The beauty of a temporary structure, Mr. Koshalek realized, is that he would only need to consult with the members of his own board. The budget would be around $5 million, a relatively paltry sum by the standards of recent museum expansions, even in today’s rough economic climate. And the design’s extreme flexibility  it can be blown up at a moment’s notice, and the interior can be easily reconfigured  could allow the museum to respond nimbly to cultural issues of the moment. (Worst case, if it turned out that people hated it, it could be packed away forever.)

The architects imagine the installation process as a performance piece in itself, something like watching event organizers blow up the balloons for the Macy’s Thanksgiving Day Parade. Two refrigerator-size air pumps would be used to inflate the baby-blue structure, which would fill the entire four-story courtyard and bulge out of the top. A smaller, globulelike form would swell out of the bottom of the building to create a public lounge overlooking the mall.

The aura of lightness  of a building that seems ready to float off into the sky  is counteracted by the structural systems that hold the addition in place. A gigantic tube of water, like an inner tube, encircles the interior of the structure to weigh it down. A series of big steel cables, tethered to the inner tube at one end and to a roof-level truss at the other, would wrap several times around the translucent form as it rises through the core of the building, making it resemble an uneven stack of donuts or an act of ritual bondage.

Image The planned Hirshhorn addition would have a lounge protruding into the Mall and an auditorium in the building’s central court. Credit... Images from Diller Scofidio & Renfro

Most visitors would enter the structure through a short, tube-shaped corridor located at the seam between the lounge and the main courtyard space. In the current version of the design, which is still being refined, the lounge’s translucent blue skin becomes progressively more transparent at the base, so that visitors will be able to see out into the mall. The inner tube that would anchor this room’s outer edge serves as an informal bench.

The main hall, by contrast, would be slightly more formal. A temporary stage would be built over the courtyard’s off-center fountain, with up to 1,000 seats arranged in a semicircle around it. Further up, a few transparent areas in the fabric would allow visitors occasional views of people up in the galleries. (Given the height of the interior, the architects might consider adding one or two levels of balcony space, which would add richness to the design and take advantage of what is now a four-story-tall void.)