I purchased a cape cod style home in Raleigh, NC a few years ago that had half of the attic converted to a bonus room. The roughly 400sf room is conditioned but it is not comfortable in the winter and summer months. In the winter, the room is often 5 degrees cooler than the downstairs (one zone -thermostat downstairs).
I've sketched a section of the attic room envelope. The previous homeowner added a 3" blue rigid foam between the 2x6 rafters and then added a 2" rigid foam running continuously under the rafters. The 1/2" drywall is attached through the 2" rigid into the bottom of the rafters with long screws i assume. The knee wall has a faced R-13 batt and the 2x10 floor joists have an R-30 batt.
I went to the U.S. Department of Energy's website and they recommend sealing the backside of the wall with rigid foam and installing a draft blocker in the floor joists (I've attached a link/photo).
The thing is, won't sealing a rigid foam which acts as a vapor retarded to the backside of the knee wall trap moisture and cause problems? Also, the way the room was built, I have 1/2" plywood running under the knee wall and stopping roughly 2' from the outside perimeter wall. Rather than cutting out this plywood to install a draft blocker, can i just continue the plywood to the perimeter and seal the joints?