Timber Flooring & Decking: Understanding Ventilation & Humidity

Optimal Sub-Floor Ventilation Helps Stabilise Timber Flooring & Decking Boards

In last month’s edition of this publication, I’d endeavoured to give precise comments of some of the problems I regularly see associated with post-installation issues.

Before I outline some of causes of poor ventilation, I think it important you, our valued customers, understand what sound ventilation will achieve. Board stability. The question is, “how do we achieve this essential aspect so it will not cause post-installation issues with your client, particularly when building a sub-floor or sub-deck?”

Essentially good ventilation creates a sub-floor/ sub-deck cavity where the flooring or decking boards experience similar atmospheric conditions above and below the flooring or decking boards.

“Essentially good ventilation creates a sub-floor/ sub-deck cavity where the flooring or decking boards experience similar atmospheric conditions above and below the flooring or decking boards.”

When this is not achieved there can be a significant build-up of humidity in the cavity below and, because wood is hygroscopic, the bottom of the boards can pick up moisture from the high humidity, leading to a difference in moisture content between the top of the boards and the bottom of the boards.

The result is board instability, particularly cupping. A sure way of detecting a poorly ventilated sub-structure cavity is a musty smell. Accordingly, I advocate for an inspection trap door to be installed for sub-structure cavities. This not only can detect a high humidity space but may also have the benefit of inspecting for termites.

“The result is board instability, particularly cupping. A sure way of detecting a poorly ventilated sub-structure cavity is a musty smell.”

In the case of a floor, the VBA rules there should be ventilators every 1 meter of the external walls of a dwelling, and each ventilator should be able to draw 6000 square millimetres of air. In addition, opposite walls should have ventilators installed so a crossflow of air is established.

In the case of a deck, I often see base boards completely closed. Again, it is essential to allow for air to not only penetrate the sub-deck cavity but it must be able escape from the opposite perimeter, so a cross flow of air is possible.

However, there can be situations where the cross flow of air is not possible or compromised. High fences close to adjacent walls can cause problems. In the case of a deck around a swimming pool, cross flow is often not possible. A solution for this can involve the use of mechanical fans or involve the services of a ventilation expert.

“(…) there can be situations where the cross flow of air is not possible or compromised.”

In conclusion, as I pointed out in a recent publication of this bulletin, humidity is the governing factor when maintaining a stable kiln dried product such as flooring and decking. If all other installation instructions have been followed*, ventilation is of paramount importance.

*Bowens includes our suggested installation guidelines with every order we receive.

I will be available for timber advice at our Belmont store on the following days:

Monday 18 March 2024 – 10am to 2pm

Monday 8 April 2024 – 10am to 2pm.

Please call me on 0412 550 740 if you’d prefer to speak over the phone.

Jeff Harvey