Breathable is a general term that means vapour diffusion. When we say a wall or paint is breathable, we are referring to the process by which vapour is released or diffused (from inside the wall) into the atmosphere.
How do you measure such a thing? Or how do you tell if one paint is more breathable than the other?
Breathability is measured via Sd value. A high Sd value means the paint is not terribly breathable. A low Sd value means the paint is extremely breathable. Sd stands for Steam Diffusion. It measures air layer thickness, ie how thick a layer of air vapour has to pass through before it is finally diffused. Or how far water vapour has to travel. The thinner the layer of air, the better.
Sd Value can be demonstrated through the Façade Protection Theory (FPT). Essentially, the FPT states that a façade undergoes no damage if its capacity to reject moisture by evaporation is greater than its capacity to absorb water. In other words, a wall will not have paint problems (such as algae and fungi, blistering and peeling, discolouration etc) if it can diffuse vapour faster than it can absorb moisture.
Acrylic paints have an Sd value of between 1-2. Meaning, vapour will have to travel through an equivalent value of 1-2 metres of air just to enter the atmosphere. Think of how tiring that must be if you’re teeny, tiny molecule. Breathe Silicate Hybrid Paint has an Sd value of 0.01m. Vapour has very little resistance passing through, making our paint extremely breathable.