A.Bill Rose, architect and building researcher at the University of Illinois in Urbana-Champaign, responds: Abandon the below-grade ductwork. Otherwise, the humidity in the ductwork will create a mold museum for the occupants, and they will be breathing a little of every water-soluble contaminant in the area. Plug the ducts at all penetrations through the slab with concrete. Route new ducts above the slab somewhere, or move to hydronic or resistance heating. No exceptions.

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