When wood is first cut or felled, its moisture content generally averages around 75%, though it can vary significantly depending on the species and felling conditions.
Initial Moisture Content of Freshly Felled Timber
Freshly felled trees, often referred to as green wood, contain a substantial amount of water. This initial moisture content (MC) is a critical factor in determining the wood's properties and its suitability for various applications.
According to data from September 18, 2018, a tree that has just been felled can have a moisture content ranging anywhere from 50% to 200%. This wide range reflects the natural variability across different tree species and even within parts of the same tree. For instance, a moisture content of 200% means the wood contains twice as much water as it does solid wood matter by its dry weight. However, the reference specifically notes that most trees typically exhibit a moisture content of about 75% immediately after being cut down.
Understanding Moisture Content Percentage
Moisture content in wood is typically expressed as a percentage of the oven-dry weight of the wood. This means if a piece of wood weighs 175 grams wet and 100 grams when completely dry, its moisture content is 75% (calculated as [(Wet Weight - Dry Weight) / Dry Weight] * 100%). High moisture content significantly impacts the wood's weight, workability, susceptibility to rot, and its burning efficiency if used as firewood.
Variability in Green Wood Moisture
While 75% is a common average, the exact moisture content of newly cut wood can fluctuate based on several factors:
- Tree Species: Different species inherently hold varying amounts of water. For example, some softwoods like pine or fir might have higher initial moisture than dense hardwoods like oak.
- Season of Felling: Trees generally have higher sap content (and thus moisture) during the spring and early summer growing seasons compared to winter.
- Part of the Tree: Sapwood (the outer, newer wood) typically has a much higher moisture content than heartwood (the inner, older wood).
- Location/Growth Conditions: Environmental factors such as soil moisture and climate can also influence a tree's water content.
Impact of Moisture Content and Seasoning
The high moisture content in newly cut wood makes it unsuitable for many applications without proper drying, a process commonly known as seasoning or curing.
- Shrinkage and Stability: As wood dries, it shrinks, primarily across the grain. This shrinkage can lead to defects like warping, checking (cracks), and splitting if not controlled.
- Strength: The strength properties of wood generally improve as its moisture content decreases, particularly below the fiber saturation point (FSP), which is typically around 25-30% MC.
- Durability: High moisture content makes wood highly susceptible to fungal decay, mold growth, and insect infestation. Proper drying significantly enhances its long-term durability.
- Workability: Drier wood is generally easier to cut, sand, and finish, leading to better quality end products.
- Energy Efficiency (Firewood): Wet wood burns inefficiently, producing more smoke and less heat, as a significant portion of the energy is consumed in evaporating the water rather than producing warmth.
Typical Moisture Content Ranges for Wood
| Wood State | Moisture Content Range (%) | Typical Average (%) | Notes |
|---|---|---|---|
| Freshly Felled (Green Wood) | 50% - 200% | 75% | Highly variable depending on species, season, and tree part |
| Fiber Saturation Point (FSP) | 25% - 30% | ~28% | Point where all 'free water' in cell cavities has evaporated |
| Air-Dried (Seasoned) | 12% - 20% | 15% - 18% | Achieved through natural drying outdoors over time |
| Kiln-Dried (Interior Use) | 6% - 12% | 6% - 8% | Achieved through controlled heating for specific applications |
Importance of Drying Cut Wood
To make cut wood suitable for construction, furniture making, or efficient burning, it must be dried. This process can be achieved through:
- Air Drying: This method involves stacking wood outdoors, protected from rain and direct sunlight, allowing it to naturally dry over time. It's a slower process but often cost-effective for large volumes.
- Kiln Drying: This involves placing wood in heated chambers where temperature, humidity, and airflow are precisely controlled. Kiln drying accelerates the process and can achieve lower, more uniform moisture content, reducing the likelihood of drying defects.
