by Denise Henkenhaf, eZED Ltd | 4 July 2014

Diffusion is a term that you don’t hear much in New Zealand, neither do you read about in the New Zealand Building Code. Yet it is a key driving force behind the moisture problems that occur in our buildings. So here we answer “What is Diffusion?”.

Diffusion, in building physics, is the mechanism of moisture transport on a molecular basis through the microscopic pores of a segment of material. This occurs due to slight changes in vapour pressure caused by temperature and moisture differences to either side of the segment.

Factors that influence diffusion are: –

  • Location (local climate)
  • Inclination (roof, wall)
  • Orientation (shaded, sun, driving rain)
  • Initial moisture content (for example building materials being exposed and wet from rain)
  • Purpose of the building (swimming pool, cool store)
  • Construction materials and how they are arranged.

In simplified terms, it is the movement of moisture vapour from the warm side to the cold side of a building element. This movement is affected by the diffusion resistance (μ) of a material. Materials that have an open structure such as fiberglass insulation, have low diffusion resistance. Conversely polystyrene has a relatively closed structure and thus has a high diffusion resistance.

Diffusion occurs irrespectively of summer or winter seasons. It always moves from warm to cold so it can reverse direction between the seasons.

Diffusion can become a moisture problem if the temperature difference is large enough, typically at differences of 10 degrees or more, so it is more marked when the climate is very hot or very cold.

The use of vapour retarders, vapour barriers and airtightness layers requires a proper understanding of how each of these materials influences convection and diffusion.

  • Airtightness layer: minimises or eliminates convection – movement of energy (and moisture vapour) through the air
  • Vapour retarder: minimises diffusion + convection
  • Vapour resistive barrier: eliminates diffusion + convection

The absolute elimination of convection and diffusion is not practically achievable. Some residual moisture levels are inevitable and won’t pose a risk for long term issues as long it cannot build up over time. Thus it is essential that moisture can move out of a building element.

A vapour resistive barrier initially appears to be the most attractive solution however the construction moisture has no means to escape hence this approach to both sides of a building element is not recommended.

The first step is to establish the predominant direction of driving forces that may lead to a long term moisture problem. It is recommended to use at least one outer layer of a material that allows adequate diffusion to occur so that the building element has the capacity to dry out.

Diffusion cannot be eliminated and should always be considered in the design process. When building in a severe climate, where temperature differences indoor to outdoor are greater than 10 degrees for a period of 3-4 weeks each year, diffusion problems will be amplified. eZED can review your construction details using the specialist WUFI® software package, get in touch if you are concerned about your project and need some help.

Copyright eZED Limited 2014