If closed cell foam is considered to be a vapor retarder, wouldn it trap moisture inside the exterior wall assembly and create wood rot?
Closed cell spray foams are used widely as a roofing system. A roofing system by design is meant to keep water out and the interior dry. Closed cell foam applied to the underside of roof decks keep the roof deck from moving due to temperature changes which result when the roofing materials on that deck are put under stress. Because the foam adheres to everything very tightly, the roof moves less and the potential for nail loosening and popping and roof deck separation and leakage is less. Should a roofing system fail due to poor workmanship or in some other manner, the closed cell foam beneath that deck will localize the leak. In other words, without the foam, the leak could be somewhere near the peak and migrate through more porous insulation materials to some other point where the natural path of least resistance will allow the water to move until it collects causing damage to roofing and roof deck materials or leak into the structure itself, creating more damage. Foam under a failed
