How to prevent moisture in modern wooden structures?

Among various building materials, wood has environmental advantages such as recyclability, negative carbon footprint, and energy conservation, making it increasingly favored by the construction industry. Modern wood-frame buildings are gradually becoming popular and are being promoted and applied worldwide. However, many people still have doubts about whether modern wood-frame buildings can withstand the test of moisture, especially in rainy or humid places. Can wood-frame buildings maintain long-term durability?

Photo: Chongqing Yuanlu Community Center

The relationship between moisture and wood

When wood gets damp, it is primarily threatened by various wood-decaying fungi and insects such as termites. Therefore, many people mistakenly believe that water is the enemy of wood. This is not the case. Wooden buildings can maintain their performance over a long period even in rainy or humid environments. The key lies in understanding how to control moisture when designing and constructing wood-based building products.

Timber with a moisture content below 19% is considered dry timber, possessing stable properties and not easily deformed or chemically reacted. All timber used in modern timber-framed buildings undergoes kiln drying to prevent most volume changes. This pre-dried timber, with a moisture content below 19%, offers moisture resistance up to 10 times that of brick-and-mortar structures. Furthermore, timber can regulate indoor humidity, making buildings more comfortable. During periods of high humidity, it absorbs moisture; during drying, it releases moisture into the air. Clearly, water and timber can coexist harmoniously.

Photo: Boat House on the Fuchun River in Hangzhou, Zhejiang

Photo: Boat House on the Fuchun River in Hangzhou, Zhejiang

The 4D Principle of Moisture Control in Modern Timber Structures

Moisture enters buildings primarily through rainwater, via three pathways: seepage, water vapor diffusion, and air leakage. Therefore, as long as proper moisture-proofing is implemented during the design and construction phases, wood can be a building material that performs well in various climates.

Image: The 4D Principles of Moisture Prevention for Wooden Structures

Deflection​

The key to moisture-proofing lies in preventing moisture from accumulating in the gaps between structural components. If a building is poorly designed and constructed, rainwater from outside or water vapor from inside can enter these gaps. Deflectors aim to deflect rainwater away from the building surface, minimizing the possibility of rainwater seeping into the building envelope and interior. The main methods include the design of roof eaves and rain collection systems. This design and construction approach can prevent moisture accumulation in the gaps between structural components, as well as related problems such as mold and rot.

Drying

To prevent condensation from causing moisture damage to wooden structures and to avoid providing a breeding ground for mold and wood-decaying fungi, an air barrier can be installed on the building's exterior to prevent moisture from condensing for extended periods through evaporation, diffusion, and airflow.

Drainage

Drainage is achieved through sloping designs on the roof and horizontal components, primarily including drainage layers, breathable paper, and asphalt felt. This allows water to flow out of the building under gravity while preventing rainwater from intruding into the internal structure. Wrapping the building exterior with a layer of one-way breathable paper effectively reduces water vapor movement caused by diffusion, making the wooden structure both effectively moisture-proof and breathable.

Image: DuPont Tyvek Breathability Paper

Durability material

In necessary locations, preservative-treated wood is used to improve the durability of critical components. Preservative-treated wood achieves its preservation by using chemical preservatives to cut off the food source. These preservatives can only achieve overall preservation by being injected into the wood under high temperature, high pressure, and vacuum conditions, allowing for a thorough treatment.

Moisture control affects the durability and comfort of buildings. With the further development of modern timber construction technology, accurate control of timber moisture content and the 4D principle of moisture control have made it possible for modern timber construction to be realized in a variety of environments.

Photo: Qingcheng Mountain Wasai Linxi Boutique Hot Spring Treehouse Inn