Foreword by Ian Thompson, Editor
The construction industry’s approach to moisture management has long been plagued by oversimplification, particularly when it comes to vapour barriers and vapour control in building assemblies. I’ve witnessed countless moisture-related failures in buildings, I’ve seen these issues intensify recently as our New Zealand building code pushes us down a path to produce airtight buildings, but fails to properly address air movement in our buildings. Let’s think it of this way, do we want to live in an airtight box with no fresh air coming in to the building?
One fundamental issue I’ve observed is how the casual misuse of terminology – specifically the term “vapour barrier” – often leads to costly mistakes in building envelope design and product specification. I must admit that even discussing the topic often leads to confusion because I’m not entirely sure what the designer is trying to achieve – and why they’re using the vapour barrier in what seems the wrong place and with the wrong wall components.
So, the very word “barrier” is misleading, as effective moisture management often requires controlled vapour transmission rather than complete blockage. My logic is a vapour barrier shouldn’t be considered an air barrier.
Modern building envelope design has evolved to recognize that some specialized materials can perform multiple functions – serving both as vapour control layers that allow assemblies to dry when needed, while also acting as water-resistant barriers against external elements like rain.
This dual functionality becomes especially crucial as buildings become increasingly airtight for energy efficiency, where proper moisture management can mean the difference between a durable structure and one plagued by condensation, insulation failure, structural decay, and dangerous mold growth.
This article by our guest author Denise Martin from BEO Building Science challenges our industry’s imprecise language around vapour control layers and presents a more nuanced framework for understanding and specifying these critical components. By examining how various international standards classify vapor control materials and exploring the implications of proper material selection, we’ll unpack why precision in both terminology and application is fundamental to creating durable, healthy buildings.
Over to our building science expert Denise Martin.
Understanding Vapour Control and Vapour Barriers
Let’s talk about a term that’s thrown around a little too loosely in the construction and building materials industry: “Vapour Barrier.” Whether you’re an architect, contractor, or building consultant, you’ve probably heard this term used to describe a wide range of materials designed to manage moisture in buildings. But here’s the thing – not all materials labelled as vapour barriers are created equal, and using the wrong one in the wrong place can lead to major headaches down the line.
Moisture issues in buildings are a serious concern. From mould growth and structural damage to poor indoor air quality, the consequences of improper vapour control can be costly and harmful. Many professionals, despite their expertise, still default to using the term “vapour barrier” for all moisture control membranes, often overlooking the nuances of vapour permeance and resistance. This broad-stroke approach can result in unintended moisture accumulation, leading to mould, rot, and ultimately, expensive remediation.
Vapour control layers should specifically refer to layers and membranes that are designed to control vapour movement. It’s important to note that every building material has a vapour permeability that falls somewhere on the scale from being highly permeable to impermeable. This includes common materials such as plasterboards, plywood, and concrete. However, specialised membranes, such as smart or intelligent vapour control layers like Proclima Intello and similar products, are designed to adapt their permeability based on environmental conditions, offering more advanced moisture management solutions.
So, what’s the solution? The key lies in understanding and adopting the correct terminology: “Vapour Control Layer.” This term better encompasses the full range of materials available, allowing for precise selection based on the specific needs of a project. Standards like DIN 4108-3, ASHRAE 160, and AS/NZS 4200.1:2017 provide clear guidelines that classify membranes based on their ability to resist or allow vapour diffusion. Instead of using the generic term “vapour barrier,” we should be talking about categories like high vapour permeability, moderate permeability, low permeability, and true vapour barriers. Each classification serves a specific purpose, and understanding these distinctions is crucial to ensuring the longevity and performance of a building envelope.
The DIN standard differentiates vapour resistance SD from 0m to 1500m, whereas in Oceania, whereas in Oceania the differentiation upwards from equivalent SD of 10 is pretty much the same.
This broad classification does not provide the level of detail needed for effective moisture control and can lead to material misapplication.
Here’s a handy table that shows how the different standards compare:
When we choose the right vapour control layer, the outcomes are clear: improved building durability, enhanced energy efficiency, and healthier indoor environments. You can avoid the stress and financial strain that comes with moisture-related failures and instead feel confident knowing your project is designed to withstand the test of time. Correct terminology and informed material selection empower professionals to create sustainable and resilient buildings.
That way, you can stop feeling frustrated by recurring moisture problems and instead feel assured that your building envelope is performing exactly as it should. Because, at the end of the day, that’s what you want, isn’t it?
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