Keeping subfloor air out of cavities

By - , Build 164

When a building has a suspended timber floor, it’s a requirement of E2/AS1 to stop subfloor air from entering the drained cavity. The Building Code doesn’t say how to do this, but we have an option.

Figure 1 Closing the base of a drained cavity for piled foundations.

THE NEW ZEALAND BUILDING CODE clause E2 External moisture performance requirement E2.3.5 states that concealed spaces must not allow external moist air to accumulate or be transferred and cause condensation, fungal growth or degradation of building elements.

Good detail needed at base of cladding

There is potential for moist subfloor air to enter the drained cavity when a suspended timber floor is constructed on a piled foundation with horizontal timber battens closing off the subfloor space.

Although the subfloor space must be well ventilated, preventing moisture movement from the subfloor into the cavity must be achieved in the detail at the base of the cladding. Unfortunately, Acceptable Solution E2/AS1 does not have a specific detail for this situation.

Aspects to consider

Two aspects need to be considered:

  • The clearance distance between the bottom of the cladding and the finished ground level.
  • How to detail the base of the cavity to ensure that it is not open to the subfloor.

Minimum clearances in E2/AS1

E2/AS1 paragraph 9.1.3 deals with separations, clearances to ground level and overlaps. These are set out in E2/AS1 Figure 65 and Table 18.

Table 18 gives minimum clearances of wall cladding to ground level depending on whether the floor is timber (see Table 1) or concrete.

Table 1
MINIMUM CLEARANCES FOR CLADDINGS WITH TIMBER FLOORS
SITUATIONMINIMUM DIMENSION
Bottom of cladding to underside of bearer or lowest part of floor framing 50 mm
Bottom of cladding to paved ground level 100 mm
Bottom of cladding to unpaved ground level 175 mm
(adapted from E2/AS1 Table 18)

Suspended timber floor requirements are described in paragraph 9.1.3.5. It requires that the base of the cladding must overlap the timber floor structure by at least 50 mm, and there must be no direct connection between subfloor spaces and drained cavities.

The base of cavities must also be verminproofed and be drained and open to the exterior.

Vermin-proofing requirements are described in paragraph 9.1.8.3 and require that cavity closure must:

  • have 3–5 mm drainage slots
  • provide a total opening area of 1000 mm² per lineal metre of wall
  • be located so that there is a drip edge of at least 10 mm at the base of the cladding.

Figure 1 shows how these requirements can be met with timber pile construction.

Figure 1 Closing the base of a drained cavity for piled foundations.

Subfloor ventilation

Where horizontal timber boards are used to close off the subfloor space, they must be installed with continuous gaps between boards (10 mm minimum and ensure the total ventilation is at least 3500 mm²/m² of floor).

Installing a flashing along the bottom of the bearers, underneath the wall underlay and over the timber batten will provide additional separation between the drained cavity and the subfloor space.

An alternative where a sheet cladding is used is to install sufficient vents.

At the top of the cavity

Cavities must be constructed to prevent air movement into roof or boxed eave spaces. Typically this is done with a horizontal solid batten.

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Articles are correct at the time of publication but may have since become outdated.

Figure 1 Closing the base of a drained cavity for piled foundations.

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