Breathable paint allows water vapour to pass through the paint layer instead of trapping moisture behind the coating. You may also see this described as vapour open, vapour permeable or moisture regulating paint.
If you've got a project that needs breathable paint, the key number to look for on the Technical Data Sheet is the Sd value. Sd stands for "steam diffusion value", also known as "equivalent air layer thickness".
The most breathable paints have the lowest Sd values, meaning it's easier for water vapour to move through the paint film and escape from the fabric of the building.
Breathable Paint Sd Classes
- Class 1 - Highly breathable, with an Sd value of 0 to 0.14 m.
- Class 2 - Moderately breathable, with an Sd value of 0.14 to 0.5 m.
- Class 3 - Limited breathability, with an Sd value of 0.5 to 1.5 m.
- Class 4 - Low breathability, with an Sd value above 1.5 m.
For older homes, lime plaster, solid walls and moisture sensitive surfaces, Class 1 is usually the best place to start.
Paint breathability is measured through water vapour transmission testing. BS EN ISO 7783:2018 is a recognised test method for measuring the water vapour transmission properties of paints, varnishes and related coatings.
