john healthyhouse

Also, remember that a moisture barrier can be one that breathes, like a house wrap (those pink and yellow ones, like tyvek); they keep out liquid water. The vapor barriers do not breath but their seams must be sealed properly.

Kaiya, one of the best available sheeting products these days is the professionally installed radon blanket material. I do not know where to get it, I have a contractor who installs it. It has hardware and adhesive tape for installation to seal it completely. It acts as a true air barrier - to keep radon gas out of course.

Terry, thanks for the welcome! I just found this group hours ago and am already intrigued enough to be involved. I too do IAQ/IEQ for many years, along with other issues with enviro health and sustainability.
For the house in question, we both still need to know what is under the floor, and why the bottom-most layer is OSB. And what the owner/builder's goals are- to use all "natural", is he willing to use plastics and other petroleum-based materials (like urethane foams, polystyrene, etc.) and what about insulation?

I am interested in your soil floor options. They are new to me.

I am not certain I agree with all completely, but it's because we can have different options at each stage for each assembly.
The real issue on the barrier location is not where the humid/wet air is, it's the surface temp of the dividing material - in other words, is it going to be at dew point (condensation occurs). The WUFI is a great resource for design professionals; I've found I've never needed it in my work of investigation, assessment, and corrections. For large complex projects it's valuable at the start of design to avoid errors.
I am a fan of the treated space. However I am not opposed to well-ventilated crawls, but usually they are not. Nor are they insulated correctly. Nor sealed against moisture intrusion. So, it's easier to design as if it's treated space instead of properly vented. (Aside: why is venting and air movement so difficult? even for roofers, insulators, hvac, and many designers?)
If one looks only at the flow of water (in all forms, vapor, liquid, solid), and starts there, it is possible to prevent both water damage and mold growth.
The number of 30% RH is not a correct number for cause accelerated mold growth. I am not sure where it originated but I do know it's been out there a long time. Many indoor environments have much higher, up to 60% and never have mold growth problems. Again, one must ask where the water ends up? And what direction it's flowing (including mass movement in air).
I hope we can get other contributions to this discussion!

Kaiya, what is under this floor assembly? dirt? gravel? concrete slab? And how much space between the bottom floor layer and the ground, is it an actual crawl space? The crawl space conditions are what is most important, more than the floor materials. The OSB as a bottom layer closest to the enclosed space (moisture) will not last very long without water damage and/or mold growth (also considered damage). You do not want moisture to 'find it's way out' thru your floor assembly into your living space unless you mean trapped water such as from a leak or flood from above (broken pipe, etc.). Your moisture and vapor barrier should be the bottom-most layer, no matter what. The floor assembly should not have a vapor barrier in the middle, since one day it will likely trap moisture/water/vapor on the wrong side. Water vapor should rise or evaporate from the bottom of the floor assembly up into the house. Vapor will not typically migrate down into the subfloor space (aka crawl space) because that space is usually wetter (higher humidity) than the living space (the direction of vapor pressure will go toward the living space). Wet goes to dry. So, ventilate the crawl or put a vapor and moisture barrier like plastic directly on the ground before you build your floor above it. Typical accepted concept is to treat the crawl like a living space, so ventilation is not usual anymore, but sealing that soil is required, as is insulating the crawl space walls, floor and foundation. then you need no vapor barrier in your floor assembly.