This is a great question concerning a very common problem.
To fully understand the solution you'll needto have a basic understanding of how the components of the roof structure work and contribute to theproblem.
Roof components and their dynamics
Concept 1: Heat always travels towards cold. The energy in the warmed air in your home is always tryingto leave your house. The greater the temperature differences between the indoors and outdoors thegreater the energy (heat) drive.
Concept 2: Moisture always travels with heat. Any moisture that is in the indoor air of your home willtravel with the heat energy towards cold. Both the heat and moisture are traveling convectively.
Concept 3: Dew Point Temperature. When the warm moist air comes into contact with a surface that iscold enough to allow the moisture to condense on the surface (dew point temperature) the moisturewill collect on the surface and eventually fall like raindrops.
- In your case the underside of the roofplywood is cold enough to allow the moisture to condense.
- The resulting moisture is absorbed into theplywood, insulation (making it less effective) and finally the ceiling drywall. If there is enough excessmoisture it might even drip from the ceiling.
- Micro condensation conditions start at around 15-degreesdifference in temperature. The warmer the air the more water it can carry.
Concept 4: Heat energy travels three ways:
- conduction (through a solid object),
- convection (in a liquid,air is a liquid) and
- radiation (with light rays).
In a cold-weather state the greatest type of heat loss isconvection. R-value is not an indicator of an insulations ability to stop convection. R-value only dealswith conduction.
So the R-value isn't nearly as important in this situation as the type of insulation.
- You'llwant to select an insulation that is resistant to moisture travel (migration).
- Fibrous materials (cellulose,fiberglass, mineral wool, etc.) are ineffective at stopping heat (and moisture) migration via convection.
Concept 5: Any structural cavity between outdoors and indoors must be constructed to reduce thepotential for moisture penetration yet be constructed to allow the cavity to dry out when moisture isintroduced.
In looking at your roof assembly, the Ice & Water shield is a water barrier. It was installed toprevent water from penetrating the roof assembly in the event that water bypassed the roof coverings.
- This product is a very effective barrier to moisture.
- While not installed as a vapor barrier it acts as onewhen the roof cavity ventilation does not remove excess heat and moisture that passes through theattic insulation.
- In your case moisture enters the attic but cannot escape the attic because the roofventilation (while adequate from a code perspective) is inadequate in performance.
- Since the Ice &Water shield is a moisture barrier vapor cannot penetrate the material from the attic side and evaporateto the outdoors.
- Visualize a terrarium with a glass top. The flat (ceiling) of the terrarium is always wetbecause the vapor that condensed on the cooler glass surface cannot escape to the outdoors.
Closed-cell insulation is the only consideration for the condition (and solutions) that youdescribed.
I would caution that I would not recommend spraying the closed-cell insulation directlyto the underside of the roof plywood. Doing so will create a different problem. Any moisture thatpenetrates the roof system could become trapped between the Ice & Water shield and the closed-cellfoam. This could result in structural damage that would take years to show up. In addition if you everhad to change any of the roof sheathing you would also have to re-insulate since the closed-cell wouldbe bonded to the plywood that is being removed.
The best option would be to install foam baffles onthe entire underside of the roof before installing closed-cell insulation.
- Conducted heat loss would thenhave an area to dissipate (ventilation) and any moisture penetration from through the roof assemblywould have a path to dry.
- Don't be fooled into thinking any fibrous insulation material is effective atsolving this problem in the long term.
Best of luck.