Smithsonian Secretary David Jan Skorton asked Congress for $726 million to renovate Air and Space, and said the Smithsonian would raise another $250 million from private sources. (Kate Patterson/for The Washington Post)

In the most costly project it has ever undertaken, the Smithsonian will need almost $1 billion to renovate and upgrade the National Air and Space Museum and to make its 22 galleries more innovative and engaging.

The 40-year-old building — one of the most-visited museums in the world — needs new mechanical systems and a new exterior, Smithsonian Secretary David J. Skorton told a congressional oversight committee Wednesday. About half of the building would be kept open during the six to seven years it would take to repair it, officials said.

The project would require $676 million from Congress for the construction and $50 million more to build storage space that would protect the collection as the repairs and upgrades are made, Skorton said. He pledged the Smithsonian would raise $250 million from private sources to “transform all 22 galleries to be more imaginative, stimulating and technologically capable.”

“We will basically have a new museum without having all the costs and loss of revenue that would occur by tearing it down,” Skorton said.

[The money has landed: Kickstarter for Armstrong’s space suit is successful]

The Smithsonian wants to begin construction in 2018 and complete the repairs in seven zones to avoid losing the retail and food revenue it brings in from its 7 million visitors each year, he said. The building’s mechanical systems — air handling, plumbing and electrical — need to be replaced, as does the building’s exterior. The marble that covers the building was cut too thin and cannot be reused. Replacing it adds $49 million to the budget, Skorton said.

Smithsonian officials estimate the cost of demolishing and rebuilding the museum would top $2 billion and would take about nine years. Air and Space would lose $131 million in revenue during the closing.

The museum cost $41 million when it opened in 1976.

The project far exceeds the cost of constructing the $540 million National Museum of African American History and Culture, set to open Sept. 24. Congress provided half of the budget for that museum, and private donations covered the other half.

Members of the Committee on House Administration, which oversees the world’s largest museum complex, pressed Skorton on the decision to fix the building rather than demolish it and rebuild. They also asked him what other projects were in the wings, and what would happen if Congress couldn’t provide the funds.

[Smithsonian asks Congress for $922 million for 2017]

“Are you going to be able to leverage private-sector investment if we’re not able to get the [amount] you’re asking?” Rep. Rodney Davis (R-Ill.) asked.

Skorton said parts or all of the building may be closed without federal funding.

“It is very difficult to raise private funds for the replacement of mechanical systems . . . and those sort of things,” he said. “We do intend to raise funds completely from private sources to change the interior, to bring the museum into the 21st century.”

Rep. Juan Vargas (D-Calif.) asked about the life expectancy of the renovated building and how the committee can be sure it will be around long enough to justify the expense. “Unfortunately, it was not built to last for the ages,” Vargas said.

Skorton said the renovated building should last 100 years. “But that’s very dependent on having sufficient funding to do important maintenance,” he said.

Originally, the project was projected to cost $250 million, and in March it was under $600 million. Skorton said the rising cost comes from a clearer understanding of the scope of the work. Currently, the Smithsonian has completed about 35 percent of the design work; it expects that to be finished next summer.

[Smithsonian outlines $2 billion plan for South Mall renovation]

Rep. Gregg Harper (R-Miss.) asked about other projects in the pipeline that would require congressional funds, and how this project affects the Smithsonian’s “deferred maintenance backlog.” In March, the Smithsonian’s inspector general estimated the institution has $785 million in deferred maintenance projects.

Skorton did not name any projects or mention the maintenance work that is part of the $2 billion South Mall plan currently under preservation and environmental review. Included in that 20-year plan are the renovation of the administration building, known as the Castle, replacement of the roof of the underground Quadrangle building (which houses the African and Asian art museums and the Ripley Center) and a joint mechanical system that would be shared by several of the buildings.

“We do anticipate in years to come that further renovations will be necessary,” he said. “The deferred maintenance backlog for the Smithsonian is a very large and daunting figure. There is nothing surprising about that. With so much square footage and so many visitors, we’d expect that we’d have a backlog of some sort.”