A B737-NG cabin climbed to 10000 ft while in cruise at FL340. The Captain timidly began an emergency descent. The cabin reached 12000 ft while the First Officer executed a high rate of descent to FL240 where the cabin stabilized at 8000 ft. Maintenance found the outflow valve frozen with leaking cabin system water.

Narrative

The flight was to ZZZ; Captain's leg; VMC. As we neared the ZZZ1 area; I noticed the cabin altimeter fluctuating down and then up; and I advised the Captain. At this time the cabin altitude had started a 1;000 FPM climb. We checked the switch positions and found all in the correct positions and the outflow valve was near closed. I switched the mode switch to alternate with no change and returned it to normal. As the cabin altitude rose; the cabin altitude warning horn went off (10;000 FT cabin altitude). We both donned oxygen masks and established crew communications (as per memory items). I advised ARTCC that we were having pressurization problems and requested a lower altitude and were cleared to FL240. Captain initiated a slow descent on the autopilot using the vertical speed mode of the MCP at about 1;000 FPM. He then directed me to locate the company radio frequency for the local area and advised the cabin crew of the problem. When it appeared to him that I was not finding it fast enough; he handed his publication to me and I located the frequency and established communication with our Dispatcher and Maintenance through company radio. At this time he transferred aircraft control to me and I initiated an emergency descent using speed brakes and the Level Change Mode of the autopilot. During this time; the Captain was discussing the situation with Maintenance on the radio. As the aircraft neared FL240 the cabin altitude stabilized at about 12;000 FT and seemed to begin a slow recovery. At this time the Captain reassumed control of the aircraft and directed me to 'find and run the checklist.' I located the 'Cabin Altitude Warning Horn' checklist in the Quick Reference Handbook and read the first 2 items which were the memory items. The next step was to place the mode switch to 'manual' and then manually close the outflow valve. The Captain directed me to not accomplish those steps. When I questioned this; he told me that using 'manual' to maintain the pressure was too difficult. The cabin pressure was stabilizing; so other than commenting that I would like to be able to remove my mask; I let this go. The remaining items in the Quick Reference Handbook were not applicable to the situation and I called 'checklist complete.' The situation now is that we are past ZZZ2 heading west at FL240 and the cabin altitude is below 10;000 FT; so we can remove our masks. The Captain (with Dispatcher concurrence) elects to continue to ZZZ at FL240 and the Dispatcher sent new fuel burn information via ACARS. The cabin altitude had stabilized at 8;000 FT but still showed some fluctuations. At one point the fluctuation concerned the Captain enough to direct me to find the approach plates for ZZZ1. The Captain was very convinced that the problem was due to a cabin seal and that the fluctuations were due to the seal 'flapping.' I asked ARTCC for what the lowest available altitudes were in the area and was provided with that; but I have forgotten the exact values. I think it was 14;000 FT; but we would have to climb for the mountain range nearer ZZZ. On the descent into ZZZ; I; on several occasions; asked for and received the lowest available altitudes in case of a recurrence of the problem. Sure enough; the system couldn't keep up in the descent and first the normal and then the alternate pressurization modes failed and I had to use the manual mode to control cabin altitude. Fortunately; we were in the descent and maintaining cabin pressure was not difficult; however; the Captain seemed inordinately concerned that I was not up to the task of using the manual system and was unusually directive (he was also flying the aircraft). The approach and landing were otherwise uneventful. After landing; Maintenance inspected the aircraft and found a leaking water line in the cabin water system. The water had leaked across the outflow valve; froze and restricted its movement. Lessons learned: First; we should have started an aggressive descent when we firstsaw the problem and that would have made the pressure differential and pressurization easier to maintain. Second; the Captain should have called for the checklist immediately instead of sending me off on a wild goose chase for the company radio frequency. Third; should have used the manual mode to close the outflow valve as per the checklist. This may or may not have worked due to the blockage; but I believe it would have stopped subsequent fluctuations. Fourth; should have diverted into ZZZ2. If the pressurization was that questionable; why continue into high terrain? Also; depressurization routes over that area would be helpful. CRM. The CRM environment was frosty. The Captain; throughout the 3 days of the trip; had demonstrated a disregard for standard company procedure and a low opinion of First Officers in general. Usual company training in abnormal procedures is to give the First Officer control of the aircraft and ATC communications (not done); so the Captain can communicate with the cabin crew; company; and run the abnormal checklists.