Modern Breathable Masonry Paint vs Lime Wash

The world of ‘breathability’ as an absolute minefield with regards to false information and data manipulation, primarily due to the misrepresentation of SD values.

What is an SD Value? The Sd value is the resistance to movement of water vapour, when compared to the resistance of a meter of air. Complicated? Yes!

The industry standard for a breathable paint is any product with an SD value below 1.

That said, it isn’t until we look deeper, at the ‘g/m²/day’ (grams of vapour moisture able to pass through a surface coating measuring 1m2 in 24 hours) that the figures really evidence why many products sold to the consumer as breathable fall far short of what can be considered as truly breathable.

Standard acrylic masonry paint – 0.7 SD – 50g/m²/day

Microporous acrylic masonry paint – 0.5 SD – 180g/m²/day

Silicone masonry paint – 0.5 SD – 190g/m²/day

Earthgborn Ecopro Silicate paint – 0.06 SD – 1254g/m²/day

Beekosil Silicate paint – 0.01 SD – 1920g/m²/day

Keim Mineral paint – 0.01 SD – 1968g/m²/day

Limewash – 0.01 SD – 1968g/m²/day

To bring the above data into terms most of us will understand.. beer garden terms that is.. a standard ‘breathable’ masonry paint will let through a double shot every twenty-four-hours, where as lime wash will let through three and a half pints!

So now we take the above data and draw the conclusion that mineral and silicate paints share the same or similar vapour permeability and characteristics as a pure, natural lime wash, and sadly that would be a mistake as I have yet to witness or be able to evidence this.

In practice, the silicates and mineral paints would show to be more water repellent and harder wearing when compared to lime wash, this is evidenced by lime wash changing colour when wet (taking on moisture) and the sacrificial nature of lime wash requiring regular recoating.

However, the one huge red flag when it comes to the validity of the data provided for silicate and mineral paints is their tendency to peel and blister, which is typically caused by….  trapped moisture!

In summary, there will always be a trade off of water repellency vs vapour permeability. Severe exposure may require the necessity for a hard wearing silicate paint, however, in practice the traditional option of a natural lime wash will far outperform its modern equivalent regards breathability and its sacrificial nature, and is always our first choice in low to moderate exposure situations.