Date: 202002
Local time of day: 0601-1200
Flight conditions: VMC
Light: Daylight
Aircraft: B757-200
Operator: Air Carrier
Operating under: Part 121
Flight plan: IFR
Mission: Passenger
Flight phase: Cruise
Airspace: Class A ZZZ
Aircraft component: Air Conditioning Distribution Ducting; Clamps; Connectors
Component problem: Failed
Reporter organization: Air Carrier
Reporter function: First Officer; Pilot Not Flying
Reporter qualification: Flight Crew Multiengine; Flight Crew Air Transport Pilot (ATP); Flight Crew Instrument
Second reporter function: Pilot Flying; Captain
Anomaly: Aircraft Equipment Problem Critical; Deviation / Discrepancy - Procedural MEL / CDL; Deviation / Discrepancy - Procedural Published Material / Policy
Detector: Person Flight Crew
When detected: In-flight
Result: Flight Crew Diverted; General Flight Cancelled / Delayed; General Maintenance Action
Contributing factors: Aircraft; Procedure
Primary problem: Aircraft

Narrative: At the gate in ZZZ I reviewed the AML and took note of the MELs and NEFs listed in the MIC Sheet and inside cover. During cockpit preflight setup I noted that the AFT temp controller was placed to the OFF position. This coincided with one of the MELs listed which was derived from an earlier writeup of higher temp in a portion of the aft zone of the cabin. We had not yet started the APU so the packs were not yet on. As we proceeded well into the passenger boarding; when ground personnel asked to disconnect PCA and electric; the Captain started the APU and told them to stand by to disconnect. It was at some point after we established the packs just before entry door was closed; I think the Purser first made mention of the lack of air in a section of the aft cabin. I remember noting that the AFT temp at this check being at 77 and so I adjusted the FWD lower to try to accommodate. The push back and taxi out was a little longer than usual. Once airborne we climbed up to altitude with little delay and no problems in our climb to FL360. I was Pilot Flying; but I included a scan of AC panel and responsibility of temperature control throughout; monitoring the temps as we cruised. They showed a little higher than ideal but never drastic. While in route through ZZZ2 airspace well past DALUD; the purser again called up to say how uncomfortable it was in this one section of the AFT cabin. I remember looking up at the AC panel temps and seeing all zones in the 70's. The CA concluded that we turn off the Right recirc fan which I reached up and did. Some moments after Captain reached over to the AC panel overhead and turned off the Left Recirc fan. We discussed the consequences of this action and consulted the manual to find a reference to get clarity on this new configuration. It was shortly thereafter that we got aural warning and Cabin Altitude light. The CA was well ahead of me in the donning of his mask so as Pilot Flying; I stayed with the instruments to fly the aircraft. As he got established on oxygen I alerted transfer of controls with 'Your aircraft' to which he responded and I commenced donning of my mask. While doing so I heard Captain command to get lower altitude. I keyed the mic to ATC and asked for; '...as low as we can get'. To which they replied descend to FL260. Pilot Flying adjusted the MCP I concurred and looked up to see the cabin altitude at about 11;000 ft. and the rate climbing. Captain called for QRC and as I was Pilot Monitoring; it was started as we started down. The Captain said we need to go further lower. ZZZ1 ATC was either talking to us or another aircraft at this time; I am not sure. But to get the attention for altitude needed I called [for priority handling] and told them we were going to descend to 10;000 ft. They then acknowledged our requests thereafter.While the Pilot Flying conducted the descent I made my way through the QRC. At the step of checking controllability of the cabin altitude I looked up to see the cabin still above 11;000 ft. and the rate was now higher than the previous check. The Captain reached for the Passenger Oxygen switch and pushed it. The rest of the QRC was completed to the end and I then elected out of habit to resort to the paper QRH. At some point in this descent the Captain decided to insert a divert to ZZZ2 in Rte2 while we descended straight ahead delaying the turn to divert while executing the descent. We were in visual conditions throughout the descent and it was clear that terrain was not to be a factor north and west of our position.At reaching 10;000 ft. we transferred control of the aircraft more than a couple of time as he communicated with the cabin; crew and passengers. There was ample time to complete all items of the checklist in preparation for an overweight landing at ZZZ1. Wind was 5 kts from the NNE so we landed flaps 30 on Runway 3. No autobrakes as per checklist and a sink rate less than 300fpm.It took about 1 hour 40 mins for the entire process of awaiting personnel and permission from the airport to finally allow us to deplane at a jetbridge. Initially were sent to a hard stand at position 14. We were lucky to have had access to a PCA during this lengthy wait to keep the cabin comfortable. The Captain opted to restart the right engine as we were towed over to the terminal to provide the backup power and air if needed.Later it was revealed that there was a leak in a conditioned air tube. This would not only explain the Hot zone but the depleted flow of air into the cabin. In an attempt to eliminate passenger discomfort while working around a MEL placard; the activation of the Overboard valve to open added to the failure of the system to maintain differential.


Second narrative: [Report narrative contained no additional information.]


Synopsis: B757 Flight Crew reported a broken duct in the air conditioning system caused cabin altitude to climb; resulting in a diversion.