A neighbor wants to get rid of the constant musty smell in his downstairs (especially during rainy periods). The smell is especially noticeable near the one smooth concrete wall that has dirt behind it (8ft high) and no sheetrock on the inside. The other walls have sheetrock (with an air gap) and much less dirt behind them (from 0 to 4ft high). He coated this wall with sealant, which helped only a little. It would be too much effort to excavate on the exterior of this wall, so the question is what to do on the inside of this concrete wall.

I suggested rigid foam board (XPS such as Foamular or Polyiso) glued directly onto the interior wall followed by furring strips or metal hat channel (creating ~1″ air gap) and then moisture resistant sheetrock (option 1). The stairwell runs against this wall and we don’t want to make it much narrower, so we will probably use 1/2″ foam board and 3/8″ sheetrock. It’s Texas, so insulation from the cold is not of primary importance.

The builder doing some other renovation on his house has a bunch of Delta MS dimpled drainage membrane from another job and suggested putting this on the wall first. It sounds like a good solution to keep moisture out. So I was thinking put this on first, then the foam board directly on top of the Delta MS membrane, then air gap and sheetrock (option 2). But the Delta service rep suggested that foam board was not necessary and could be a fire hazard. So the current plan is the dimpled drainage membrane, then an air gap with furring strips or hat channel, then sheetrock (option 3).

One thing to keep in mind is that he is not going to remove the stairwell or remove any flooring, so the dimpled membrane will stop at the floor or stair level. It will not extend below the floor to a french drain the way it would if it were installed on new construction.

Which of these options is best?