Can interior latex paints perform as a vapor retarder?
Joe Lstiburek's Builder's Guide book series about residential construction promote the use of latex finish paints as the air barrier and a Class II semi-impermeable vapor retarder (perm rating between 1.0 and 0.1) allowing residential exterior walls to dry in both directions. When I first researched paints, as a result of Lstiburek's books, that provided any data to support their use as a vapor retarder, I could find none.
Recently the question was posed by a client firm researching some significant moisture problems in an occupied building. The interior paints were specified as a solvent-borne epoxy as the base bid and acrylic water-borne epoxy as the alternate bid. After bidding the owner selected the acrylic epoxy. Of course, now, perm rating is a primary consideration.
When the project was originally specified, solvent-borne epoxy paints were, by their chemistry, were expected to provide some vapor resistance. Although none of the paint manufacturers had tested the paint for permeance. Acrylic paints by their nature are breathable, allowing vapor to pass through the paint film. This is why acrylic elastomeric paints are a popular choice for coating concrete and masonry. The substrate is never "dry" when the paint is applied.
Latex paints are water based with a variety of resins. The most common resins in the construction market are acrylic and vinyl-acrylic blends. The acrylic component will continue to allow the paint film to breath and pass water vapor.
Styrene-butadiene rubber (SBR) resin will provide water vapor resistance. SBR is substituted for the typical, less expensive, acrylic or polyvinyl-acrylic resins used for most latex paints to provide vapor resistance. The resistance is usually in the 0.5 to 0.6 perm rating, although some only publish that it is less than 1 perm. The paints that are advertised with a perm rating are primers or combination primer/finish paints.
My concern about relying on paint as the vapor retarder is the continuity of the vapor retarder. Every receptacle outlet, wall switch, and other penetration disrupts the vapor retarder if the paint is serving that purpose. Sealing all these penetrations is difficult and not always effective.
The current trend is to move the vapor retarder to the exterior side of the exterior wall. The vapor retarder is combined with the moisture barrier and air barrier. Often all three functions are performed by a single material, simplifying the interfaces and helping ensure better continuity.