By Dustin Jepson, PermaTint Limited, Concord, ON, Canada
It is common knowledge that trapping moisture inside the walls of a straw bale structure is a recipe for disaster – for this reason, mineral based paints are the ideal choice.
Mineral based paints are completely different than standard acrylic/latex based paints. They do not stick to the surface and form a film that traps moisture inside the wall. Rather, they soak into the surface and bond to the inside through a chemical reaction (silicification). Instead of being a coating that is adhered to the wall, they become part of the wall itself. Because they are comprised of greater than 95% natural minerals, mineral based paints are completely breathable and will never trap moisture in the wall.
Key Advantages of Mineral paints:
- Natural, matte finish
- Inhibit the growth of mildew
- Environmentally friendly
- Zero VOC and non-toxic
When choosing a paint for your straw bale home/building, no characteristic is more important than breathability. Every paint product has a perm rating which measures the vapor permeability of the coating – typical acrylic/latex based coatings have a perm rating of 1-7. More advanced acrylic/latex paints that claim to be breathable reach 15-20 on the perm scale. Mineral based paints typically have a perm rating of 75 or higher – significantly higher than other coatings on the market.
The high breathability is due to the microcrystalline structure of mineral based paints – when viewed through a microscope, the coating appears to have millions of tiny holes. These holes are not large enough for a water drop to pass through, but are big enough for a vapor molecule to escape. Thus, mineral paints will prevent water ingress but will never trap moisture on the inside of the wall. These claims have been proven by test results from numerous manufacturers and is often referred to as micro-porosity.
During the summer months, a cementitious wall painted with standard acrylic/latex paints will heat up during the day. The heated exterior will attract moisture in the wall causing it to migrate to the surface. Upon hitting the back of the vapor impermeable coating the moisture will be trapped and will begin to apply pressure on the backside of the coating. As the pressure builds, water sacks or blisters will appear in the coating. Eventually they will burst and this leads to cracking/peeling and failure of the coating. This will never happen with mineral paints as they will not trap the moisture in the wall.
Durability was the underlying motivation for the invention of mineral based paints in the mid-late 1800s. After viewing the beautiful lime frescoes of Northern Italy, King Ludwig I. of Bavaria began applying similar paints to the buildings of his kingdom. With a much harsher winter season in Bavaria (Northern Alps) the paints quickly failed and he ordered his team of scientists to develop a paint that would last in their climate. Mineral paints were invented and their durability is proven by buildings in Europe that were painted in the 1890s and have never had any maintenance (The city hall in Schwyz, Switzerland and Gasthaus Weiber Adler in Stein am Rhein, Switzerland as well as others in Oslo and Germany). Since their invention, mineral based paints have been used widely across Europe and are understood to be the superior coating for exterior building surfaces. This knowledge has not transferred to North America as well where many builders/architects are unaware of their benefits.
Chemistry is at the heart of the incredible durability possessed by mineral based paints – while standard acrylic/latex paints rely on organic binders which decay over time, mineral paints bond through crystallization of the inorganic mineral potassium silicate. Rather than being a film on the surface that slowly dries out/cracks and peels, mineral paints soak into the surface and petrify within it – they can never peel.
Of further importance, is the high resistance to UV light – mineral paints do not rely on organic resins (as standard paints do) that dry out over prolonged exposure to UV light causing failure. The potassium silicate binder in mineral paints is exceptionally resistant to UV light and remains stable through decades of exposure. Furthermore, mineral paints are colored only with inorganic mineral pigments which do not fade in UV light either – the result is a paint coating that is incredibly durable and will never fade.
Mineral based paints promote a healthy building – they prevent water penetration from the exterior, but allow vapor to be released from the inside of the building. This promotes a dry building and greatly reduces the chance of mold/damp from taking hold of the inside of the walls. Furthermore, mineral paints are alkaline on the pH scale and this naturally inhibits the growth of algae or fungus on the exterior of the building. Lastly, mineral based paints have a very low thermal-plasticity – meaning they don’t get ‘sticky’ when heated up. Coatings with a high thermal-plasticity tend to attract a lot of dirt as it adheres quite easily – mineral paints are the opposite and keep the building clean and easy to wash.
Mineral paints are comprised of >95% natural minerals, all of which are highly abundant on Earth. They are sustainable products that are natural and completely non-toxic. In fact, leftovers can be left to harden and disposed of in the landfill safely – essentially all that is being thrown out is rock. Because they do not contain chemical solvents, mineral paints are zero VOC and pose no health risks to those occupying the building.
For use on new straw bale walls, it is generally recommended to apply 1 coat of mineral based primer (dilution) and 2 coats of mineral based paints. The primer is much different than a paint primer and it acts to equalize absorption across the surface and strengthen the render to lock up any loose particles.
Old surfaces that were previously coated with mineral paints should be lightly washed with a TSP to ensure the surface is chemically neutral, and then 1-2 coats of new mineral paint can be applied to change the color of the walls.
Disclosure: PermaTint is a sponsor of TLS and is not endorsed by the Journal. They are a steadfast supporter of our magazine and therefore deserve consideration during your choice of finishes. We encourage feedback on their products in the form of comments below.