I have a house built on piers. Originally it had a plywood skirt down to the ground to help stop the wind. The bottom of the skirt rotted away. I replaced the skirt with 0.5in. plexiglass 32in. below grade. Using 2X6 PT lumber at the base and 2X4 PT studs stopped the wall from being pushing in. I installed closable vents with a negative pressure fan to pull moisture out. I did not install a vapor barrier. Several yeas later I noticed some mold. I used two applications of vinegar and one of hydrogen peroxide do deal with that issue. I also removed the 3.5in. fiberglass batts from between the floor joists.
I am planing to put down a 6 mil. plastic vapor barrier and insulate the walls with 2in. NXPS Dow foam board. Reading reviews on your site I only see applications of foam board against concrete or concrete block walls. Also I just saw that you do not advise installing XPS foam board below grade. I was planing on taping the 6mil. plastic to the plexiglass 8-12in. up the wall and then wedging the foam board between the studs. I will need to apply a mold and mildew resistant dark paint to the foam board so it does not show through the plexiglass. I worry about moisture being trapped between the plrxiglass and the foam board and growing mold. At the end I will install 1-2 dehumidifier, space is longer then wide with a narrowing between two sections. Is this the wrong way to proceed?
The newest part of the house has 3.5in. fiberglass batts installed between the floor joists with 0.5in. foil faced foam board nailed to the bottom of the floor joists. I have not removed this insulation. Is this a problem waiting to rear it's ugly head?