Hi everyone! I live in SE NC. We have a vented roof and I am finishing an attic space. When the insulation was installed, I knew very little, and as a result some mistakes were made. First, I had them put r19 insulation in a 2x4 wall cavity. Second, there was no sheathing on the back side of the knee wall. So i got some 1/2" rmax and have finished backing the kneewall. I sealed every nook and cranny. I also stuffed insulation with garbage bags under the floor where the knee wall meets the roof joists. It has made a huge difference even without the drywall on yet. I have been working on this project 9 years and just have to move forward. Opinions on whether or not this will be sufficient? Any moisture issues? I just plan to add drywall with no barrier to the front. Thanks for weighing in.

By "roof joists" do you mean "rafters", or are you talking about where the floor joists go below the knee wall? With a vented roof you don't want to block air movement in the rafter bays, but you DO want to block air movement from the mini-attic space under the floor of the room. What do you have in the slanted roof area for insulation?

Plastic bags stuffed with insulation aren't likely to be an adequate air seal for the floor joists- it usually takes some half-inch rigid foam board foamed in place at the edges to truly seal them off. It can also be done with corrugated cardboard stapled then foam-sealed in place. You can leave the insulated bags in place to ensure that the insulation is fairly continuous.

The floor of the mini-attic is usually the ceiling of conditioned space below, which needs to be air sealed (any electrical boxes, plumbing stacks, flues, recessed lighting cans all need to be treated appropriately), then insulated to R49 (for the US climate zone 4 parts of NC) or R38 (the climate zone 3 parts of NC.) Using blown celluolose works better than low density fiberglass, since it's inherently more air retardent, and it's opaque to infra-red

http://www.greenbuildingadvisor.com/sites/default/files/images/DOE%20climate%20zone%20map.preview.jpg

http://publicecodes.cyberregs.com/icod/irc/2012/icod_irc_2012_11_sec002.htm

Compressing the R19s in place to 3.5" in a 2x4 studwall performs at R13, which is fine. (A low density R19 is basically a "fluffed" R13- it's the same weight per square foot.) Using half-inch foil faced polyiso is good- but air seal the seams with a high quality temperature rated foil tape (eg Nashua 324a, found in most box stores), and run a bead of can foam to seal the top & bottom edges to make it air tight.

The air tight drywall with no vapor barrier is the right thing to do for he interior side of the kneewall. Be sure to caulk the bottom plate of the kneewall framing to the subfloor, and air seal all electrical boxes & wiring penetrations. Since the foil faced polyiso on the back side is a true vapor barrier, be sure to use only latex/acrylic paints, no vinyl or foil wallpaper, no alkyd paints on the interior side or it'll be moisture trap. The risk of trapped moisture isn't high in a kneewall, but there's no need to take even that low risk.