Young, dumb and broke, we built the original part of our house (CZ6) on pressure treated piers over bare ground 35 years ago. The floor was insulated with R-19 fiberglass covered with OSB. A few years later we replaced the piers with a shallow concrete block perimeter wall. The exterior from footing to house sidewall was covered with an inch of poly iso protected by a layer of pressure treated plywood.

We covered the rough ground beneath the house with two layers of 6 mil plastic as best we could, but it is not sealed to the concrete block. Needless to say, the cold ground causes condensation on the plastic. We used to ventilate the crawlspace in summer, but now keep it sealed which seems dryer, but we still have condensation and a few black spots on the rather soft OSB.

I am looking for easy to implement solutions – there is only 18″ to 3′ headroom and lots of pipes and wires to work around. Could I add a layer of mineral wool on top of the plastic and add another layer of plastic sealed to the foundation wall? Mineral wool would be flexible enough to follow the contours (and cushion the rocks). Or am I better off removing the OSB and fiberglass and having the underside of the floors closed cell spray foamed? That doesn’t resolve the condensation, just adds a better barrier.

Are either of those a viable solution or do you have other suggestions?