Underfloor Ventilation - Suspended Timber Floors

Purpose and Installation

In order to prevent an excessive build-up of moist air in the underfloor space in a suspended timber floor, underfloor ventilation is necessary.

An even distribution of vents will ensure the elimination of stagnant air pockets where moisture might accumulate.

A ventilated air space measuring a minimum of 150mm to the underside of the floor or the bottom of the insulation if insulation is provided must be provided. The measurement from the concrete to the underside of the wall plate must be at a minimum of 75mm.

Diagram B41 - Underfloor ventilation - minimum ventilated air space

Installation Details

To prevent ventilation of the cavity, any ventilation to be provided must be sleeved through cavity, the typical details for such an installation are shown below:

Diagram B42 - Typical ventilation detail

Diagram B43 - Detail A - Typical ventilation detail across the cavity

Diagram B44 - Typical sleeper wall detail

Diagram B45 - Typical detail for an air vent sleeved across the cavity

Ventilation Openings

Ventilation openings in external walls for underfloor void must be provided so that there is a free path between opposite sides and to all parts of the void.

The minimum requirement for openings is that they equate to 1500mm^2 for each metre run of wall.

Minimum diameter of pipes needed to carry ventilating air is 100mm. The fact that the available free area of the vent is reduced to the area of the pipe used should be taken into account.

Detached house

Below is an example of how to calculate the number of ventilation opes required, for the house illustrated below.

Diagram B46 - Calculating the number of ventilation opes for detached houses

(30m) x (1500 mm^2)-r(5400 mm^2)*= 8.33 so a minimum of 9 no. vents are required.

To ensure a balanced air flow the same number of vents should be used on each opposing elevation and as such in this instance 10 no. vents should be provided as follows:

  • 2 no. vents along the 5m elevations.

  • 2 no. vents along the 10m elevations.

This figure will vary depending on the design of the vent and from manufacturer to manufacturer, and on how the vent is sleeved. In this example the figure for a typical purpose designed telescopic wall ventilator is used, i.e. 5400 mm^2.