Controlling condensation from air leakage and vapour diffusion

Figure 3: This table, taken from the Building Science Digest 163, Controlling Cold-weather Condensation Using Insulation (John Straube), illustrates the ratio of interior to exterior insulation to mitigate interior air leakage condensation.

Exterior to interior insulation
Despite most efforts to reduce interior air leakage, designing an airtight building and actually constructing one are two different things. Constructing a building to meet strict Passive House standards requires the building shell achieve 0.6 ach @ 50 Pa. Most buildings are between 5 to 9 ach, with the better ones being between 2 and 3 ach. Therefore, most buildings will have some amount of air leakage.

The key to preventing condensation from occurring within a wall is to keep the condensing surfaces in the assembly—typically the sheathing, as it is usually the outmost layer—above the dewpoint temperature. By placing an adequate amount of insulation on the exterior side, the sheathing remains warm and condensation is avoided.

Although placing interior cavity insulation increases the thermal resistance of the wall assembly, it also resists heat flow to the sheathing, lowering its temperature. To ensure the sheathing remains above the dewpoint, the correct ratio of exterior to interior insulation is required to control air leakage condensation for various outdoor and indoor conditions, as shown in Figure 3.

Understanding the fundamentals of heat, air, and moisture transfer, along with the properties of materials, is critical in avoiding moisture-related issues. Constructing walls with double-vapour barriers or a vapour barrier on the wrong side of the building should be a thing of the past.

When considering the realities that may impact a building’s moisture equilibrium (e.g. water intrusion, air leakage, and vapour diffusion condensation), it is important to choose materials that can control and limit the amount of moisture introduced into the system, but at the same time allow for adequate drying when needed.

The goal is to design buildings resilient to the realities of aging, leaks, and imperfect construction by encouraging drying mechanisms or by ensuring the drying potential is greater than the wetting potential in the enclosure. As heat loss slowly disappears with increasingly stringent requirements for continuous exterior insulation, so will the requirements for vapour barriers in the ‘milder’ cold climates of Canada. Only time will tell.

Vincent Chiu joined ROXUL’s building science team as a building science specialist in May 2016. He has a master of applied science degree in building engineering from Concordia, where he focused on building diagnostics and rehabilitation, building modelling, wind engineering, and building aerodynamics. Chiu now works extensively with architects, engineers, design consultants, and contractors, providing building enclosure solutions to the architectural community and promoting energy-efficient enclosure designs and sound building science. He can be reached at

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