I’m generally less concerned about vapor diffusion—not because it’s unimportant, but because air leakage is far more critical. Around 97% of moisture issues stem from air leakage, while only about 3% are caused by diffusion. That said, understanding vapor diffusion is valuable to avoid getting into a panic over it.
In Joe Lstiburek’s paper, “BSI-049: Confusion About Diffusion,” he discusses the complexities and misunderstandings around diffusion, condensation, and moisture behaviour in building assemblies.
The following is a summary of Joe’s paper. Follow the link below my summary to read his full article.
While diffusion is often simplified to a movement of particles from high to low concentration, real-world application is far more nuanced. Diffusion is typically a blend of diffusion, capillarity, and surface interactions, all influenced by fluctuating temperatures, vapour pressures, and material variations within structures. Joe paper argues that focusing on surface behaviours is more practical than attempting to decode every intricate detail of molecular movement.
Joe critiques the traditional dew point calculation often used to predict condensation, which implies that moisture should condense in the middle of wall insulation. However, he points out that condensation generally accumulates on the backside of the sheathing, (or in the case of Australian construction, the back of a non-vapour permeable wall wrap)—the actual “condensing surface of interest.” This discrepancy results from the dew point location merely indicating where moisture can begin to condense, not where it will remain. Once water vapour reaches a cooler surface (like a non-vapour permeable wall wrap), it condenses and remains there, driven by temperature gradients.
The principle extends to other parts of buildings. For instance, in a room, the interior surface of windows acts as a dehumidifier, influencing condensation behaviour. Similarly, in attic roof space and sub-floor spaces, surface conditions often dictate moisture dynamics. The roof cladding underside, or the underside of a non-permeable sarking, for example, is where condensation primarily collects, not within insulation. In sub floor spaces, surfaces such as the underside of floor joists or the top of ground covers often become the “condensing surfaces of interest,” governed by the ground temperature, which typically aligns with the location’s average annual ambient temperature.
Joe emphasises that practical risk assessments can often be achieved without complex calculations, as professionals in the past evaluated real-world building conditions through observation and basic temperature plotting. By monitoring average temperatures on critical surfaces, we can predict condensation risks more effectively than with computer models that may not accurately simulate airflow and other variables.
In summary, Joe’s article reveals that understanding moisture control in buildings comes down to surface conditions and practical assessment rather than reliance on overly simplified or theoretical models. This surface-centric approach offers a more intuitive and effective way to address moisture issues in construction.
Joe’s article is worth a read. Link provided below:
https://buildingscience.com/documents/insights/bsi-049-confusion-about-diffusion
Lastly, what about exterior rigid insulation board fixed to the outside of the frame? This isn’t vapour permeable. Won’t moisture condense on the inside face? The short answer: Exterior insulation can actually mitigate condensation by warming the interior surface and keeping it above the dew point, thereby reducing or eliminating condensation risks…but more about this in another post.