Every home must handle three very different forms of moisture:

Liquid water (rain and leaks): The most visible, controlled by cladding, flashings, and drainage layers.

Air: Less visible, but a powerful carrier of moisture. Airtightness and deliberate ventilation keep it under control.

Vapour: The smallest particles, constantly moving through materials. Whether vapour can safely escape depends on material properties like µ-value and sd-value.

When we understand these layers, we can design walls and roofs that both protect and breathe.

Different manufacturers describe vapour behaviour in different ways: some use perms, others use µ-values, sd-values, or vapour resistance. Without conversions, it can feel like comparing apples to oranges.

The good news? Once you know the simple relationships between units, you can translate them into common ground — and suddenly the fog clears. Designers can make confident, science-based choices, and clients can feel secure that their homes will last.

Other countries have already navigated this challenge:

The UK sets sd-value classifications in BS5250.

North America requires specific roof ventilation ratios and climate-zone-based vapour retarder placement.

Germany leads with dynamic hygrothermal modelling to ensure assemblies can dry.

These aren’t roadblocks — they’re roadmaps.They show us what’s possible when science and design work hand in hand.

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Higher insulation standards mean homes are warmer and more energy-efficient. But they also shift where condensation can form. By integrating vapour control and ventilation now, we can prevent hidden problems and move confidently into the next era of performance design.

homes are warmer and more energy-efficient, but they have also shifted where condensation can form

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