Some houses in cold or high humidity climates have had moisture problems, where the OSB roofing absorbed moisture above installed spray foam and rotted out. As a result, IECC climate zones 5 and higher building codes require the use of a vapor retarder if open-cell spray foam is used. A number of builders in cold climates use closed-cell foam instead because of its lower water vapor permeability, which means it doesn’t need the extra step of installing a vapor retarder. However, closed cell is much higher priced than open cell.
But where does the moisture come from? One thought was that it comes from wet weather on the outside migrating inward. However, tests have shown there was no measurable effect of roofing underlayment permeability on inward moisture driving through the roofing assembly. Vapor impermeable closed-cell foam does not suffer the same fate as open–cell, so it makes sense that moisture from above wouldn’t be the culprit.
Another possibility is that the moisture is infiltrating into the attic from outdoors via leakage. Roofs that are not sealed during construction can have many rim air gaps and in a cold or humid environment absorb outside air and moisture that wicks between the OSB and foam. It is difficult to place insulation at the roof rim were these gaps can exist. The biggest benefit of spray foam insulation is its air-sealing quality, so if air is infiltrating into a spray-foam attic, then the installers missed some spots.
Another hypothesis is that moisture from indoors can diffuse through the foam and find the cold roof sheathing, where it accumulates and eventually rots the roof. This is possible in a humid or cold environment as the warm or humid conditioned air will rise into the attic and again leak through the foam until it saturates the cold OSB material. Of course if the home is being heated, or the air conditioning system was correctly sized and the duct system sealed neither of these problems should exist as humidity should not exist. However, if the home is cold and damp and showering creates a lot of moisture that can rise into the attic the result could be a rotting roof.
The conclusion from a number of sources is that moisture generated indoors or that infiltrates into the home or attic is responsible for the bulk of the moisture in an attic insulated with spray foam on the underside of the roof sheathing. It’s not coming from above the roof and it’s not some new moisture source resulting from the spray foam. So what do you do to make sure your roof insulated with open-cell spray foam won’t rot? Deal with the air in the attic.
Yes, some homes with open-cell spray foam have had moisture problems. Many more have performed perfectly well. It should be noted that open-cell foam is said to be vapor permeable up to 3 to 4 inches in depth and most roof insulation specifications require an insulation level of between R38 and R49 which equates to between 11 and 15 inches in depth. So is this really a problem, or just poor installation?