WHY PRECISION MATTERS WHEN TALKING ABOUT MEMBRANES

Pamela Howat, Senior Technical Advisor at Proctor Group, explains the similarities and differences between membrane terminology.

Construction membranes have a host of applications that directly impact on building performance and the comfort of building users. As a result, a lot of terminology comes with specifying and installing membranes. Over time, the term ‘vapour control layer’ (VCL) has come to be replaced by ‘air and vapour control layer’ (AVCL). This change has been driven by improvements to airtightness requirements in building regulations, increased adoption of standards like Passivhaus, and updates to codes of practice such as BS 5250:2021 (management of moisture in buildings).VCLs are layers, usually membranes, installed on the warm side of the thermal insulation layer.

They significantly reduce the quantity of moisture vapour able to pass into the rest of the wall or roof construction. Less moisture vapour in a build-up means less likelihood of surfaces on the cold side of the insulation allowing condensation to form unseen within the construction.

While moisture vapour can diffuse through materials, it is also carried by warm air. The movement of warm air into and through a wall or roof acts as another form of transport for moisture vapour. Not only that, it increases heat loss from the building’s interior – increasing energy consumption and reducing comfort. Specifying an AVCL instead of a VCL is therefore a recognition of the importance of limiting air movement as well as moisture diffusion. In practice, VCLs are airtight anyway and can, therefore, be classed as AVCLs.

HOW ARE BREATHER MEMBRANES AND VAPOUR BARRIERS DIFFERENT?

A common search term on our website shows people looking at the difference between ‘breather membranes’ and ‘vapour barriers’. The term vapour barrier typically means a vapour control layer so, really, the question is trying to understand how a breather membrane differs from a VCL/AVCL.A ‘breather’ or ‘breathable’ membrane allows the passage of moisture vapour through it. We refer to this as a vapour permeable membrane. When ‘breather membrane’ is used to describe a pitched roof underlay, it is generally referring to membranes that are low resistance (LR), ie, vapour permeable.

DO ALL LR BREATHER MEMBRANES PERFORM THE SAME?

Roof underlays such as Proctor Air are air permeable as well as low resistance, or APLR. This is a crucial distinction, as they create a more uniform flow of air from roof spaces and eliminate the need for roof ventilation products. Despite this, APLR underlays are often grouped with LR underlays using the generic ‘breather membrane’ terminology. This is not necessarily incorrect, but it is less precise and doesn’t accurately convey how a roof specification is intended to perform.

WHAT IS THE BEST POSITION FOR A BUILDING’S AIRTIGHTNESS LINE?

We’ve covered membranes that are both air and vapour tight. We’ve covered membranes that are both air and vapour permeable. We also need to address membranes that are airtight while being vapour permeable, like our Wraptite system.Given the importance of airtightness to building performance, there are a variety of reasons why an external airtightness line is preferable to an internal one. For example, internal airtightness layers are easily damaged or disrupted, with no guarantee of being properly reinstated.

A membrane like Wraptite provides airtightness, while also being vapour permeable. This helps to manage moisture risks in a wall build-up.An external airtightness layer does not necessarily negate the need for a layer that also controls moisture vapour. An internal membrane is often still specified, therefore. While the internal layer has the performance of an ‘AVCL’, for the application in question it is being specified only as a ‘VCL’.

This points to an important overall message: precision matters. Using generic terminology when membranes are serving a specific function risks clouding understanding around why particular solutions are needed within project-specific applications.