The roiling $15 million cost disagreement between the Minnesota Sports Facilities Authority (MSFA) and Mortenson Construction will go to mediation for three days beginning Nov. 9.

Michele Kelm-Helgen, MSFA chairwoman, announced the mediation dates at Friday’s board meeting, which also included the usual updates on new spending, construction progress and minority hiring at the site of the $1.1 billion Vikings’ U.S. Bank Stadium, the largest public-private effort in state history.

The MSFA, the public authority with appointed members, oversees the project on behalf of taxpayers, who are covering half the cost of the project on the eastern end of downtown Minneapolis. Team co-owners Mark and Zygi Wilf are covering the other half.

“I think everybody is looking forward to getting into that proceeding and moving past that process,” Kelm-Helgen said of the mediation.

Whether Mortenson shares her optimism was unclear. In his update to the board, construction executive Eric Grenz didn’t discuss the mediation.

On Aug. 5, Mortenson requested mediation over $15 million in costs the Minneapolis-based firm claims it’s owed for changes to the original stadium design. The MSFA says Mortenson isn’t owed anything.

The mediation request is highly unusual — almost unheard of — for Mortenson, which prides itself on resolving issues privately.

Mediation is the first step. If an agreement isn’t reached, an arbitrator could be called in to make a deal that would be binding to all.

The dispute already has proved stubborn.

MSFA and Mortenson have been in closed-door discussions for more than a year. Two subcontractors recently wrote to the MSFA pleading for a resolution, saying the lack of payment was pushing them to bankruptcy. Mortenson has since paid the subcontractors itself rather than wait out the mediation.

Progress updates

The project remains on time to open for the 2016 season. Grenz said 75 percent of building work and materials have been billed. The workforce also has reached its zenith of about 1,400 workers daily. Those numbers will steadily decline.

The latest development on the site: Workers installed and manually opened one of the signature pivoting doors on the western side of the stadium. Grenz said the glass will go in next month. Grenz also said the first freight elevator has been installed and is working.

In another action, the authority approved allowing the Vikings to spend an extra $130,546 to upgrade water fountains. “The water will be colder and you’ll be able to refill your water bottles,” MSFA executive director Ted Mondale said, adding that the change is significant for roller-bladers and youth baseball players who will use the space.

The board also received the equity update indicating the project remains ahead of the targets. At the end of August, 37 percent of the workers were members of minority groups, 9 percent were women and 4 percent were veterans.