A system for sealing large volumes or gaps includes a flexible envelope that can assume the shape of the volume when filled with a foaming composition that expands the envelope to the boundaries of the volume. The foaming composition may be integral with the envelope or delivered in bulk by an external device. The foaming composition may be one or multiple parts, typically two parts such as a poly isocyanate and a polyols, in which case it is necessary to keep the two parts separate until foaming is desired. Separation may be accomplished by providing multiple compartments that deliver components via a mixing device into the interior. The envelope may include ribs for structural strength and shaping, and leak pores around its perimeter to enhance bonding and sealing. It may also include perforations for bulk, modular seals or for separation of portions of the envelope to seal irregular shapes.