Scott, thanks for reply, but I'm still searching for definitive IRC code citation/interpretation that prohibits flexduct thru floor/ceiling assembly penetrations in residential construction. I'm beginning think that my city's code enforcement dept. is confusing firestopping with fireblocking where the former applies to rated assemblies and the latter to non-rated typical residential construction. Clearly, flexduct is not permitted thru a fire-rated assembly as in commercial construction or in residential multi-family. I have not, however, found a clear prohibition in IRC that prohibits flexduct in passing thru floor-to-ceiling or wall-to-soffit penetrations.