CDX between drywall and vapor barrier–a problem?

A homeowner asked a building science forum whether it's a problem to put half-inch CDX plywood between drywall and a vapor barrier (a layer meant to control moisture movement through a wall). The worry was that sandwiching a solid material like plywood between two other layers might trap moisture, especially since drywall, even painted, still lets some vapor pass through.

An expert who answered explained that this setup isn't a concern. What matters for moisture control is where the vapor barrier sits in the wall, not what materials are layered inside of it. As long as the vapor barrier stays on or near the interior (warm) side of the wall, the plywood, drywall, and everything else on that same side can be treated the same as any other interior wall material. The real risk for condensation is when warm, moist indoor air reaches a surface that's cold enough to be below the dew point — the temperature at which water vapor turns to liquid. Since the vapor barrier, plywood, and drywall in this case are all on the warm side of the wall, none of them get cold enough for that to happen.

For homeowners planning similar wall assemblies during a renovation or new build, this is reassuring: adding a layer of plywood behind drywall, even with a vapor barrier already in place, doesn't create a hidden moisture trap, as long as the vapor barrier itself is positioned correctly within the wall.