Sub Floor Ventilation by Isa Stralian

The purpose of venting the area beneath a floor, particularly a timber floor, is to prevent the incidence of wood rot and consequential odour arising from high humidity levels caused by lack of sufficient ventilation as may be required, to remove moisture presence.

One needs to remember that all space needs to breathe (ventilate) The space under a timber floor is one such area, however it should be noted that a vented sub floor area will always be cooler as a consequence.

A moisture barrier can be employed to partially address this problem however the necessity to vent the area will always be required.

Obviously the most desirable form of aspiration or venting of an area is a natural one, where thermal and pressure differentials about the building perform the task of ventilating the area as is required.

Natural ventilation is often referred to as 'passive' by those wanting to sell you a powered 'positive' ventilation solution inferring that passive is equal to submissive and therefore not as effective.

There's eating and there's overeating....and as with anything overdone there are always unpleasant consequences

The more open the sub floor area is, then obviously the more efficient the ventilation, however there is a level to which sufficient ventilation is considered appropriate and going beyond that point can often lead to adverse results. The more the better attitude regarding sub floor ventilation, or any form of ventilation in fact, is not a wise one.

In many cases the sub floor area is going to be difficult to assess due to inaccessibility and this is where you may find yourself at the greatest disadvantage because the assessment of the problem may simply be the result of conjecture and supposition.

There are tell tale signs of certain conditions arising from poor ventilation being obvious to someone who is experienced in these matters. You will not know, so it is important that the assessment be presented to you in writing, so that if you need to get or want a second opinion you are not trying to think of what was said previously.

When a sub floor venting problem presents itself it is imperative that the prevailing dynamics causing the lack of appropriate ventilation be understood before any solution is proposed or implemented.

Get rid of the problem, not the symptoms

In any efficient natural venting solution there are ingress and egress points as may be required to facilitate a balanced displacement of air in the area.

A ventilation professional will never propose a fan be used from the outset, as a ventilation fan can never be considered a solution. It can be used to address the symptoms but will never be the solution.

Should a fan need be employed it should be momentary, and not constant, so as so create an artificial high / low pressure in the void

If the problem is caused by poor water drainage, as are most, then the obvious solution is addressing the drainage, not putting in a so called 'turbo vent fan' or 'Viro Quad' as a band aid. This product will fail at inopportune times and require continual replacement due to moisture degradation .

The obvious outcome being the salesperson's got the money and you're still stuck with the problem....'Oh yeah but',.... 'we tried', ....'wait 'til next season and see how it goes'....'give it a chance to dry out', .......doesn't quite do it.

A product is something that is purchased and employed where a problem arises out of poor building design and no means of rectification is available.

The so called solar, or packaged products, require careful consideration as their performance efficiency and work life needs to be questioned.
A packaged product is one that is sold and claimed to be suited to all situations.

If it is not a solution designed specifically for your situation it becomes a little like buying a pair of shoes and being confronted with cutting a little off the toes, or filling the toes of the shoes.

Forums are useful up to a point but there is no guarantee....just opinion, and are you going to spend money based on this 'free' opinion.
Forums are good in the sense that they are able to provide alternative approaches that can be considered whereas they may not have been.

Very few will admit to having deluded themselves.

Where fans are used for assisting in the venting of such areas you have a choice of 'negative' or 'positive' modes

Negative venting is where the fan is employed in a exhaust duty in drawing the air volume out of the sub floor area and the supply is via fixed vents of various size to position are used for ingress.

Positive venting is where a fan is used to fill the sub floor area and discharge vis displacement occurs at the same time

Ideally the fixed vents should be of stainless steel irrespective of aesthetic colour

There are other more specialized venting themes one can use depending on the circumstances and nature of the building along with the solution not giving rise to other problems.

Back to Architectural Notes