I am installing engineered hardwood flooring over concrete slab below grade in the basement. The flooring installation instructions mention that if using 6-mil poly underlayment, it should be lapped up the wall, but not touching sheetrock. However, I'm using a 2-in-one underlayment (vapor barrier as well as a bit of foam cushioning), and the instructions for that don't mention lapping, they say to just cut to the size of the room. Some other random sources have mentioned lapping 6-mil up so far that it's hidden by the base molding (so it's definitely touching sheetrock).
What's the deal? From what I understand, all of the barriers are meant to block moisture coming from below. Lapping makes sense for creating a more complete "seal", but it makes sense that if it touches sheetrock, then condensation could travel from the barrier to the drywall. The instructions I've seen that say not to lap are implying that rather than blocking the moisture at the edges, this instead gives it some room to evaporate.
Does anybody have a definitive answer?