This is a subject that i am really interested by and also a subject that seemingly is an impossible one for people to agree on. Some people swear by using 'microporous paints' and some don't, but it is an issue which needs a little more explanation to end up with the correct answer and i am probably opening up a can of worms by even broaching it, but here goes!
Micro-porous paint.
The term 'Microporous Paint ' was apparently adopted/invented by Dulux, and has become a huge marketing tool for many paint manufacturers. The term micro-porous, or breathable seems to indicate a paint that is somehow good for us, the wood or plaster, and also good for the environment. It is also a term which is very misleading.
What is the definition of Micro-porous?
Micro-porous materials are classified as materials which have a pore size of less than 2nm (nanometer) which will allow the free passage of water vapour..
Micro-porous paints therefore allow water vapour to pass through them, but do not allow water molecules to. They are therefore termed as 'breathable' by some manufacturers which is where part of the confusion begins.
There are an awful lot of materials which allow water vapour to pass through them. Even ones you would not think, but when it comes to paint - it seems that most paints you would use on your houses today would be classified as micro-porous. From Acrylics to oils the basic fact is, they all allow a certain amount of water vapour to pass through them in greater or lesser extents. Lead paint is defined by the SPAB, as breathable and even glass has a certain amount of water vapour permeability.
Some paint manufacturers label their paint as being 'microporous' and 'breathable' but this really seems like it is a bit of a con, if indeed most of the paints available have some degree of micro-porosity.
What we should really be asking ourselves is ' is our paint vapour open and by how much?
Breath ability:
The breath ability of paints is measured in an sd value. This is a calculation of the resistance of the paint (in this case) to water vapour movement through it. It takes into account the thickness of the paint to work this calculation out. An sd value of 0.0 means that there is no resistance to the passage of water vapour - and i guess this means that this would be thin air. The greater the sd value the less 'breathable' our paint.
What does this all mean for our paints?
For a paint to be classed as breathable it seems that it needs to have an sd value of around or below 0.1.
The substrates i am going to discuss below are mineral and timber, as metal and plastic, although used a lot in building materials, are less affected by water vapour diffusion.
- Mineral - plaster, renders, brick
- Timber - Internal & external joinery and woodwork
Mineral Substrates:
eg: Cement based, Lime Based and synthetic resins for the exterior. Lime based, Lime cements, Gypsums/Gypsum cements & Clay plasters for interiors.
All of these would benefit to varying extents from having a vapour permeable paint on them, but with the case of limes and clays it is essential to have as much as possible. Lime or clay plasters or renders allows moisture to pass into it and hold moisture, so it has to have a coating that is as vapour open as possible, and therefore a coating with high breath ability and the lowest sd value possible.
What is best?
For Lime render the best choices are really a Lime wash or a silicate mineral paint such as Keim, which both have sd values of 0.01 as has Keim Mineral paints. The Lime wash is breathable because it is basically water and lime with a little bit of pigment, and Keim is breathable because it is a water glass solution (silicate) and it chemically bonds to the substrate. Clay paints such as Earthborn , distempers and casein's are next with sds of 0.02 -0.04 followed by paints like Classidur Tradional 0.05 and Classidur Superclassic 0.04. Various Lime Paints like St Astier have 3% Acrylic content added to them to improve their performance, but they still retain a low sd value. It is also possible to get acrylic blend paints with sd value of around 0.1, so still within the realms of breath ability. The fact is that nowadays there are various breathable systems which we are using more and more of in our homes: plaster systems, insulation's, membranes and backer boards that we are installing to help with the passage of moisture and water vapour both inside and out of our houses, so breathable paints are becoming far more of an issue than they once were.
Timber Substrates:
On timber surfaces the issue of breath ability becomes a little bit more complicated. Paint is used to protect our timber from degrading. This happens in a number of ways, but the main elements that cause timber to fail are moisture and infection from either insect or a fungal attack.
Timber starts to rot once it has a moisture content above 21%, so the aim is to coat it in something that prevents this happening. We also need to stop various forms of parasite and fungal attack, moulds etc so our paint needs to contain certain things to stop this from happening.
One of the issues with timber is that it is hygroscopic so it expands and contracts with increase or decrease of moisture in the air so it has a constant transfer of water vapour in and out of it as Relative humidity rises and falls - so from this point of view a good open pore paint system sounds like a good idea.It also contains (depending the sort of timber used)differing levels of tannins, which are water soluble, which can leach out of the timber causing staining. Although this can be cleaned off with a good fungicidal wash, it can ruin your paint work. So a open pore paint from this perspective sounds like a bad idea! However some micro porous paints hold staining back - it is not the fact that the paint is or is not labelled as micro porous, it what it is made of that makes the difference.
Tannin staining
What is best ?
If you ask one of my Swedish suppliers whether micro porous paint should be used on exterior timber he would think you are totally mad! His view is that timber should be totally protected from any sort of moisture ingress - and i can see his point. If you take the case of linseed's - a classic Swedish coating for exterior woodwork which when applied correctly lasts for years. The oil floods the timber fibres with natural resins which not only feeds it, but protects it as well, chalking over time as the linseed wears, when all you do is apply another coat. Linseed however, is a polymer and therefore has very little breath ability eg: f you add more than 3% linseed to Lime wash, it increases its sd value to something more akin to a standard masonry paint.
Other alternatives range from 100% and high Acrylic paints like eicó, Mythic, or Little Greene. Acrylic content paints degrade rather than go brittle and crack like their oil based contemporaries so behave a bit more like the old lead paints, so are very good contenders for a hard wearing finish. Opaque stains both oil and water based by companies such as Osmo, Solignum, Sadolin or Owatrol, are available in both water and oil based versions (with the acrylics being more micro porous than the alkyd). These generally need only 2 coats, but these need to be put onto bare timber to really get the best out of them so to use them retrospectively is a waste of time. Higher spec still, you have the commercial grade acrylic Polyurethanes from companies such as Sigma and Jotun and which are superbly hard wearing. These are now becoming more readily available targeted at the domestic market, with products by ican and Bedec - so your list is endless.
A Conclusion of sorts
I guess the conclusion and an answer to the question 'what is best to use' is....... there isn't one!
The best paint to use is the most appropriate system and the one that suits your timber and condition best. It can be oil or water based, Opaque stain or linseed, 100% Acrylic, or natural oil, they are all micro porous to a degree. This may seem like a cop out, but as far as i can work out after having asked Chemists, Merchants, Suppliers, Technical departments and looked on the trusty old Internet, there is no decisive answer!
The truth is, unless you can get an sd value from your paint company, then the term 'microporous' is really nothing more than marketing terminology, to make you think you are doing yourself, the environment and your substrate more good than if you were to put on a paint that does not make the claim!
If you want any further information on this subject, or more information on some of the products listed, then do contact me at South Farm and i will do my best to help!
Paints and Interiors are stockists of eicó, Keim, ican, Osmo, Owatrol, Scala and Bedec paints.
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