What Exactly Does "Breathable" Mean?

To understand, it takes a little bit of chemistry and physics but the concept is fairly simple. Technically, for those of you who want to impress friends, the scientific term is vapor permeability, which is the ability of water vapor to permeate through a film.

Polymer films are composed of long chains of molecules that are bound together in a three-dimensional matrix. This matrix, in films like polyurethane and alkyd coatings, is quite dense and there is not much space between the polymer chains. These polymer chains prevent small water vapor molecules from making their way through the film. On the other hand, Lifeline finishes form a less dense matrix with room enough for water vapor molecules to penetrate through the film, but not enough space to allow liquid water to make its way into or through the film.

The water vapor transmission rate is measured to determine the extent of vapor permeability through a film. It can be thought of in terms of a film with many small holes- or pores- that filter out large objects but allow smaller objects to pass through (see illustrations below). These types of films are known as semi-permeable membranes, but they are often referred to as breathable films. Our Lifeline finish system, the image below on the right, is one such system.

In addition to the chemical composition, another factor that influences vapor permeability is the thickness of the film. The thicker the film, the more difficult it becomes for water vapor molecules to permeate the film. That is one reason we constantly remind people to apply our finishes in thin layers. When applied too thickly, even acrylic latex films cannot breathe or become a vapor barrier.

Why Breathability Is Important

Logs and timber that cannot breathe end up with trapped moisture inside the wood. This moisture can bring about mold, which eventually leads to rot, and moisture makes the wood inviting to wood-eating insects like termites. But by being protected by a breathable finish, logs and timbers avoid the conditions where mold grows and the wood attracts harmful insects.