Airtightness, Breathability, and Vapour Control: How to Make It All Work Together? – CIJA? | Episode 4

Airtightness, breathability, and vapour control are three concepts that get conflated and confused in equal measure, and understanding how they interact is fundamental to designing a building fabric that performs well and doesn’t accumulate moisture damage over time. Airtightness means eliminating the uncontrolled bulk movement of air through gaps in the fabric — between components, around windows, through junctions. It has nothing to do with ventilation: an airtight building can be very well ventilated, either manually or mechanically, but the air exchange happens in a controlled way rather than wherever the fabric allows. The reason airtightness matters so much is that moving air carries a substantial amount of moisture with it — far more than vapour diffusion alone — and uncontrolled infiltration deposits that moisture inside the fabric where it causes damage.

The question that trips people up is how a material can be airtight while still being breathable. The answer lies in scale. Vapour molecules are extraordinarily small relative to air molecules — imagine a rubber dinghy in the North Sea, where the sea represents the pore structure of the material. To a vapour molecule, many materials that are impermeable to bulk air movement look mostly like open space, allowing diffusion through while blocking convective air flow. This means a material can simultaneously provide airtightness and vapour permeability — or airtightness and vapour resistance, depending on what’s needed. In an internally insulated solid wall, where the wall needs to dry inward, the membrane or plaster finish should be airtight but vapour permeable, maintaining the drying capacity toward the room side. In a roof structure, where inward drying isn’t typically needed, an airtight vapour retarding membrane makes more sense.

The distinction between a vapour retarder and a vapour barrier is also important. Most vapour control layers used in well-designed roof and wall assemblies have an SD value of between five and around 100 — they slow the flow of moisture into the structure to a rate the assembly can manage, while remaining dynamic enough to allow some inward drying during summer when the diffusion gradient reverses. A true vapour barrier with an SD value of 1,500 or more on the inside of a roof that needs to dry inward will cause condensation to accumulate on its back face over time. Choosing the right membrane for the right application — and understanding why — is where vapour control strategy either protects a build-up or gradually undermines it.