Firstly, if you want a really fantastic set of articles to help “debunk” the minefield that is breathable interior paints – Patrick Batys blog is a great source of Technical Information and the best I have seen at explaining the issues. This article is a shorter less technical overview of the subject based on our own experiences;
We are asked a lot whether Farrow and Ball and other such fancy emulsion paints are suitable for lime plaster and other breathable surfaces. Put simply, Lime Plaster takes a long time to carbonate and fully dry out and an impermeable paint will restrict this process. At least during the drying out phase it is highly advantageous to have either no finish whatsoever, or a highly permeable one. This is why it was historically commonplace to provide “temporary” finishes such as distemper for a period prior to full decorations with oil based paints which were not breathable.
We now have an array of paints available many claiming to be breathable and suitable for old buildings….but are they all suitable and worth the extra cash ?
The most commonly adopted measure of vapour permeability is SD vale and it stands for steam diffusion or air layer equivalence. It is a measure of how much of a barrier a paint coating is to water vapour and how easily the vapour can pass through this barrier, it is measured in meters. The lower the SD Value means that more moisture is able to pass through, the higher the SD Value, the lower the moisture transfer, and less moisture can pass through. We can use this measurement to giver an overview of what may be breathable and what may not be.
If we take Limewash for example – this has an approximate SD value of 0.01 and is highly breathable and permeable. If we take a standard external masonry paint , this will be over 1. A permeable paint is generally considered as having an SD value of less that 0.05. Leyland , Dulux and other such internal paints do not publish their SD values and due being Vinyl Paints are not suitable and likely in a similar region of permeability to masonry paint..
Farrow and Ball Estate Emulsion – 0.01
Clay Paint 0.02
Earthbourne Eco Pro Emulsion 0.02
Earthbourne Silicate – 0.06
We can see from the table above that Claypaint and Limewash have low Sd values and are very permeable as we would expect. However although not marketed as “breathable” Farrow and Ball Estate Emulsion is suitably permeable, as is Earthbourne Eco Pro Emulsion, which is not marketed as breathable, but actually has the same SD value as clay paint.