Bowens - The Builders Choice

Understanding Humidity and Timber Performance

Building Advice
Jeff Harvey

Flooring & Decking

Whenever I inspect a problem with flooring or a deck, I usually try and establish the Equilibrium Moisture Content (EMC) of the sub-floor or sub-deck cavity, particularly if the floor or deck is affected by poor ventilation. In cases like this, it is not uncommon to measure high humidity readings at 80% plus.

The meter I use measures not only humidity but temperature as well. I usually place the meter in the cavity to determine the EMC. I then record the findings and relate them to the chart above. Where the two points meet is the EMC for that micro-climate.

On close examination of the chart, you will notice a change in temperature has little effect on EMC, the governing factor is humidity. Below I will try and explain how this can impact the timber products concerned.


Even though the Australian Standard for “LIGHT DECKING” SECTION 4 AS 2796.1—1999 allows a moisture content of a minimum of 10% and a maximum of 18%, I would suggest most decking kiln-dried products would be around 12% when they are delivered to site. However, if the deck is poorly ventilated and the humidity is around 85%, the chart suggests a moisture content of about an untenable 18%. This can often lead to the boards coming together from expansion and cause severe cupping.

Of course, the opposite will occur in the summer months when the boards can experience prolonged hot weather when the humidity can drop to very low levels perhaps averaging 40%. Again, the chart would indicate moisture content levels of about 7%-8%. If a builder at the point of installation allows a wide gap to avoid the boards coming together in the winter, they could experience excessive (untenable) wide gaps in summer.

In both unwanted scenarios, OHS issues could arise.


Figure: EMC humidity reading of 59%. 

The major flooring producers gear the moisture content of their products to a moisture content of about 10% to 11%. This is important to understand as a good proportion of their floorings are going into new homes where the heating and cooling appliances are not installed at the time of installation. In effect, this means the producers know the Equilibrium Moisture Content for Melbourne and Sydney averages about 11%. Again, referring to the chart would suggest the average relative humidity for those capitals to be about 60%.

Accordingly, this can call into question the need to acclimatise. I advise no acclimatising is required if the flooring is going into a home where the boards will experience a microclimate similar to what they will in their service life. On the other hand, if the flooring (when installed) is exposed to a region where the atmosphere differs to the major capitals, that could cause the boards to adjust greatly to an EMC that could cause stress on the boards. Then acclimatising to that region should be considered. Note, proper acclimatisation can only happen if the boards are stacked so air can make contact with each board on all surfaces.

Air Conditioning

There are 2 common types of air conditioners used in homes, refrigerated and evaporative systems.

The refrigerated split system, whether it is cooling or heating, takes the humidity out of the atmosphere. This means if used for prolonged periods it can cause flooring boards to shrink resulting in unwanted gapping.

The evaporative system has the opposite effect by putting humidity into the atmosphere. This means if used for prolonged periods it can cause the boards to swell or expand, resulting in tenting of the boards.


  • Look after your floor over the holiday period, particularly if going away for an extended time. In other words, don’t let your house become a hotbox.
  • Leave some windows open if possible.
  • Leave buckets of water in the house, this will again release moisture into the microclimate atmosphere as the water evaporates.
  • Avoid having the aircon on 24/7. Give it a spell and open your house regularly to let some fresh air in.
  • During hot spells consider drying your washing in the house. This will release moisture into the microclimate.

Any questions regarding humidity and timber performance, please do not hesitate to contact us here.

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