Can I drywall over fiberglass batts touching the roof decking in my attic?

Slapping drywall directly over fiberglass that's touching the roof decking is a recipe for rot in an unvented assembly. Since your soffits are alredy foamed shut, you’ve essentially committed to a 'hot roof' design, but fiberglass alone doesn't provide the air seal or the thermal break you need. Without a layer of **rigid foam** or closed-cell spray foam against that wood, warm interior air will migrate thru the batts and condense on the underside of the cold roof deck during the winter months. The most effective way to handle this now is the 'cut and cobble' method. You’ll want to cut pieces of rigid foam board—XPS or polyiso—to fit snugly between your rafters against the decking. Leave about a half-inch gap around the perimeter of each piece and fill that gap with canned spray foam to create a true airtight seal. This moves the condensing surface from the wood roof deck to the face of the foam board, wich can handle the moisture much better. Once that foam board is sealed in place, you can put your fiberglass batts back in and then hang your drywall. But you also have to address those gable vents. If you've got furnace registers running up there now, that space is fully conditioned. Leaving the gable vents open is basially like leaving a window cracked while the heat is on. Seal those vents up tight from the inside with more foam board and spray foam so your insulation envelope is continuous from the walls up to the peak.

Your local climate zone dictates how thick that rigid foam needs to be to keep the decking above the dew point. If you're in a northern climate, you'll want at least two inches of foam before the fiberglass goes in. Also, make sure you aren't sandwiching the fiberglass between two vapor barriers. If your batts have a kraft face, you might want to slash the paper if you're worried about trapping moisture between the drywall and the new rigid foam layer.

That cut and cobble method sounds like a lot of work but definatly better than a rotting roof. One thing I'm stuck on is how to handle the electrical wiring running trough my rafters. If I'm jamming rigid foam in there, do I just notch it out around the cables? I'm worried about gaps letting air through right where the wires are.

Also, since I've got the furnace registers pumping air up there now, do I need to worry about the humidity levels? shoud I be running a dehumidifier during the winter so the moisture from our breathing and stuff doesn't mess with the foam seal?

For the wiring, just notch the foam roughly and use a can of spray foam to fill the voids. It doesn't have to be pretty, just airtight.

Since you've got registers up there now, you're basicly living in that space. Check your rafter depth. If they're standard 2x6s, you'll have a hard time hitting the R-value code requires for a ceiling once you lose space to that rigid foam. I'd think about furring out the rafters with 2x2s. It gives you way more room for thicker insulation and makes the electrical runs alot easier to manage without compressing your batts.

Don't overlook the drywall itself while you're at it. If you aren't sealing the drywall to the top plates and using gaskets on your electrical boxes, you're still going to get air bypass. Look up the airtight drywall approach. Also, check your recessed lights if you're putting any in that ceiling. If they aren't airtight and IC-rated, they'll leak warm air right into the rafters. I had a buddy ignore that and he ended up with moisture stains on his brand new ceiling within one winter.

Don't forget about thermal bridging through the rafters themselves. Even with the cut and cobble, those wood studs act as heat highways straight to the roof deck. If you’ve got the headroom, I’d suggest putting a continuous 1-inch layer of polyiso over the face of the rafters before the drywall goes up. It breaks that bridge and makes the whole assembly way more efficient. Also, make sure your shingles are in top shape before you close this up. Once you seal the underside of the deck, any small leak from the outside won't show up until the wood is completely mush becuase it can't dry out anymore.