Wall Breathability & Moisture: The Sd Coefficient in Architectural Paints

In modern building physics, "breathability" does not mean the wall allows air to pass freely. It refers to the microscopic diffusion of water molecules in vapour phase through a solid material's pore network, driven by the difference in vapour pressure across its two faces.

The Sd coefficient (Equivalent Air Layer Thickness) is measured in metres (m) and expresses the thickness of a theoretical, still air layer that would present the same resistance to water vapour. The lower the value, the more easily the material "breathes."

Sd is not a marketing advantage - it is the physical link between a paint's chemistry and the building's longevity.

The cup method specified in EN ISO 7783:2018 measures the rate of water-vapour transmission through a standardised porous substrate. The results divide coatings into three strictly defined categories.

Applications demanding extreme moisture management - such as heritage building restoration - mandate exclusively V1 coatings.

A paint's ability to "breathe" depends on the molecular structure of its binder. Acrylic and vinyl (plastic) paints form an elastic film over the wall - highly washable but with a high Sd. Mineral silicate paints bond chemically with the substrate through silicification, leaving pores open.

The right chemistry alone is not enough: if applied in thick coats, even a silicate paint can turn into a vapour barrier.

The belief that "breathable" paints are only relevant for exterior facades is incorrect. Proper use of ultra-low Sd paints indoors is a critical factor for air quality and the long-term health of the building.

In bathrooms, kitchens and ground-contact rooms, an impermeable paint creates a permanently damp film - an ideal substrate for mould.

The market promotes breathability as a panacea, but from a strict engineering perspective there are clear cases where the cost of a premium mineral paint offers no measurable return.

The decision for a "breathable" paint is not taken dogmatically - only where the building's physics demand it.

A masonry wall is not an impervious boundary; it functions as a dynamic hydraulic system. Water vapour always moves from the warm, humid side towards the cold side. In winter, this vapour drive runs from inside to outside. If the exterior coating is impermeable, moisture condenses within the wall.

In the Mediterranean climate, masonry must accommodate both the winter flow (inside → outside) and the summer reverse flow (outside → inside).

Translating the laws of thermodynamics into practical material choices separates durable structures from those that deteriorate rapidly. The right Sd balance between interior and exterior coatings determines everything.

There are no "good" or "bad" paints - only correctly or incorrectly engineered hygrothermal systems.

The Sd coefficient is not just a marketing buzzword. It is the non-negotiable physical link bridging polymer chemistry with building sustainability. In high-humidity spaces and traditional structures without modern barriers, extreme breathability is vital. In internal partitions without hygrothermal load, pursuing zero Sd is an expensive over-specification.

In the Mediterranean climate, interior paints must act as mild retarders, while exterior coatings must remain strictly breathable. And always, the choice of primer must be an organic part of the same chemical system.