Russian cosmonauts, station commander Sergey Kud-Sverchkov and flight engineer Sergei Mikaev, attempted to fix the problem. Their escape route was the separately docked Soyuz MS-28 spacecraft.
Retired Canadian astronaut Chris Hadfield, who was a commander of the ISS in 2012, said it had always leaked around half a pound of pressure a day.
"When you have an area that's leaking a little more, you get up to a pound a day, maybe a pound a half or even two, then we hit a threshold where, okay, we've got to do something about this," he told BBC Newshour.
"You're always one breath away from having to take shelter somewhere if the station has a problem. It's just a matter of fact of living on board a spaceship."
It is not the first time the station has had to deal with this latest leak - the cracks responsible have persisted on and off for around six years.
However, following the arrival of a Russian cargo ship last month, the Russian space agency Roscosmos noticed a fresh slow pressure drop in the tunnel, prompting the decision to move beyond patchwork fixes and attempt a more extensive repair operation on Friday.
But it was the method they were planning to use that prompted the order to take shelter, according to news agency Reuters.
Kud-Sverchkov and Mikayev were said to be using a saw to try and get into an area to access the crack that was leaking air.
Nasa disagreed with the method they were using and mission control in Houston ordered five crew to take "safe-haven" procedures on the Dragon ship.
