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I need to run electrical and network cables (cat6) from the third floor of my house down to the basement where there are different breakers and different computers.

So I did a relative measurement in both places that I hope lines up so I can drop the cables down.

From the basement there is a hole that looks like this going up inside the wall in the house:

Image looking from my basement up the hole

Upstairs I tried drilling a hole in the wall (and it's an inner wall between my and the neighbors side of the house) but oddly I found a sheet of insulation blocking my way (see below):

Peekaboo! Insulation says hello! Through the hole I drilled in the middle wall

Since the insulation is in an interior wall (see below),

enter image description here

I don't know why it is there. What I do know is that when my neighbor hangs things on the wall on the interior wall the screws come through the other side, so I was very careful not to drill through her side of the wall, which is how I found the insulation.

I don't know why it's there, but it's preventing me from getting my cables run.

Does anyone know why it's there or what I can do about it?

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    Note that if you're in a duplex or apartment building, that wall needs to be fire rated under current building codes...so I'd be thinking about a different route for your cabling, as well as repairing the drywall where you already made the hole
    – ThreePhaseEel
    Dec 6, 2020 at 23:51
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    Insulation is commonly used for sound dampening between adjoining units. Is that the extent of your question?
    – isherwood
    Dec 7, 2020 at 0:40
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    Think about if you want your unit to be cool and your neighbor wants theirs to feel like a sauna.
    – whatsisname
    Dec 7, 2020 at 0:55
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    @leeand00 search for "plaster and lath" to understand how your walls were built. The inside of your walls looks exactly like the inside of mine... It's got advantages and drawbacks for the renovator.
    – FreeMan
    Dec 7, 2020 at 12:16
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    Your "wide" pic looks like that used to be an exterior wall, then they added on. Of course, someone could have just used siding as an interior wall finish, there's nothing wrong with that. This would account for the "very thick wood" you noted above. TBH, plaster & lathe makes hanging pictures dead easy: "You want it here?" Drives screw, job done. No worries about finding a stud or using a molly/anchor. As noted, I've done that in my house for years with P&L walls. Finally - I like your network rack!
    – FreeMan
    Dec 15, 2020 at 19:50

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The first 3 comments are all correct answers.

Comment #1 by ThreePhaseEel regarding a code required fire wall is correct, but it’s for fire rated draftstops between living units.

Comment #2 by Isherwood regarding sound control is also correct. The code requires a minimum STC rating between units and insulation can fill the voids and help with that requirement.

Comment #3 by whatsisname regarding a thermal barrier is also a code requirement. The requirement is for the envelope around a unit...there is no exception for common party walls.

Do not remove the insulation just to extend some cables from your attic to the other floors.

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