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Subfloor Ventilation: The Ultimate Guide

Do you need a Subfloor Ventilation System?

It’s easy to overlook the importance of having proper ventilation in the area beneath your home. Although you may not be directly aware of the air quality there, it can significantly impact your living environment. If there are ventilation issues in the subfloor, you might notice signs such as a musty smell or sunken floorboards. These problems occur because excessive moisture can lead to timber decay and the growth of mould.

Moreover, damp or rotted timber creates an inviting environment for termites to establish their nests. In the worst-case scenario, the dampness from beneath the home can even cause health problems. To ensure the well-being of you and your family, it’s crucial to prevent or address subfloor dampness promptly. Implementing proper subfloor ventilation is an essential step toward resolving these issues.

How can you tell if there’s a problem in your Subfloor space?

mould growth in subfloor

Did you know that sometimes there can be a problem under our house, but we don’t realize it until it starts causing problems in our home?

Sometimes, we don’t notice that something is wrong in our subfloor space until it starts causing problems inside our homes. There are signs that can show us if there’s a problem under the floor. We might notice strange smells or floors that sink down. But there are other things to look out for too! If you see a lot of water on the windows or if the house feels really humid and sticky, it could mean that there’s too much moisture under the house. That’s why it’s important to check the ventilation in the whole house, including under the floor, to make sure everything is okay.

If it’s possible, try looking under your house to check if there’s any wetness, mould, or rot. It’s important to investigate any signs of water under the house, such as discoloured wood. Also, try to figure out where the water is coming from:

  • Is there a leaking pipe?
  • Are there signs of condensation?
  • Is there any visible mould or mildew?
  • Can rain water come in?
  • If there’s a musty smell, is it stronger in the crawl space?

Where is the source of the moisture?

Moisture can find its way under the house in different ways, some of which are not so obvious. It’s easy to notice a drip from a leaky pipe or a puddle where rainwater has entered through a gap in the building. However, it’s harder to determine if the moisture is coming up from the ground. Even if the surface soil appears dry, it can still hold a significant amount of moisture. As this moisture evaporates, it rises through the soil and underneath the house. Without proper subfloor ventilation, this trapped moisture remains under the house with no way out. That’s why the air under the house might feel damp, even if there’s no visible water source.

How do we fix damp sub floor?

If you discover a leak under the house as the cause of the problem, the initial action is to seal it. After ensuring that all leaks have been properly patched and any excess water has been drained, it’s time to focus on addressing the subfloor ventilation.

Without proper ventilation, the moisture trapped under the house has nowhere to escape. Even if there are existing vents, the airflow may not be sufficient to remove the moisture effectively. In such cases, the use of an exhaust fan to mechanically extract the stagnant air becomes necessary. The concept of subfloor ventilation is straightforward: ensuring a well-ventilated space that allows fresh air to enter from different areas, resulting in a healthier subfloor environment.

The most common method to achieve subfloor ventilation is by strategically placing simple air vents, typically brick vents with small holes, around the perimeter of the subfloor, (as shown in the image below). These vents facilitate natural cross airflow, allowing air to circulate. It’s important to keep these vents unblocked and positioned above ground level to ensure unobstructed airflow. However, in most cases, relying solely on these vents may not be sufficient. Factors like house extensions or attached decks can obstruct the vents. In that situation, some mechanical ventilation may be required in addition to these vents.

how to ventilate subfloor area

How to ventilate subfloor area – Image from National Construction Code

If you’re thinking about adding an exhaust fan to your subfloor area, there are several factors to consider and different ways to set up the system. The right approach may vary depending on the size of your subfloor area, the number of existing passive vents, and any specific areas that lack external vents. Let’s explore a few options to consider below.

Option 1. Using Wall Mounted Sub floor fan

This is perhaps the simplest method and only requires mounting a decent fan on the wall. One wall-mounted fan is used to exhaust the air in the subfloor space to the outside in the example below. The home in the example below has passive vents on one side that allow air to enter. Remember that you will need to build a power outlet in the subfloor area close enough to the fan so that it can be plugged in. Therefore, you will need to engage an electrician to complete this task unless one already exists.


passive vents subfloor

In situations where there are no passive vents available, certain models of exhaust fans can be configured to also supply air. This means you can install one fan to supply air while another fan exhausts it, creating a balanced pressure system. This configuration ensures proper air circulation and ventilation in your subfloor area.

Option 2. Using a solar-powered subfloor fan

solar subfloor vent fans


Installing a solar-powered subfloor fan for your subfloor space is also an effective solution. These subfloor fans utilise solar energy to circulate air, improving the overall ventilation in your subfloor area. Not only are they environmentally friendly and emission-free, but they also effectively reduce moisture buildup, preventing issues such as mould and rot. Installing a solar subfloor fan provides a sustainable and cost-effective solution to enhance the air quality and promote a healthier home environment.

Option 3. Ducted Inline Fan System


A ducted solar subfloor fan from Solar Whiz

With a ducted inline system, you have the flexibility to direct ducting to specific “problem areas” within your subfloor. This option works well for larger spaces, as it allows for multiple intake points. In the provided example, one passive vent is used alongside the inline system. The inline system features two intake points strategically positioned in the subfloor area that is farthest away from the passive vent. This setup ensures effective subfloor ventilation and targeted airflow where it is needed most.

How to Calculate the Required Subfloor Fan Size?

To compute the required extraction rate, multiply sub floor length x width x height. This will give you your sub floor area in m3.

We advise aiming for 6 to 10 air changes per hour for subfloor ventilation, but the exact number will depend on how bad the issue is. You will require a subfloor fan with a higher extraction rate if there is a lot of moisture under the home.


how to calculate needed subfloor fan extraction rate

How to Calculate the Required Subfloor Fan Size – Image from Pure Ventilation.


Example of how to compute the needed subfloor Extraction rate

Let’s consider a subfloor space that is 0.8 meters high, 20 meters long, and 10 meters wide. This results in a total floorspace of 200 square meters (m2), which is slightly larger than the average size of an Australian house (approximately 186 m2). The overall volume of this subfloor area is calculated to be 160 cubic meters (m3).

To ensure proper ventilation, it is recommended to achieve 6 air changes per hour. This means the extraction rate should be 960 cubic meters per hour (m3/hour). This rate is necessary to effectively remove and replace the air within the subfloor area, maintaining a healthy and fresh environment.

Maximising the Benefits of Subfloor Ventilation

When setting up a subfloor ventilation system, it’s important to keep a few factors in mind:

  1. Proper fan capacity: Ensure that the fan you choose matches the requirements of your specific subfloor area. The fan should have sufficient power to effectively circulate the air.
  2. Ventilation from multiple sides: It is advisable to have vents on at least two sides of the subfloor space. This arrangement encourages cross ventilation, allowing air to flow smoothly throughout the area.
  3. Unobstructed vents: Check that the vents remain unblocked and unobstructed. It is crucial to maintain a clear pathway for air to move in and out of the subfloor, preventing any restrictions to the airflow.
  4. Straight ducting: If you use ducting in your ventilation system, make sure to keep it as straight as possible. During installation, handle the ducting with care to avoid any tearing or damage that could hinder its performance.

By considering these factors, you can ensure that your subfloor ventilation system operates optimally, effectively improving the air quality and moisture control in your subfloor area.


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