The word "microporous" is often used in marketing literature for building materials. The idea promoted is that microporous surfaces or coatings are impermeable to liquid water, but permeable to water vapour. In other words, they are supposed to keep out the rain, but still allow the building to dry out, or ‘breathe’. This might or might not be true, because the word is often used rather loosely, and manufacturers can be reluctant to produce evidence to support their claims.
The notion of "microporosity" stemmed from the need to control and reduce the buildup of moisture in wood joinery to below 21%, generally accepted as the point when timber starts to become susceptible to attack from wood-destroying organisms.
Before the advent of microporous coatings, alkyd paints were the main protection strategy for window joinery. However, despite being easy to clean and less prone to sticking or blocking when freshly painted, they became brittle with age, and progressively less responsive to dimensional movements in the woodwork. When this occurs, cracking of the paint film around joints is the usual result, with the consequence that rainwater can be drawn into the wood by capillary attraction at the point of fracture.
Once inside, the water has no effective means of being vented out since it cannot reemerge past the alkyd paint. The result is that moisture build-up within the fabric of the joinery occurs to a point where it can both trigger the development of rot and also interfere with the surface adhesion of the intact coating, causing its eventual delamination in the form of peeling and flaking – a scenario which is all too familiar in the context of restoration work.
Coatings manufacturers were quick to realise that one of the most effective ways of controlling moisture in joinery was through the formulation of paints that were more permeable and which could allow any entrapped moisture within the joinery a ready means of escape.
Permeability is the amount of water that can pass through a given area in a given amount of time and is usually expressed in terms of grams per square metre per day (g/m²/day). There are pitfalls in the use of practical guidance offered by permeability data insofar as the measurements may differ according to whether they have been calculated on the basis of water loss or moisture absorption. In practice, permeability may be influenced by the density and interaction of the substrate, as well as being significantly influenced by variations in temperature, humidity, the moisture gradient present in the joinery and the age of the coating.
As with many things there is a balance to be struck. If coating permeability is too high, it may lead to splitting and deformation of timber components. If it is too low, it may not allow moisture to escape readily and lead to decay. The real problem from the user’s standpoint is that no agreement currently exists on what the optimum figure for any given end use should be. Consequently, providing data on permeability becomes largely academic since there is little of practical value to guide the consumer.
Over the years there has been a tendency to use permeability as a marketing tool, referring to it in terms such as "open"," ventilating", "breathing" and "microporous" to promote sales. This has led to the misperception that micro-porous paints somehow constitute a specific category or generic class of coating. In reality it can be argued that most paints can be said to be "microporous" to a certain extent, being capable of allowing at least some moisture to pass through the cured coating.
It is also a common misconception that "microporous" paints can allow water to escape from painted wood but do not allow it to enter. In essence, if a coating is permeable it will allow moisture in vapour form to cross it in both directions, as dictated by the relative vapour pressure differential that exists on both sides of the paint film.
Good design
Much has been written and said about the issue of microporosity and the illusion still persists that it is in some way a "cure" for many of the problems that are a common feature of painted woodwork. Yet there are other issues to consider and measures to take here. For instance, the use of careful detailing and good design can also help, going a long way to excluding water uptake in the first place.
One can also make a case for using a permeable paint system as a measure to offset or delay the inevitable consequences of bad design. However, one can make an equally valid argument for using less permeable paint systems if, because of their greater properties of extensibility, they can effectively prevent moisture penetration and resist the onset of premature cracking – white lead paint gives that option. It’s ironic, however, that conservation work deals with the restoration of original building features irrespective of whether they work to the benefit of their long-term function.
Modern coating technology is able to offer the conservationist a range of alternative systems which are neither very permeable nor toxic to health. However, it is important to select coatings based on a logical treatment of the specific issues in a given project – and to take steps to avoid choosing a coating which may not be right for the job.
