Its time for new shingles. I have a split level with about 1/2 the home having a cathedral ceiling. The home was built in 1990. I assume there is polyethylene under the drywall. I want to add exterior foam insulation to the roof. What do I need to do to prevent moisture problems in the ceiling since the poly will prevent drying to the interior.

the short answer is that you need to put enough insulation above the sheathing to warm the space enough to prevent moisture from condensing. the thickness depends on your climate. When you add the foam insulation, it is critical that it be installed so no air can get past the foam, so the joints need to be taped and you may need to use caulking in places. Air infiltration into the foam layers will bring in cool air which will cool the sheathing which can lead to condensation & mold.

Bob

I understand that. If a person were building a wall new you would not want 2 vapor barriers in a wall. The old it needs to be able to dry in at least 1 direction, either to the inside or outside. The rigid foam will prevent drying to the exterior. I believe there is already poly to the interior which will prevent drying to the interior.

In these situations does anyone pull up a row of roof decking along the bottom My thought it to air seal the soffit vent intakes and to remove the bottom portion of poly. This would allow at least some drying to the interior. I think it would be beneficial to air seal all the seams of the roof deck.

EPS foam (well sealed of course) plus an air gap would allow drying to the exterior. The waterproof layer can be on top of that. Preventing condensation is an important goal, but go beyond that and also prevent conditions suitable for mold. Let any wall or ceiling breath in at least one direction.

With EPS you'd be limited to about 4-5" ( R16-R20) before it becomes too vapor tigtht, and you'd need to leave at least a 1.5" vented gap above it and the finish roofing with the waterproof underlayment. You need to strip the old roofing down to the bare sheathing, and put nothing less permeable than 15# felt between the decking & foam, and only use UNFACED EPS, not XPS, not polyiso. If you need more than R20, your best bet is rigid high density rock wool panels, which are very high-perm.

It's also important to fill the remaining air gap below the roof deck after you seal up the old venting, to prevent convection from concentrating what moisture DOES find it's way into the rafter bay at the cold end (typically the eaves.) This can be done fairly inexpensively drilling from the exterior and squirting in some cellulose. Density doesn't much matter, as long as you have a decent amount of R on the exterior, which reduces the seasonal humidity cycling within the fiber layer (the cause of settling in non-dense-packed cellulose.)