Subfloor Ventilation Facts for Every Homeowner


Subfloor ventilation systems are installed in buildings to eliminate moist air and replace them with fresh air. These systems play a significant role in maintaining the structural integrity of the building as well as the health of the individuals residing in the house. Though many of today’s buildings are equipped with these systems, not many homeowners are aware of the basic facts related to their subfloor ventilation systems. This brief discussion shares some of the most fundamental subfloor ventilation facts that will make it much easier for the homeowners to ensure optimum performance of their ventilation units.

How to Identify Subfloor Ventilation Issues?

Most people find it difficult to identify subfloor issues simply because they don’t spend much time there. Sunken floors and odor are two primary indicators of a problematic subfloor. However, there are other signs, too. If you experience the presence of moist air in your living space, its source could be the subfloor. Therefore, thoroughly check the ventilation of the entire building including the subfloor area, if there is a lot of condensation on the windows. If possible, check your crawlspace for signs of rot, mold, or condensation. The presence of musty smell and discolored timber are two strong indications of water under your home.

What are the Sources of Subfloor Moisture?

Moisture may accumulate under your house in many different ways. It is relatively easy to detect rainwater seepage through a gap in the structure or water dripping from a faulty pipe. However, it is much harder to detect moisture that comes through the soil just beneath the home. As the moisture evaporates from the soil, it enters the subfloor area and gets trapped. Without proper ventilation, it remains trapped as there is no way out for it.

How to Fix Subfloor Moisture Problem?

If a leakage under the home is responsible for the problem, seal it immediately. After fixing the leakages and draining out all the water from the subfloor, the next step is ventilation.

Please note that the existing vents in the subfloor may not be able to create enough air flow for moisture extraction. If that is the case, you will have to use an exhaust fan. If there is lot of moisture under the building, the extraction rate of the fan should be higher. In general, five to ten air changes per hour are recommended for this purpose.


How to Ensure Optimum Subfloor Ventilation?

While installing a ventilation system for your subfloor, please take the following factors into consideration.

How to Calculate the Fan Size?

As mentioned briefly earlier, depending on the amount of moisture, five to ten air changes per hour is recommended. Let us assume that we are looking to achieve seven air changes per hour. This means that the system should be able to exhaust the overall area of the underfloor area seven times per hour. Now you need to measure the volume of subfloor area by multiplying its length, width, and height. If the volume is 70 m3, this is the volume to be exhausted by the fan for each replacement. For seven replacements per hour, the volume comes to 490 m3. Therefore, the fan must have an exhaust capacity of 490 m3 per hour or more.

How Long Should I Run the Fan?

It has been observed that maintaining good ventilation requires the fan to be run for approximately four hours per day.  Generally, it is always a good idea to run the fan during the day when the air is warm and dry. However, if you have too much moisture in the subfloor, you may have to run the fan longer, at least during the initial stages. As things start getting better with time, reduce the number of hours gradually.

How much does it Cost to Run a Subfloor Fan?

In general, most of the subfloor fan systems are energy efficient. As a rough estimation, it can be said that the annual cost of running a fan with a 100w motor for four hours per day will be approximately $51. Most of the subfloor ventilation kits provide this information in their listings.

Are the Subfloor Ventilation Systems Noisy?          

This will depend entirely on the size of the fan. The fans with larger capacity tend to produce more noise. In general, however, these fans do not make a lot of noise at all. You can avoid making noise at the night by running the units during the day. Also, you can install them under the unused parts of the house to minimise the sound you hear during the day.

These were just a few of the many questions people ask about subfloor ventilation systems. If you have any other questions or concerns related to your underfloor area, please feel free to contact us. At ExtractAir, we have a wealth of experience in designing, installing, and maintaining subfloor ventilation systems for all types of buildings.