There is a seam in our upstairs ceiling where the drywall tape is peeling up. I know I can repair this, but I am worried about the cause. It's just in one little section, maybe two feet long, right above this ceiling is the attic. Should I be looking into attic humidity levels? Is there a way to test that? Our attic has a vent on the side but no roof mounted vent. I just want to make sure I'm not covering up a potential problem by repairing the dry wall tape.
No need to read so much into a loose tape seam. It rarely indicates a serious problem. Ceilings in particular expand and contract along seams as well as intersections with walls, causing tape to loosen.
To fix this, you need to cut out the old tape with a utility knife, because this loose tape will never reseal. Next, apply fiberglass drywall tape, which is mesh and perforated. You can trowel and spackel across and through that. Finally, apply a drywall compound on top of the fiberglass tape to restore the seam. Once that's complete, you can prime and repaint.