Should I use rock wool or fiberglass for these exterior bathroom walls?

Rockwool is a massive upgrade over fiberglass, especally in a bathroom. I've used it in several mid-century renos and it's much easier to friction-fit around those weird 1950s stud spacings than standard batts. It doesn't sag over time, it's fire-resistant, and it won't mold if it gets damp, which is exactly what you want behind a shower wall. It's more expensive than fiberglass, but for a single bathroom, the cost difference is negligible compared to the performance boost.

Regarding that double-faced paper you found, that was common back then but it's actually a recipe for rot because it can trap moisture from both sides. When you switch to modern rockwool, you're dealing with an unfaced product. Whether you need a poly vapor barrier depends entirely on your climate zone. If you're in a cold northern climate, you'll want a **6-mil poly** sheet or a smart vapor retarder on the warm side of the studs (the interior) before the drywall goes up. If you're in a hot, humid area like the south, you actually want the wall to breathe more so you'd skip the plastic entirely.

For the shower area specifically, don't rely on a plastic sheet behind the rockwool as your primary moisture defense. You should focus on a waterproof substrate like **cement board** with a liquid-applied membrane or a foam board system. This keeps the water from ever reaching the wall cavity in the first place. If you're doing the tub surround for sound, stuffing those interior walls with rockwool will definitely kill that 'hollow' bathroom noise.

One thing to watch out for in a 1957 build is the depth of those exterior studs. Sometimes they used true 2x4s and sometimes they're slightly off. Rockwool is rigid, so if you have shallow cavities, you might have to trim the thickness of the batts with a serrated knife so you aren't compressing it. Compressed insulation loses its R-value, so keep it snug but not squished.

While the previous poster correctly identified the potential for compression, one must also account for the inevitable presence of bypasses around the plumbing stack; in my own 1954 restoration, I found that meticulously carving the mineral wool to accommodate the vent pipe's specific diameter—rather than merely tucking the material behind it—resulted in a significantly more uniform thermal envelope. Furthermore, regarding your query on the vapor retarder, if you opt for a smart membrane like Intello, ensure you are sealing the perimeter with an approved adhesive rather than standard staples alone, as maintaining the integrity of the air barrier is arguably more critical than the specific perm rating of the insulation itself in a high-moisture bathroom enviornment.

Before you commit to the mineral wool for sound, have you ruled out the noise coming from the plumbing itself? In those '50s homes, the cast iron pipes are quiet, but if you've replaced them with PVC, the 'whoosh' is what people ussually complain about, and insulation doesn't always fix that. Also, are you sure about the vapor barrier? If you have original wood siding or a brick veneer with a tight air gap, adding plastic inside might cause more harm than good by creating a double vapor barrier. Have you checked if your exterior sheathing is actualy breathable before you seal the inside?

I’m just starting my first-ever DIY reno on a tiny bungalow and reading this is getting me so hyped to tear into my walls! I’ve been obsessing over Rockwool videos all week. Since you're dealing with a 1950s house too, have you looked into what’s up in the attic above that bathroom? I’m terrified of finding weird old loose-fill stuff when I pop the ceiling. Does anyone know if you can just blow new cellulose right over that gray dusty insulation or do I have to vacuum the whole attic out first? I really want to keep the heat from escaping thru the exhaust fan vent!

Quick update—thanks for the advice on the plumbing noise. We actualy still have the original cast iron pipes, so that's a relief! I decided to go all-in on the rock wool for the exterior walls too. You were right about the wierd stud spacing; the friction fit is making the install way easier. Now I'm just curious if a smart vapor retarder woud be better than poly?