I am working on a wall design for a super efficient home (near Passive House standard). The double stud wall with cellulose insulation, and a fibreboard exterior sheathing (no polyethylene vapour barrier on the interior) will- in places- have a stone veneer as the exterior cladding. Because we live on the west coast of British Columbia (similar climate to Seattle), and because some of the walls are south and west facing, I am concerned about inward solar vapour drive.
So, I am wondering the following: In such a wall assembly, where one has a brick or stone veneer attached to either cement board or hard-panel, which is itself placed over a vented rainscreen gap, could all inward solar vapour drive be prevented by placing a layer of 6 mil poly ON TOP of their rain screen directly behind their cement board backing for the stone veneer? I am wondering if the stone or cement board itself might over time be compromised by losing its ability to dry inwards into the rainscreen cavity, but do want to retain the high vapour permeability of the framed wall itself, hence the fibreboard sheathing. Just wondering if anybody has any experience with stone/cementboard/hardi-panel being moisture saturated for too long, or if this poly on top of the rainscreen may in fact be a good preventative measure for inward vapour drive. Thanks in advance for any thoughts and advice!