Hi all,
We’re in Zone 5, Northern California mountains, with very dry summers and variable-humidity winters (rain/snow raises humidity).

We have a dirt crawlspace which we plan to put a vapor barrier plastic, but not completely encapsulated.  It will have automatic passive vents that close at 40 and open at 70, and will not have power-venting.

We are going to install engineered hardwood flooring, and the non-antimicrobial Quietwalk from Green Building Supply, which is promoted as a breathable vapor barrier.

Will it impede breathability to put Rmax Thermasheath (polyiso sheet insulation) in between the joists?  Is there any risk with doing that?

The subfloor is 1959 shiplap, which we’ll be leveling with plywood and wood shims, so there will be 2 layers of wood between the quietwalk and the polyiso.

Is there enough breathability between the sheet insulation and the Quietwalk?

The house has a ductless heat pump which provides both AC and heat (it’s a small house).  

Also, for the vapor barrier we’ll be installing on the dirt crawl space floor, is it best to attach that to the concrete foundation walls?  And when wrapping the posts (which are wood on top of concrete), is it best to keep it to the short concrete piers, or can the crawl space vapor barrier go up to the wood on the posts?

Thanks!