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Humidity Buffering

Solid wood constructions can buffer variations of indoor humidity, by adsorbing moisture when the humidity of the indoor air is high and desorbing moisture when the humidity is low. In other words, solid wood can act as a passive dehumidifier and a passive humidifier.

Solid wood construction can improve the comfort and quality of the indoor environment, by increasing the frequency that the indoor air is in the desirable range of 30-60% relative humidity.

Improvements can be significant in bedrooms during the night, especially for houses in humid climates such as Auckland.

There is evidence the moisture buffering with solid wood reduces peak cooling loads in air conditioned buildings by up to 30%.

There is also evidence that moisture buffering slightly reduces building energy use (=5%).

Energy saving is through reduced ventilation requirements and relaxed heating and cooling temperatures rather than latent heat flows associated with the adsorption and desorption of moisture in the building fabric.

The rate of moisture exchange between a solid wood construction and the indoor air depends on many factors, including:

  • Water vapour permeability of wood, which varies with wood species, grain direction with respect to the moisture flow and wood moisture content.
  • Water vapour permeability of internal linings and/or coatings applied to the wood.
  • Area of exposed wood.
  • Moisture content and flow rate of air (from outdoors and other parts of the building) entering the building space.
  • Air temperature of the building space.
  • Moisture generated within the building space.

Moisture exchange depends on the external climate and the behaviour of occupants, in addition to building design.

Predicting the effect of solid wood construction on the indoor environment and thermal performance of a building is a complex problem involving heat and mass transfer through the building structure.

The latest generation of simulation tools are capable of modelling moisture flows in buildings, making it feasible for designers to exploit the moisture buffering benefits of solid wood construction.