Q. Will Open-Cell Foam Cause Moisture Problems?
A. It’s important to understand that neither open nor closed-cell foam will result in moisture problems in common building assemblies if the insulation system is designed and installed correctly (see Vapor Barrier Requirements in Question 17). The code recognizes both open and closed-cell foam as meeting the requirements of a true air-barrier. Since most vapor is transported with air-flow, an air-barrier will eliminate the vast majority of vapor movement. However, in conjunction with this principal, it is essential that adequate R-value or thermal resistance be achieved to prevent a dew point and condensation on the surface of the foam. This required R-value is discussed in the latest supplemental code criteria from the 2006 IRC. The research shows that an R-20 of open or closed-cell foam in a roof deck is necessary to prevent a condensing surface in our climate zone (under normal operating conditions).
Since open-cell foam is more water permeable than closed-cell, many people incorrectly assume that open-cell foam will result in moisture problems. Open-cell foam has been successfully used for over two decades with nearly a perfect track record. While open-cell foam is not appropriate for all applications, in general it provides excellent cavity insulation in most conditions and climates. It is simply a fallacy that open-cell foam will wick moisture out of the air or will wick water like a sponge. Water will only penetrate through the foam with pressure (i.e. submersion, flooding, gravity, sub-grade hygric pressure, etc). In fact, in 96 hour water submersion tests, 0.5lb open-cell foam was found to have a water absorption rate of approximately 25% – no more than typical plywood. Further, 0.8lb foam has a water absorption rate of 2.53%, below that of plywood and OSB. Can you imagine the water absorption of fiberglass batts! Many building science experts agree that the water absorption of insulation is irrelevant. In fact, many argue that the water absorption and distribution characteristics of insulation can be beneficial to the structure.
Joseph Lstibureck, Phd, considered one of the foremost building science experts, writes in a letter we obtained from him: “Where spray foam is applied directly to the cavity side of exterior sheathing, the water absorption of the spray foam is irrelevant…In fact, it could be argued that just the opposite characteristic is desirable in this location – some water absorption is in fact beneficial as it allows the material to act as a “hygric” buffer that allows some liquid phase water redistribution. For example, in roofing applications, wood roof decks traditionally outperform steel decks from a condensation control perspective for this reason. The key material and system characteristics for a cavity insulation are convection suppression, thermal resistance, air impermeability and vapor resistance – not water absorption.” Joseph goes on to write that water absorption only becomes relevant where foam will come into contact with concrete and masonry.
“Unvented attics require water/moisture control.
Water moves in (or out) of buildings three
main ways. The greatest amount of
moisture is moved as bulk water
(rain or any kind of water flow). Less
moisture is moved by moist air such
as with infiltration. The least amount
of moisture is moved by moisture
migration through materials.”
2006 IRC Commentary