It is axiomatic that water in and around our built environment needs to be controlled and the architectural profession and the various building trades have developed many clever and elegant ways to exert this control.
One of the ancillary controls at their disposal is through the use of coatings.
Coatings are primarily mixtures of film formers (polymers, oils, waxes, resins and their like) and greater or lesser amounts of pigments. It is the film formers that provide the greatest resistance to moisture transmission and as a general rule, micron for micron, clear films give greater restriction to moisture movement than pigmented films.
Further the nature of the film former plays an important role with certain polymers such as PVA allowing much freer passage of moisture than polymers such as chlorinated rubber, bitumens or waxes.
The term moisture has been used as an attempt to differentiate behaviours of water as a vapour and as a liquid (this subject was covered in detail in Architects Memo No. 44).
It is inappropriate to think of coatings as waterproofers (although a lot of coating literature, including our own, would gainsay this); it is more appropriate to consider coatings as a useful method of reducing the flow of moisture to levels that create no nuisance or danger.
Most well-designed, resin-rich coatings should be able to achieve this level of control on porous but relatively continuous substrates such as brick, fibre-cement sheeting, solid plasters etc. The greatest danger that can exist with such systems is the presence of gaps in the film due to 'holidays' or occluded air bubbles. (Paint chemists often cringe when their carefully de-aerated paints are shaken over-vigorously - an air bubble is like a gateway to water). This is one of the most powerful reasons for applying paints in multi-coats - it is highly unlikely that such flaws will occur at exactly the same spot on subsequent applications.
Textured substrates, such as many plaster finishes and stucco, are difficult to cover completely with standard low-build paints and the use of high-build coatings (applied at lower spreading rates) can more easily fill the occlusions in such surfaces. Without all of the occlusions being filled, conduits will exist in the surface that will allow the relatively free flow of water.
Other substrates, such as concrete blocks, have quite large pores and channels running through them. Control of water with typical coatings can only be achieved if all of these pores and channels are completely filled with multiple coats of high-build coatings.
There are, of course, areas of controversy. One such area is whether elastomeric coatings are required or is flexibility enough. It is true that the best elastomeric coatings can maintain an intact film even over a propagating crack. There is a restriction however - for a film to remain intact over a crack that propagates to 1mm, 2mm (2000 microns) of coating are required - nice business for us paint manufacturers! Elastomeric coatings do have a role. They are at their best over structures that are permanently flexed such as concrete road bridges (to reduce carbonation) or buildings close to heavy traffic roads that could also be subject to flexing. There are generally some compromises in long-term durability with elastomeric compared to non-extensible but flexible coatings.
A greater area of controversy is that high build 'membrane' coatings, although they are designed to keep water at bay, will hold the water inside a structure by those very same mechanisms should water breach the defenses through cracks, faulty flashings etc. One school of thought suggests that open porous films are a safer option as these will allow the free passage of moisture from inside out.
The only definitive study on this (JOCCA 2001) was done in Iceland and showed that structures painted with porous paints stayed wetter for longer than identical structures painted with high-build elastomers. It was considered that porous films sucked water into the structure through capillary forces. However, as the structures were solid concrete and did not contain cavities, it could not be said to duplicate timber framed structures.
It is also likely that, except in the heaviest driving rain conditions, hydrophobically-modified porous coatings will not draw water through them as hydrophylic coatings will.
In summary, it is clear that coatings have an important role in the control of water ingress into buildings but they must always be seen as an adjunct to more permanent claddings. The design of the coating and the mode of control it exerts needs to be sympathetic to the type of cladding chosen.
The Resene architect's memo section provides technical information on a variety of topics relating to paints, finishes and coatings.