Installing solar sub-floor ventilators

Installing solar sub-floor ventilators

This page is still in preparation.  Call us for any questions you may have.


Sub-floor Ventilation Installation Guide – draft copyright 

Sub-floor ventilation must be provided to enclosed sub-floor spaces to prevent damage and illness. Moisture escapes from the soil and is absorbed by the air above it, raising the relative humidity of the air. This in turn will raise the moisture content of the building framing members and the flooring.

Ventilation is necessary to reduce the relative humidity of the cool air in that space, by replacing damp air with new warm drier air, drawn from outside the space. The quantity, distribution and efficiency of any vents provided is extremely important for the stability and performance of timber floors, and to reduce the risk of decay and termite hazards

Where the sub-floor is enclosed by solid masonry, (blockwork, brickwork, stone, etc), permanent vents must be installed in the masonry during construction.  The BCA, Part 3.4 outlines sub-floor ventilation requirements to limit the moisture content of framing timbers in the sub-floor space. The minimum level of ventilation required to achieve this is dependent on the location and the sub-floor conditions

The aim of the sub-floor ventilation system is to provide cross-flow of air in both directions, ie. from side to side, and end to end, of the dwelling. Vents shall be placed in all walls that enclose the sub-floor space. Corresponding openings shall also be placed in any internal walls in the sub-floor space. To prevent the formation of ‘dead-air’ pockets in the sub-floor space, vents are to be placed within 600 mm of corners of each wall and then at the required spacing in-between.

The drainage system provided to the dwelling site should ensure that all run-off water will drain away from the building perimeter (not towards it) and that run-off water is prevented from entering the sub-floor space. The ground beneath a suspended floor should also be graded so that no ponding is possible. Where springs or acquifers are present, (e.g. exposed by earthworks on sloping sites) and causing water to enter the sub-floor space, a closed drainage system should be installed under the dwelling to remove this water.

No ventilation system will cope with this level of moisture inside this space.

The most common approach to sub floor ventilation is constant or timer operated electrically powered fans. These fans are designed to remove moisture from the sub-floor area – with the aim of allowing replacement air to enter under the house via vents located around the building. The problem with constant or timer operated fans is that they also operate when there is high humidity e.g. when it is raining – which means that they actually bring damp air into the sub floor area. Due to the normally lower temperature under the house this causes condensation and very high moisture levels – which over a period could lead to destruction of your subfloor area through mould, rot and termites.

The Solar sub-floor ventilator draws in fresh dry air through existing wall vents (or new ones that may need to be installed in addition to Solar ventilator, if not available or insufficient) and draws it through the sub floor area across the damp earth, effectively drawing out the moisture and venting it to outside of the Sub Floor area. Ducting may be added to the fans and is generally extended into the center of the house or building. This allows the air to be drawn evenly from all sides of the house, to the centre, where it is drawn to the outside by the solar ventilator fan, through the duct.

If the area under the house is restricted in height and won’t allow the installation of the large solar ventilator fan, we offer an alternative 150mm diameter solar fan. It comes in two versions with capacities of 300 or 450 cubic meters per hour. We generally recommend using the more powerful one, for best performance.

Solar fans offer a couple of obvious advantages over the traditional mains-powered and timer operated systems:

  • The systems only operate during sunny conditions – ensuring that the replacement air generally is quite dry and therefore contributes to keeping the sub floor area dry.
  • Solar fans are fairly quiet, and cost nothing to operate.

A solar air heater may be added to the system to further heat the incoming air (see diagram).

Suib-floor vantilation diagram

Suib-floor vantilation diagram

The fan is designed to suck not push, so for maximum airflow it is important to place the fan as close as possible to the outlet. The fan is marked with the direction of flow.  (Arrow towards the outlet) Use the flanges mounted on the fan to attach the fan to the air outlet with approximately 400 mm of acoustic flexible duct for good performance with minimal fan and airflow noise. You may tape the duct to the fan – and also use the tape to ensure no air escapes from the joints.

Wiring of the solar panel and fan

Wiring up the air extraction system is very simple.

Connect the positive (Red) wire from the PV panel to terminal #1 on the switch.

Connect the positive (Red) wire from the fan to terminal #2 on the switch.

All the negative wires (Black) are joined together (but not to a terminal)

SolarWhiz – Wiring Diagram with night operations pack


If the sun is shining and provides enough power to operate the fan (about 10 volts with load) the solar ventilation system should now be operating – and be drawing warmer dry air into the space underneath the building.

Remember, if you have a unique sub-floor ventilation problem, or you just want some friendly assistance with your design, we are always happy to help.   Send us a site plan with any existing air vents marked on it; that can help to clarify what you are trying to do.

Sub-floor ventilation is relatively simple, and usually there is no wrong way or right way about doing it.   Some systems will work better than others, however it is wise to remember that the fresh, dry air needs to be dragged efficiently over all the wet areas, to pick up the moisture, before being expelled.

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