If the bottom chord is buried in insulation, all cool and comfy and the top chords are up in the high heat and lower humidity... and then if you add some reaction wood that shrinks lengthwise as it dries into those top chords... truss uplift.I have heard that a truss can bow upward taking a non-bearing wall upward, but have never seen it, could be non-fiction. Could be a rumor started by a framer that dislikes truss construction?