Flight crew reported pressurization failure in cruise and returned to the departure airport.

Narrative

Cruising alt was FL340; we were just passing through FL330 and I noticed the cab alt on the EICAS was higher than usual being at 8100 ft. when it's not supposed to be higher than 8;000 ft. on EICAS. So it appeared the pressurization was no longer holding as it kept creeping up as well as the pressure differential which indicated 8.1 at the time. So I quickly told ATC we needed to do an expedited descent to 10;000 ft. Oxygen mask was donned as it reached 9;700 ft. in the cabin from pressure failing. Upon reaching 15;000 ft. the pressure was stable so we stopped our descent and leveled there. Calculating our fuel; determined we didn't have enough to stay at that alt. So I notified Dispatch; ATC; FA (Flight Attendant) and Passengers we would be returning to ZZZ as we did not have a alternate planned. Stayed at 15;000 ft. and continued back to ZZZ and landed uneventfully and taxied to the gate. Never declared emergency; just requested priority and no assistance needed. Dispatch was helpful and responsive as well as ATC.