Breathable Masonry Paint

The topic of Breathable Masonry Paint is probably one of the most common questions that I'm asked about. To understand it, you have to delve into the meaning of 'breathable' and then understand how this relates to allowing water into or out of a building. 

Breathable Masonry paint is almost an oxymoron. Manufacturers sell paint as 'protective, waterproof, seal your walls, weatherproof' - and yet they tell you that it is breathable. Oh dear. Its not really.

I'm going to assume that we are all talking about one thing here - water. The trouble is, water exists as several states. Ice, Liquid, and Gas. It's all the same stuff really, made up of multiples of Hydrogen and Oxygen atoms. Specifically, two hydrogen atoms to every oxygen atom. They are bonded together to form what everyone knows as H2O.  Water molecules are quite small when they are a gas - a bit like naughty kids really - they are small, move very rapidly, are very high energy, and can get into all sorts of very small spaces and hide. But.. they don't do a lot of damage, and we can all live with them. 

Enter 'breathability'.

H2O as a gas can get through most things - wood, stone, brick, cement, some paints - provided that these materials have even a very small pore sizing - and we are talking VERY small - at an atomic level. So long as water is a gas, it isn't a problem - it's not wet, it doesn't cause rot, damp walls, mould or anything else really - it's fine.  So - for something to be breathable, it has to allow water as a gas to pass through it. OK - that's not rocket science - so lets take paint - if you make some paint, and smear a thin film of it over a surface, the odds are that there will be micro holes and bubbles in it that allow gaseous water to pass through it unhindered. So what happens if we smear another coat of it over the first - less holes, less space, and less H2O gas can get through. Another coat of paint, and the odds are that nothing will get through. So breathability is something that can only be defined in very scientific terms - if someone says 'My Paint is Breathable' - either he is telling a lie, or the paint has been tested in a lab as a very thin coating in a test rig, probably around a micron or less - a micron is a thousandth of a millimetre. When I make cider for example, we filter it down to 'less than 3 micron' using a cartridge filter to take out any remaining yeast cells.  So - these paints are applied in practise, in coats up to a millimetre thick.  I think you can get the picture that they are not going to allow a lot through them at normal thicknesses, and certainly not when they are things like 'sandtex masonry paint', loaded with lumps of sand or gravel. Most paints are plastic coatings of one form or another, and plastic isn't that good at leaking unless it's got a darned great crack in it..

This applies equally to PVA coatings, or silicone chemicals - things like stormguard / stormdry, thompsons water seal and so on - all these things you paint on and which claim to waterproof the wall, probably do waterproof it to a degree, for a while - but they certainly won't let water back out again and in so doing, they are going to damage brickwork and stone. Remember the damp industry and all those 'injection damp proofing chemicals' that magically created a damp course that never worked? Walls just got wetter, and flaked, spalled and disintegrated. Same chemicals.

The Elephant in the Room:

Temperature.  Water as a gas will move through some small holes. The bonds between hydrogen atoms and oxygen are very strong, and as the energy of the molecule drops (the gas cools down and the naughty kids get exhausted) the powerful bonds begin to take over - individual gas molecules clump together to form liquid water. Now, we've multiplied the size of the animal from something that will easily pass through tiny gaps in a plastic film, to something that has no chance. It's condensed - the wall got cold, dropped below dew point, and the gas became a liquid. Now it cannot get out, and the wall gets wet. This is why paint peels - water is trapped, behind it - and tries to get out, pushing the paint off the wall, or timber. What we need in ANY building, is a paint that will perform two functions - first, it physically stops moisture from getting into cracks for example - and second, that is magically able to allow liquid water to evaporate out of the wall via either capillary action, or as we've seen above, part water, part gas transferring through the pore structure of the wall. Plastic paints of any description will not allow this water as a liquid to pass through, and the wall gets wet. Its one of the causes of so called 'rising damp' where water condenses in the wall and is trapped. 

Is there a solution?

Yes - its been there for centuries, because, amazingly - it works. Its called limewash!

There are a variety of related terms that are used to quantify breathability:

Vapour Permeability or Resistivity.  Vapour permeability is the ability of a material to allow water vapour to pass through it. Here the water is in gaseous form (vapour), and passes through material at a rate that is determined by the pore size and thickness of the material. Vapour permeability is referred to using the symbol r. This is a material property and is not dependent upon size, thickness or shape of the material. The units of r are either GNs/kgm (Giga Newton seconds per kilogram metre) or MNs/gm (Mega Newton seconds per gram metre).

Vapour Resistance.  Vapour resistance is a term mainly used in the construction industry. This is vapour permeability (r) multiplied by the thickness (in metres) and has units of GNs/kg or MNs/g. A material can therefore have a high r value, but a low G value if it is only thin (such as paint).

Water Vapour Diffusion Current Density.  The water vapour diffusion current density indicates how many grams of water vapour can diffuse per square metre area in one day through a paint layer. This can be calculated from the Sd value.

Water Vapour Resistance Factor (µ) This is a measure of resistivity as a ratio of the resistivity of still air. µ is a relative quantity, and hence is expressed as just a number with no units. This is a property of the bulk material and is not dependent upon size, thickness or shape. The lower the µ value, the more ‘breathable’ the material.

Equivalent Air Thickness (sd) This is a useful measure of breathability as it measures a materials resistance to transmission of water vapour through it. This can only be quoted for a particular thickness of material and is measured in metres. The units of Sd are given in metres and it refers to the resistance shown by the equivalent thickness of air, so the lower the number the better the breathability. Paints with Sd values below 0.1m are considered to have high breathability. The diffusion equivalent air layer thickness specifies how thick a layer of air with the same diffusion resistance would have to be. In practise, these Sd values in the lower range are NOT very breathable at all.

Water Absorption Co-efficient, W, Kg/m2.h0.5. Capillarity refers to the absorption/ desorption of water as liquid. Capillarity is a function of pore structure. It can be altered by coatings and additives and many of these act as hydrophobic agents by blocking these larger pores, but still allowing the smaller pores to remain open. In this way the pore structure may be kept open for hygroscopic and vapour permeable transfer of moisture but closed to capillary transfer of moisture.  Capillarity is measured by placing a standard cube of material in water, with all sides sealed except the bottom. The weight of the material is then measured from time to time and this is expressed as a co-efficient w kg/m2.h0.5.  W is used to measure the amount of liquid water absorbed by a material by direct contact. 

All these terms do, in reality, is provide a fertile breeding ground for paint manufacturers to play with numbers and confuse the public. Test methodologies vary, Standards are deliberately vague, with the result that it is impossible to compare apples with apples. We asked B&Q to tell us how their 'breathable masonry paint' was tested and certified. After weeks of back and forth emails and phone calls, they gave up and admitted they could not guarantee any breathability.


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