LVP over decking in 3 season room

Hi all,

I have a client that has selected lifeproof LVP for his enclosed 3 season space on his cottage, an application that lifeproof claims is suitable. This enclosed space is part of his deck. My client has proposed putting 3/8" or 1/2" plywood over the 2x6 deck boards as a fresh subfloor and then installing the LVP on that.

This method would be fine for interior, however, I have concerns about moisture from underneath the deck getting trapped between the plywood and deck boards and causing problems. I have thought about attaching 2" extruded foam to the underside of the deck joists as an insulated layer and vapour barrier, but fear that this could be worse.

Thoughts?