Advantages and Disadvantages of Microporous Paint
Unprotected wood becomes easily susceptible to damage from organisms when moisture accumulates in the joints; a fact that prompted the concept of microporous paint. But is it really a better choice?
What it is
The term “microposity” is applied to a material that supplies a measure of water resistance. Pores of the material would have an average size of between 0.1 and 1.0 microns to be considered microporous. This feature is ideal for paint, both interior and exterior. Surface coatings such as paint can either adhere to the surface to which it is applied, or it can soak into it. In some cases, a coating may do both. For those which adhere to the surface, usually polymers, a skin like feature is formed that is less permeable than the surface it is applied to. It is important for coatings to be breathable; polymer breathability can be greatly enhanced by adding certain filler ingredients that develop a more porous coating.
How it works
Wood is generally a highly porous material. Unless a barrier is provided against moisture and humidity, wood will absorb the moisture and may swell with the excess. Organisms that seek this moist, dark environment can take up residence and begin to destroy the wood base. Before microporous paint was created, most coatings were made of alkyd resins, which were a reaction between polyesters and drying oil. These alkyds presented a strong, hard film that was easy to clean, resisted water and resisted sticking during the drying process. While this sounds perfect, there was a major drawback in that it became brittle as it aged; resulting in cracking and peeling around joints which allowed moisture to permeate the wood surface. Once situated within the wood, the moisture was trapped by the still viable alkyd paint. Wood rot soon set in, ruining the window, door or joist.
It was quickly realized that the joints would require a more effective means of protecting the wood through controlling the moisture. Paint that is more permeable would allow moisture that became trapped within to escape. This special paint was developed, and began to be marketed using catch phrases such as “breathable”, “ventilating” and “microporous”; appealing to the homeowner who saw this as a means of preserving the quality of their windows, doors and walls.
External surfaces of structures face the direst threat of the invasion of moisture and, as result, damaging organisms. Masonry, wood, soft brick and friable stone are all prone to decay when exposed to wind, rain, humidity and sun, and it can be argued that their high porosity requires a coating with a permeable nature. Interiors are also subject to humidity, but at a much lower threat with the exception of doors and windows that are exposed to both conditions.
While it is true that a microporous nature allows trapped moisture to escape, it must also be considered that the reverse, then, must be true; it would allow moisture to enter within. This presents a special dilemma for the homeowner. Is it better to invest in a surface coating that is water resistant, and accept the fact that while it keeps moisture out, it can also prevent trapped moisture from escaping? Or would the breathability of the coating be a better choice, even though this same feature could allow moisture in the form of vapor to enter in?
There are no clear answers as to the advantages of microporous paint over alkyd paint. It is important for homeowners to understand the properties and drawbacks of each type, and to consider those against the type of application planned. Disregarding marketing hypes, the real decision should be based on the common sense of the application.