Hi Murph - I've installed 2 entire basements with Dricore so am quite familiar with it. The underside of the chipboard is a rubber/plastic compound that allows air to flow through all of the channels underneath the subfloor. When installed tightly against each other via the tongue and groove, the plastic parts push up against each other creating the vapor barrier. I am also willing to guess that the sealant they put on top of/into the chipboard also helps add to the vapor barrier effect. Feel free to PM me if you have any specific questions. I'll trade you for some grouting advice!
