 3 pm in January and July. The season with the highest relative humidity is used. Generally this will be July for southern Australia and January for northern Australia.
specifies the minimum amount of subfloor ventilation openings and height of subfloor framing members above ground level for the three climatic zones illustrated in Figure 3.4.1. The table allows subfloor ventilation rates to be halved if the ground within the subfloor space is sealed by an impervious membrane because humidity levels in the space will not be affected by moisture from the soil.