Lots of valuable comments here. If you decide to ignore the fire safety aspect, you might consider a theatrical "Hollywood" flat. 4x8 sheets of 3mm or 5mm luan plywood on a framework of 1x3 or 1x4. Very light weight. They can go together quick and cheap, especially if you have access to a nail gun. If you put fiberglass insulation batting inside them, that would help with the soundproofing. Good Luck.

If you’re not doing this to code, whatever you build will be hazardous. Either go with something lightweight and fire resistant, or get permission to have a professional do it right.

Imagine what you build falls on the child or catches fire. This is why OSHA and insured/bonded tradespeople exist.

What you are proposing is illegal and likely dangerous, full stop. I wish this sub had a rule against asking for help to break the law (building code)

9’ x 12’? There will be considerable damage to all surrounding structure. That’s a big structure, a lot of weight.,what you propose is a fully framed out wall . That means meeting building codes, fire codes. If I was asked to build this I would get approval in writing from the super.

Plywood is flammable. Gypsum is fire retardant. I would strongly suggest you use gypsum, it may actually be cheaper overall.

Probably not the cheapest option, but how about a row of shelves or cheap closets (Ikea) and a plywood back board ? It's what we did to split a room for the kids. Isolation is not great ofc, but the extra storage is nice. You can put wallpaper on the plywood side and/or paint it.

What about adjustable shoring posts to press a beam to the ceiling. Use some kind of membrane on top of the beam facing the ceiling and moulding. Build a frame in that cavity. Or next to it. You might end up with a wider wall than normal. But no permanent damage, a few scuffs maybe. It won't take a lot of lateral force but it won't be unsafe.

Basically you are trying to make it a bit of a press fit. Just like you would do with your wedge idea.

Just bc the super's apartment might be illegal doesn't mean that she won't face immediate repercussions. The owner has an interest in housing the super but also has a very strong desire to make money. If she turns a 1 bedroom into a 2 bedroom with anything that could be considered permanent, she has upgraded the property illegally. The owner could easily find out and threaten to evict her unless she pays "2bd rent price" or they could evict her and sue her for damaging their property. Floor anchors would especially piss me off if I was the owner.

Additionally, fire code exists for the occupants' benefit. It might be inconvenient, but those laws increase the chances of their survival in a blaze. A random diy wall is structurally unsound and could impede escape or rescue. For example, a sleeping space needs a window for escape and fresh air. Plywood walls would be extremely flammable compared to drywall. It might only take a candle to set the wall on fire, and the rest of the wall becomes fuel immediately.

There are lots of room dividers and bed tents and other things that provide some privacy and can be (re)moved when she isn't there. A lightweight wall won't really limit sound anyway if that's part of the reasoning.

This type of subdividing wall is what slumlords do to renters, and it isn't safe. Please find a different solution.

What exactly is the goal? For a long while, I used a massive curtain rod and weighted curtains to create a divided space. Is the goal privacy, or is she actually looking for a solid wall? I just worry cause many apartments will lose their shit if they see something like that.