Those familiar with Joe Lstiburek appreciate how he gets right to the point with short, declarative sentences: "Lots of attics insulated with open-cell low density spray foam are having problems—in hot humid climates, mixed humid climates, and cold climates. The problems are moisture related. The attics are 'unvented'— open-cell, low-density spray foam is installed directly on the underside of roof sheathing. The attics are humid. Very humid. Unacceptably humid ... "
In this recent article, Lstiburek gives us a thorough understanding of the problem (that's where the "ping pong" part comes in), layering on lots of complicated vocabulary (terms familiar to a chemical engineer), but in the end, Lstiburek's perfectly clear: "From my perspective the explanation is less important than solving the problem," and the solution is super-simple: "It is ok to use open-cell low-density spray foam in conditioned attics … but the attics need to be conditioned." Conditioned. Not by leaky ductwork, but intentionally brought inside the building envelope.