If you’ve ever wondered how a building can be airtight and breathable at the same time, you’re not alone. It’s one of the most common—and confusing—questions in sustainable building.
In this article, we’ll break down the key ideas behind airtightness, breathability, and vapour control. What do these terms actually mean? How do they work together? And why does it all matter so much when you’re designing or renovating a home?
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. 👇
What Is Airtightness, and Why Do We Need It?
Airtightness means stopping unwanted air from leaking through gaps in your building. That includes cracks around windows, holes in walls, or tiny gaps between materials.
Now, that might sound a bit extreme—don’t we want our buildings to breathe? But here’s the thing: when air leaks in or out of a building without control, it doesn’t just waste energy. It also carries moisture with it. Over time, that moisture can get trapped inside walls or roofs, leading to damp, mould, and even structural damage.
So airtightness isn’t about sealing everything up like a Tupperware box. It’s about making sure you decide where air comes in and out—usually through windows or a proper ventilation system—not through random gaps.
What Do We Mean by “Breathable”?
Here’s where things get a bit confusing. In building terms, breathability doesn’t mean the same thing as letting fresh air in.
Instead, it means that materials in the building can allow water vapour to pass through them. Not liquid water (like rain), and not bulk air (like a draught)—just tiny molecules of moisture in the air.
Think of it like this: vapour-permeable materials act a bit like a wool jumper. They let moisture escape, but still keep you warm and dry. This helps your building “breathe” in a healthy way—letting moisture out without letting the cold in.
So How Can Something Be Both Airtight and Breathable?
Great question! It all comes down to scale.
Airtight materials stop bulk air movement—like a breeze whistling through a gap. But many of those same materials still allow individual water molecules to move through. That’s because water vapour is made up of incredibly tiny molecules, which can travel through the microscopic pores in some materials—even ones that are airtight to air.
For example, lime plaster and some modern membranes are completely airtight but still breathable. They stop draughts, but let moisture vapour slowly pass through. Clever, right?
What’s a Vapour Control Layer?
A vapour control layer (VCL) is a material—often a membrane or board—that slows down how much water vapour moves into a wall, roof or floor.
Why slow it down? Because too much moisture building up inside a structure can cause problems. Timber might rot, insulation can become soggy, and cold surfaces might develop mould.
In most buildings in the UK, moisture tends to move from the warm inside of the building towards the cold outside, especially during winter. So placing a vapour control layer on the warm (indoor) side helps prevent too much moisture entering the wall or roof where it could condense.
But here’s the important part: we usually want a vapour retarder, not a full-on vapour barrier. Vapour barriers (like plastic sheeting with very high resistance) can trap moisture inside if conditions change. Vapour retarders, on the other hand, let a small amount of vapour through—and even allow some back-drying if needed.
Real-Life Examples
- Solid masonry walls: If you’re adding insulation on the inside of an old brick or stone wall, you need to allow the wall to dry back into the room. A lime plaster or a vapour-open membrane can help you do that while still keeping things airtight.
- Roofs: Since roofs don’t usually take on moisture from the outside (as long as they’re built properly!), you can often use a more vapour-resistant membrane here—as long as it still allows for some inward drying during summer.
- Timber frames and lightweight walls: These need special care. Timber and insulation can soak up moisture during construction, so it helps to use materials that can buffer and manage that moisture. But you still need to limit how much vapour enters from the room side—especially in winter.
Key Takeaways
✅ Airtightness helps control moisture and heat loss
✅ Breathable materials let vapour out, not draughts in
✅ Vapour control layers manage moisture flow—not block it completely
✅ The right approach depends on your building type, location, and materials
Understanding how moisture and air behave in buildings isn’t just for engineers—it’s something every builder, designer, and homeowner should get to grips with. The good news? It’s not as complicated as it first seems.