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Moisture is one of the most influential factors affecting the performance of timber in buildings. Whether used externally or internally, timber will continually interact with ambient humidity and temperature, adjusting its own moisture content to reach balance with its surroundings. If this natural behaviour is not understood and properly accounted for during specification and installation, it can lead to problems such as warping, cupping, gaps, joint failure, or premature coating breakdown.
This page explores:
- What equilibrium moisture content (EMC) is, and why it matters.
- How timber shrinks and swells with moisture change.
- The risks posed by moisture on site, and why acclimatisation is important.
- Practical specification strategies to help timber perform reliably over its lifespan.
Definition and Relevance of EMC
Equilibrium Moisture Content (EMC) is the point at which timber’s moisture content stabilises in line with the surrounding environment. At EMC, the timber is neither absorbing nor releasing moisture. Because timber is hygroscopic, it constantly adjusts toward this balance in response to the local relative humidity and temperature.
Why it matters
- Timber installed at a moisture content far from its in-service EMC will continue to lose or gain moisture, leading to shrinkage or swelling after installation.
- By specifying and installing timber close to the expected EMC of its service environment, you can minimise dimensional changes and reduce the likelihood of defects.
For typical Australian conditions:
Internal EMC generally ranges from 8-14%, depending on whether spaces are air conditioned or naturally ventilated.
External EMC is usually higher, around 12-18%, varying by climate zone and season.
Figure 1: Relationship between Relative Humidity and Equilibrium Moisture Content in timber at various temperatures
Key considerations
- EMC is dynamic and will change with seasonal or long-term shifts in ambient conditions.
- The timber’s MC at the time of manufacture or delivery may differ from what is ideal for installation, so checks or additional conditioning may be needed.
Timber is hygroscopic, meaning it naturally absorbs and releases moisture from the surrounding air. As it does so, it changes dimensionally-mainly across the grain-through shrinkage when it dries below fibre saturation point, and swelling when it absorbs moisture.
Typical behaviour
- Shrinkage starts once timber dries below around 25-30% moisture content (the fibre saturation point), and continues down to equilibrium.
- This dimensional change is not uniform:
- Greatest tangentially (across growth rings),
- Less radially (through the rings),
- Minimal along the length.
For most species, tangential shrinkage can be roughly 6-10% from green to oven-dry, while radial shrinkage is often half of tangential shrinkage.
Figure 2: Tangential and Radial Shrinkage.
Why it matters
If not accounted for, this natural movement can result in:
- Gaps in flooring and lining, or boards cupping.
- Doors and windows sticking or jamming as frames move.
- Cracking and checking in larger sections or at connections.
Different species respond differently
Species vary in shrinkage rates due to their cellular structure. Denser hardwoods often shrink more in absolute terms, but may still provide stable performance if properly dried and detailed.
Figure 3: Common Shrinkage Rates - TDG 14 p60
Moisture challenges don’t end with correct specification-site conditions often introduce unexpected risks. Timber may pick up additional moisture during transport or storage on site, or be exposed to rapid drying once installed in air-conditioned spaces.
Typical site risks
- Rain during transport or storage.
- Stacking timber directly on the ground, exposing it to rising damp.
- Covering timber in plastic wraps without ventilation, which can trap condensation.
Good practice
- Store timber off the ground on bearers, under cover, with sides open for airflow.
- Avoid wrapping in non-breathable plastic for extended periods.
- Allow time for acclimatisation: let timber sit on site in the conditions it will be installed in, so it can reach a stable MC closer to in-service EMC.
Because timber will inevitably move with changes in moisture, good design and specification focus on managing this movement, rather than trying to eliminate it. By understanding timber’s response to moisture and specifying it to suit both the expected EMC and the site conditions, designers and builders can significantly reduce issues like shrinkage cracks, swelling jams, or coating failures. The key is to anticipate movement and design to accommodate it.
Allow for seasonal movement
- Gaps and tolerances: Design joints and interfaces to accommodate seasonal expansion and contraction. For example, leave expansion gaps in flooring, lining, or cladding to allow boards to swell slightly without causing buckling or cupping.
- Slip joints and flexible sealants: Use detailing that permits differential movement, particularly where timber interfaces with more dimensionally stable materials like steel or masonry.
Figure 4: Left: Poor detailing, no gap - timber movement and failure. Right: Good detailing, sufficient space for timber movement.
Source: Timber With Ted Stubbersfield Newsletter July 2025
Specify appropriate moisture content
- For internal joinery and floors, timber is typically specified at 10-12% moisture content, aligning with average indoor EMC.
- For external applications, slightly higher MC (around 12-15%) is acceptable and better matches the typical external EMC.
By selecting timber dried to a moisture content close to where it will stabilise in service, you reduce the extent of post-installation shrinkage or swelling.
Choose compatible coatings and finishes
- External finishes should slow moisture ingress and egress, helping timber adjust more gradually to humidity changes. Penetrating oils and microporous paints are often recommended.
- Avoid impervious films in external applications unless they are well maintained, as breakdown can lead to localised moisture traps.
Detail for water run-off and ventilation
- Use design details that shed water and allow for ventilation, reducing prolonged wetting and encouraging drying. This includes flashing, drip edges, and ventilated cavities behind cladding.
Summary
By understanding how timber interacts with moisture - from the seasoning yard to the finished building - designers, specifiers and builders can make informed decisions that safeguard both appearance and performance. Ultimately, planning for movement and specifying to suit the timber’s final environment ensures the material’s natural advantages are fully realised, delivering projects that stand the test of time.