Inexplicably, some well-built vaulted ceilings, complete with vents and fiberglass insulation, have moisture problems. The drywall becomes stained, the insulation becomes damp and the roof wood can even begin to rot.
Foam insulation is an excellent, although expensive solution. But it’s not a DIY project. There are two types available: “open” and “closed” cell. The terms refer to whether the foam bubbles burst during curing, making the foam soft like a sponge, or remain intact and firm, like those in the rigid foam panels at home centers.
Closed-cell foam in a vaulted ceiling offers substantial advantages over open-cell. First, closed-cell foam has a 60 percent higher R-value per inch than open-cell (6.3 vs. 3.9). The higher R-value reduces condensation when moist interior air hits the now “less-cold” ceiling (vice versa in summer with humid outside air and A/C inside). Second, since the closed cells in the foam prevent air movement and moisture absorption, the foam acts as both an air barrier and a vapor diffusion retarder (the new name for a vapor barrier). Open-cell foam restricts air movement, but it doesn’t prevent vapor diffusion. You have to install a separate vapor diffusion retarder. Even then, if moisture gets past it, the open-cell material will hold it—not a good thing. Finally, closed-cell foam is more rigid, so it actually strengthens the roof deck. That added strength restricts flexing from snow loads—the kind of flexing that creates leak points. Closed-cell foam costs 40 percent more than open-cell.
But know this. Once water gets into a foam-insulated space, no matter what type of foam, it doesn’t leave, and rot happens quickly. So your shingled roof has to be in good shape and well maintained.