Closing the leaks in supply and return ductwork (mastic, tapes, or injected sealant). Typical homes leak a meaningful share of conditioned air into attics and crawl spaces — sealing recovers capacity you already paid for and evens out room-to-room comfort.
Air-quality terms map to humidity control in muggy Chesapeake summers, filtration (MERV), ventilation for tighter renovated homes, and duct condition in houses that have seen decades of retrofits.