Airtightness, Breathability, and Vapour Control: How to Make It All Work Together? – Can I Just Ask? – Episode 4

Airtightness, Breathability, and Vapour Control: How to Make It All Work Together

When it comes to building performance, few topics generate as much confusion—and controversy – as airtightness, breathability, and vapour control. Can you really make a building airtight an breathable? Do vapour barriers belong anywhere in modern construction? And how do you prevent condensation and mould without compromising energy efficiency?
Let’s unpack how these concepts interact, and how to detail them successfully in real-world buildings.

If you’d rather hear Andy and Chris talk through the ideas in this article, you can watch Episode 4 of Can I Just Ask? below. 👇

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Why Airtightness Matters (Even More Than You Think)

Airtightness isn’t about stopping fresh air – it’s about controlling it. Uncontrolled air leakage through cracks and gaps in the building envelope not only results in energy loss, it also carries significant amounts of moisture into walls, roofs, and floors. This kind of convective moisture transport is one of the most damaging and difficult to predict.

By creating an airtight envelope, we’re reducing those uncontrolled air paths. That means fewer cold draughts, improved thermal performance, and most critically, better protection for the structure itself. From there, ventilation – whether natural or mechanical – can be properly designed and controlled to meet the needs of the occupants.

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The Big Misunderstanding: Airtight but Still Breathable?

A common question we hear is: “How can something be airtight but still breathable?” The key is understanding the difference between air movement and vapour diffusion.

Airtightness refers to bulk air movement. Think wind whistling through a gap in a window frame. Breathability, in contrast, refers to a material’s ability to allow water vapour to diffuse through it – molecule by molecule, not as part of a draught.

Materials like lime plaster or intelligent vapour control membranes can be completely airtight while still allowing moisture vapour to pass through. It all comes down to pore size, structure, and how the material interacts with water. Imagine water vapour as a rubber dinghy floating through the North Sea – the sea being the pore structure of the material. From a vapour molecule’s perspective, it’s mostly open water.

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Choosing the Right Vapour Control

Vapour control isn’t about blocking moisture entirely – it’s about slowing it down to a manageable level. This is especially important in timber frame and other lightweight constructions, where moisture can accumulate within insulation layers.

In most UK buildings, the vapour drive is from inside to outside for much of the year. That’s why the internal vapour control layer (VCL) is so important. But not all VCLs are created equal. Many so-called “vapour barriers” have an SD value of 1500 metres or more – effectively stopping vapour in its tracks. That’s not always desirable, particularly in natural or moisture-buffering wall systems.

Instead, intelligent vapour retarders with SD values between 5 and 100 metres allow for dynamic moisture movement. They slow vapour enough to prevent build-up, but still permit inward drying when needed—like during summer conditions when the vapour drive can reverse.

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Detailing for Drying: Context is Key

It’s not just about the membrane you choose, but where and how you use it. For example:

• Solid masonry walls being internally insulated require vapour-permeable finishes on the warm side to allow inward drying. Here, lime plaster or a vapour-open membrane can provide both airtightness and breathability.
• Roofs, on the other hand, usually don’t face wind-driven rain from above, so a more vapour-resistant membrane may be appropriate—provided it still allows some drying to the inside during warmer months.
• Timber frame structures benefit from materials that can buffer incidental moisture during construction. But they still need a VCL to resist vapour ingress from the interior during the heating season.

Simulation tools like WUFI are invaluable here, helping designers and builders assess drying potential and moisture risk based on actual site conditions and material choices.

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Getting It Right in Practice

The key takeaway? Airtightness, breathability, and vapour control are not in conflict. When approached with care, they’re complementary strategies that protect both occupants and building fabric.

Think of it as a balancing act: airtight enough to control air movement, breathable enough to allow vapour to diffuse, and smart enough to adapt to the building’s context, climate, and materials. Done right, it’s not just possible to have an airtight, breathable, well-ventilated building – it’s essential.