ASBP Paints & Finishes Group – Briefing paper series – ‘How to read a data sheet’ | Published – June 2026
What is breathability?
When we talk about ‘breathability’ in regard to paint, we are referring to how easily water vapour can permeate and transfer through the surface of the material. Using a ‘truly’ breathable paint in tandem with breathable building materials allows water vapour to transfer freely and prevents moisture becoming trapped beneath the surface of the paint.
This paper aims to help specifiers and practitioners understand how to interpret SD values on paint data sheets and make informed choices about vapour permeability.
‘Breathable’ is often used to describe this behaviour. It is sometimes described as ‘two-way’ movement (in and out), although in practice vapour movement is driven by differences in vapour pressure. This is particularly relevant for wall paints, or paints suitable for lime renders, stone and brick, as those substrates need to be able to absorb as well as release moisture.
Some paints, such as linseed oil paints, behave differently by shedding liquid water while allowing limited vapour movement, sometimes described as ‘wicking’. This is particularly important when painting wood and metal, where you don’t want the substrates to absorb moisture, just release it (our forthcoming paper on linseed paint will set this out in more detail).
Why does it matter?
Trapped moisture can lead to damp, causing paint to bubble, blister or peel as water attempts to escape. In more serious cases, pressure from trapped moisture can cause coatings or render to fail.
It can also contribute to wider issues within the building fabric, including mould growth, condensation and rot. If left unresolved, these issues can have significant consequences for both occupant health and the durability of the building.
Breathability is a particularly important consideration for older, solid wall buildings. The materials used in this type of construction (e.g. lime mortars) are also breathable, and it is vital to retain these ‘vapour open’ qualities across the building fabric.
However, vapour permeability is also relevant in modern construction, particularly where moisture risk is present or drying potential is limited.
But shouldn’t paints be waterproof?
A “waterproof” paint is designed to block moisture and act like a film around its surface. While this may initially prevent moisture from entering, it can also restrict drying.
As walls naturally move, tiny cracks can develop. Moisture can then enter via these cracks and become trapped beneath the paint surface and within the building fabric, increasing the risk of damp-related issues.
Aren’t all paints ‘breathable’?
The vast majority of paints can technically be classed as “breathable” as they will eventually allow some moisture to escape. However, what’s important is how breathable the paint is.
The term “breathable” is not regulated and is often used inconsistently in product marketing. For this reason, it is important to rely on measurable performance data rather than product claims alone.
Can breathability be measured?
There is no defined European Standard as to what constitutes breathability. However, when it comes to paint, the most accepted and widely adopted metric is the SD value.
What is a SD value?
SD value stands for ‘steam diffusion’ or air layer equivalence. In regard to paints, it is a measure of how much of a barrier a paint coating is to water vapour and how easily the vapour can pass through this barrier.
The SD value represents the diffusion-equivalent air layer thickness and is expressed in metres (m).
- The lower the SD value, the more moisture is able to pass through
- The higher the SD value, the lower the moisture transfer
The metric is derived from European standards for water vapour diffusion (EN ISO 7783) and is widely used across Europe (originating in German practice).
“If you are looking for a ‘truly’ breathable paint, a lower number is better. Aim to choose a product with an SD value ranging from 0.01 to 0.5. If you can’t see an SD value referenced on a data sheet, it can be difficult to verify vapour performance and the product should be treated with caution.”
Why and where is it used?
The SD value is probably the easiest methodology to understand, interpret and compare, making it a useful tool for specifiers and practitioners.
An SD value may be listed on a data sheet under headings such as:
- Water vapour permeability
- Diffusion-equivalent air layer thickness
- Breathability
- SD value – H₂O
If you are unable to find the SD value listed on the data sheet, it can be difficult to assess vapour performance.
What is a typical SD value?
An SD value is measured in metres. Conventional masonry paints will often have an SD value of 1 m or above, meaning they significantly restrict vapour movement.
What is a ‘good’ SD value?
An SD value below 1 can be classed as breathable. However, a ‘truly’ breathable paint should have an SD value ranging from 0.01 to 0.5.
This equates to moisture having to travel approximately 1 cm to 50 cm through an equivalent air layer, meaning it encounters relatively little resistance.
| SD Value (m) | Interpretation | Typical Use |
|---|---|---|
| <0.1 | Highly vapour open | Heritage, solid wall, lime |
| 0.1–0.5 | Vapour open | General breathable systems |
| 0.5–1 | Moderately breathable | Some modern systems |
| >1 | Vapour resistant | Conventional masonry paints |
Examples of natural paint products with low SD values include:
- Keim Innostar: ≤0.01 m
- Earthborn Claypaint: 0.02 m
- Edward Bulmer Emulsion: <0.1 m
- Beeck Maxil Pro: 0.01 m
Conclusion
Understanding and specifying SD values allows for more informed decisions and helps ensure that paint systems support, rather than hinder, the moisture performance of the building fabric.
Rather than solely relying on the term ‘breathable’, specifiers and practitioners should prioritise measurable data to support effective moisture management and long-term durability.
References
- https://cornishlime.co.uk/wp-content/uploads/2016/06/Cornish-Lime-Breathable-Paints-Explained.pdf
- https://earthbornpaints.co.uk/faq/what-are-the-moisture-transmission-values-mvt-and-steam-diffusion-values-sd-of-earthborn-paints
- https://www.ecologicalbuildingsystems.com/post/q-and-a-breathable-paints
- https://www.keim.com/fileadmin/content/Documents/Additional_brochures/EN-EN/Technical_digests/Breathability___Mosture_Vapour_Permeability.pdf
- https://www.edwardbulmerpaint.co.uk/inspiration/whyusebreathablepaint/