An expandable air filter device. The air filter device includes first and second air filter units and an expansion joint. The air filter units each include a filter media assembly and an outer frame assembly. The filter media assembly defines a perimeter. The outer frame assembly is arranged around the perimeter. The expansion joint is connected to and extends between the outer frame assembly of the first air filter unit and the outer frame assembly of the second air filter unit. In some embodiments, the expansion joint is expandable from a collapsed state to an expanded state. A gap between the first and second air filter units is greater in the expanded state than in the collapsed state, with the expansion joint spanning the gap, serving to impede flow of un-filtered air through the gap during use.