I don't know much about insulation except that it's very important that the right one be selected and installed correctly. Otherwise the problem will persist. The cold water pipe obviously needs insulation. But it's quite possible that the hot water pipe might also be contributing to the problem and should therefore also be insulated. I would also consider the possibility of installing small vents so that the air around the pipes doesn't get stagnant. Is that possible?

Take your time to make sure you get the right kind of expertise to fix this. Since insulation is a blind item, this means ultimately a leap of faith.

Happy hunting.

On further reflection, I think your problem may be more intractable than I originally thought. I'm not sure putting insulation on the pipes will correct the problem because, when the pipe get wet from condensation, the insulation will get wet and lose its effectiveness. Over time, it will degrade. In fact the insulation may be a bad idea because it will conceal the problem, unless you like the idea of out of sight, out of mind...

I think your solution lies in getting the closet as ventilated as possible. Perhaps you can replace the solid door with one with louvers so that air can pass through it. Even with that, you might need to install a small (silent) ventilation fan, strategically places so it can pump out the air in the closet and into the room. That humid air will be a benefit in the winter, when heat tends to dry out the air too much, but I'm not so sure about summer time.

And you can replace the rusty door frames with frames made of vinyl or fiberglas, or some other synthetic. Once you paint it, no one will ever know what it is.

Thanks for getting back to me Haus. I believe I got what I was looking for in the plumbing forum.

Closed cell foam insulation would not breakdown or get wet and should do the job. As for venting the wall, the bldg is 80 yrs old and closed by design. With a 2 year old living in that room, I don't think I'd want those pipes to provide moisture in the winter. It wouldn't help the clothing either.

The door frame is original 1927 and I will try to repair as it's are pretty much imbedded in the wall.

Thanks again.

"I'm not sure putting insulation on the pipes will correct the problem because, when the pipe get wet from condensation, the insulation will get wet and lose its effectiveness."

If you use pipe insulation designed for AC lines there will not be any condensation to speak of.
For already installed lines get the split insulation, and buy the required glue to fasten the ends of each section to the next.

This insulation is used on AC suction lines (the bigger pipe) to prevent condensation on the cold pipe.

Thank you brickeyee

I'll look into this as soon as I can get the wall opened up.

Ultimately, the problem was fixed last month. One of the risers had been leaking for some time and was replaced at the co-op's expense.

Originally, it was believed to be the upstairs neighbor's branch line coming off of that riser. After this was repaired, the closet wall was to be repaired/replaced. The painter discovered a still wet wall and the riser was replaced.

So far, everything is MUCH better. No musty / damp smells and the new wall (moistureproof sheetrock) is spaced farther out from the hot/cold pipes so this shouldn't happen again. The door frame was repaired with bondo, primed , and painted.