> For the complete documentation index, see [llms.txt](https://docs.betterbuilding.io/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://docs.betterbuilding.io/support-and-training/other-support/faqs/design-and-specification/materials/control-layers.md).

# Control Layers

While thermal performance often takes centre stage when discussing high-performance building envelopes, it’s just one part of the picture. A building envelope is made up of several control layers that separate the internal environment from the outside. These layers include everything from external finishes like render or cladding, to internal finishes such as paint. Each plays a distinct role in ensuring comfort, durability, and performance.

There are four main types of control layers:

## **1. Water Control Layers**

The primary role of a water control layer is to prevent liquid water from entering the building. It forms the first line of defence against rain and other external moisture. This layer is made up of several materials and is designed and installed to form a continuous rainwater barrier.

* In **face-sealed systems**, it’s the outermost layer of the envelope.
* In **concealed barrier systems**, it sits just behind the outer surface.
* In **drained systems**, it’s located behind a drainage gap or drainage layer, acting as the drainage plane.

## **2. Air Control Layers**

Air control layers are essential for managing moisture movement caused by air leaks. Warm, moist air from inside the building can leak into wall or roof assemblies, where it may condense and cause hidden damage over time. These layers help maintain durability by limiting the movement of air and the moisture it carries.

Because walls and roofs are often closed systems, issues like mould may only become visible on surfaces after damage has occurred internally. An effective air control layer is critical to preventing this.

## **3. Vapour Control Layers**

Vapour control layers manage the movement of moisture vapour through materials, something air control layers don’t fully address. For example, plasterboard can stop air, but vapour can still pass through it. That’s where vapour control comes in.

These layers need to strike a balance: preventing moisture build-up while allowing enough drying potential to keep assemblies healthy. This is particularly important in colder climates, where higher insulation levels reduce the energy flow that would otherwise help dry out materials. Many modern materials also have lower vapour permeability compared to traditional ones, making this consideration even more important.

In many cases, the air and vapour control functions are provided by the same material, but this isn’t always the case, and the distinction matters.

## **4. Thermal Control Layers**

Although thermal control layers are less critical to durability, they attract the most attention due to their role in energy efficiency and occupant comfort. These layers help regulate heat flow through the building envelope.

In heating-dominated climates, thermal layers should generally sit outside the air and vapour control layers. This prevents warm, moist interior air from reaching cold surfaces where it could condense.


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