Mould and underfloor ventilation - solutions

Date: 22 August 2016

I would like to present a brief overview and hopefully provide some useful information in dealing with subfloor ventilation, a problem pest and building inspectors all encounter at times when carrying out inspections.

I am a licensed pest controller with 38 years experience and have gained most of my knowledge from hands on experience in upgrading subfloor ventilation systems as part of termite and borer control programs.  I do not hold any formal qualifications in engineering, science or building, although some knowledge from all these fields is required when trying to identify and fix the problems caused by inadequate ventilation. 

I will start with some often sighted or sensed circumstances we encounter, that indicate there is most likely a ventilation issue within a building.


To start with smell.  Most of us have encountered the scenario of entering a building and noticing a mushroom type odour.  This generally indicates that there is a mould problem somewhere in the building, which is caused by raised moisture levels. In South Australia because of our generally dry climate, apart from the Hills and South East areas, it is not all that common to have a mould problem caused by climatic conditions.  So unless you are in one of these areas with higher humidity, you can assume that there is most likely a building defect causing raised moisture levels within the structure.

At times the occupants can also be the source of the problem with some of their activities inside the building.  Examples being not opening windows and running high heat in the cooler weather, not using the exhaust fans in bathroom and kitchens and growing indoor plants etc. with hydroponics.

An initial visual survey of the building should identify if there are any visible signs of mould growth on internal surfaces, which when located, enables us to identify a possible cause of the problem.

A good example is mould on a ceiling, not being in a wet area.  We would usually look for water ingress through the roof from damaged materials, poorly installed flashings, overflow from blocked gutters or a leaking pressure pipe, anything that can provide a source of moisture to allow mould growth to occur. 

When there is no visible sign of growth evident, it usually indicates a moisture issue is in a hidden cavity area, which can range from wall cavities to the rear of a built in cupboard or suspended subfloor area. We use an electronic moisture meter to check surface areas especially where we would expect possible water leakage, such as the rear of showers and baths etc. This can generally identify some hidden sources of moisture which enables mould growth. When we get a surface reading of 17% and above there is enough moisture for mould to grow, form and multiply, producing the spores we can smell.  

To fix mould problems usually requires removing the moisture source and reducing the humidity from the internal areas by air exchange.

The black mould we often see in these situations usually does not cause structural damage to the building but can cause health issues for some people living in the building, especially respiratory and allergy symptoms. The staining caused by mould can be difficult to remove from some materials.



The other main problem area we deal with is inadequate subfloor ventilation.  Most older houses that have suspended wooden floors are the main buildings where we encounter ventilation problems. Due to the enclosed foundations with dwarf walls and solid internal walls it is usually difficult to access the subfloor area.  So to identify a subfloor ventilation problem can be difficult without carrying out an invasive inspection.

However there are a number of visual signs which can give some indication of a possible ventilation problem. Some being staining around nail heads in floorboards, cupping of boards due to swelling, and springing movement in floor joists.  These point to possible subfloor ventilation problems.  If you look for floor areas which receive sunlight through the windows, you will often find higher moisture readings there, as higher concentrations of condensation occur in these areas.

If you have identified that there is a ventilation issue in the floor, moisture readings should be taken to identify the extent of the problem. If a reading of 28% or higher is found, fungal decay (dry rot) is present and the structural integrity of the boards would be compromised.

At times over the years I have been called out to quote on termite jobs where someone has felt a floorboard give way under the floor covering. When we have arrived to carry out an inspection we have found that the board has failed because of fungal damage as much as termite damage.  In one case a number of years ago, I inspected a shop in Glenelg, where the owners had had a termite treatment carried out by a pest control company and repair work done to the floor twice over a period of 5 years. However, we identified the problem was fungal decay and no real signs of termite damage were evident, which shows that either the company staff that carried out the inspection and treatment had little knowledge of what they were dealing with, or were just interested in selling a treatment.       

The cause of subfloor issues can be mainly put down to the same as the mould situation in the internal areas of structures, that is, high moisture levels creating humid conditions for fungal growth to occur.  This is fixed by reducing or removing the cause of the moisture where possible and the regular exchange of air to reduce the humidity.  

Steps to reduce the subfloor moisture may include fixing drainage, particularly stormwater run off, as well as leaking pipes.

However there is often a limit to what can be achieved in reducing the moisture levels of the soil, especially in areas where ground water is close to the surface.  There is also the problem of creating further structural problems with cracking and movement in foundations when soil moisture levels are altered.  

Improving the exchange of air is often the most effective way of eliminating fungal growth and reducing the moisture in structural timber.

The majority of problems encountered are caused by building alterations which compromise the original passive ventilation systems installed at the time of construction.  These generally rely on cross ventilation through a venturi type effect with air movement over the vent, drawing air from one side of the building to the other.  At times we find blockages of the original vents from salt damp issues or vents that have been sealed over to keep out rodents, bees etc.  Over the last 10 -15 years rendered wall finishes have been popular when renovating and people unwittingly cover the vents when rendering.

In the case where the external vents have been compromised, it often just requires their removal and replacement. The subfloor areas may also need to be opened up to align the airflow path, especially when the external vents are set above floor height or are off set, which is often the case in a brick cavity wall. 

When we find issues with renovated homes the majority of the problems are caused by incorporating new concrete floors into sections of the original structure and blocking sections of the crossflow air movement, or laying a slab down one side of a building and preventing the crossflow of air.  In these situations we normally have to use a mechanical form of ventilation.



This is where we have to work out where we can draw air from to pass through these dead pocket areas under the building. This can be by using wall cavities which are connected to the roof space, or by using ducting and flues drawing the air from one area into the dead area and then using another fan to expel the air to an external vent. Sometimes fitting a panel vent into a wall lining to allow the draw of air from the interior area is the only solution.  However, occasionally there is no solution without major works.  These issues are often caused by inlaid flooring being placed over the original floor.

Air naturally moves in layers, but to achieve movement otherwise we need to use fans.  We have been using solar powered fans for a number of years and have found them reasonably reliable. The advantages they have are that they are low voltage and have low noise when in operation compared to a number of other units on the market. The solar panel life is at least 10 years and once installed, the units have have no ongoing running costs.

We also at times install a fan unit with a 12 volt transformer using a timer.  This is generally where a power point is readily accessible or the installation of the solar panel may not be possible.  I do not trust transformers to be running 24hr in a confined space. We use a timer when we install one of these units and set the running time in between 9 am- 4pm to minimize any noise impact issues.

In the majority of our installations we use a 4 motor Envirofan vent, which moves 5500 litres of air per minute.  This is adequate for approximately 80 square metres of floor coverage.

Being solar powered, the most frequently question I get asked is, how does it run in winter when we have cloudy days. The answer is that it only needs approximately 4 hours running per day to achieve adequate exchange of air under the floor and even with cloud cover we find that the unit will run, albeit at not full capacity.

The noise level emitted is 27db, which is around the same of a pedestal fan, which generally is not noticed by the occupants of the building as the unit only runs through daylight hours.

We also occasionally use an inline fan when we have multiple areas that we need to extract air from using ducting.  This model fan produces a 58db noise level, which makes it not all that suitable in domestic situations.  However its efficiency at moving 7000 l/m makes it a more cost effective proposition in situations where noise is not such an issue.