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The wall behind the toilet in the bathroom I am renovating is cinder blocks

When this was finished the wall was first covered with some black paper (like tar paper but thinner) then they installed 1x4 furring strips and between these they put regular expanded polystyrene (the white stuff) and on top of the furring strips they added 1" Styrofoam and then drywall. There was no other framing on this small section behind the toilet and that was because in the old days (60's) the basement was not used for living but there was a service toilet there just in case you need. When the finished the basement there was not enough room behind the toilet to do proper framing so the went with the above

Since the toilet I am installing is a bit longer and it requires 30cm (11 3/6") from the center of the drain to the wall I need to get thinner on insulation so I removed the insulation and I did not like what was behind

When they built the window frame (see the picture) that is just above the toilet sit they left an empty and unfilled space under the base of the frame. I suspect that cold air went in and in time condensed and the humidity made it downward to the top side of the furring strips that are below that window. As a result these got rotten (no mold but the wood was crumbling) Also the black paper that was there looked decayed -I can't say why but it looked thinner than the same paper just below, lower on the same wall. Maybe it was rain water that got in one time ?! Not sure when and why the wood deteriorated

enter image description here

I decided that since I got this far I will redo it, it is an area of 65"x30" so not a big deal

What is the right way to do it ?
Should I continue to use that black paper ? What is the name of it ?
Should I add vapor barrier ? If yes where -near the wall or just behind the drywall
I am planning to use 1x4 pressure treated furring strips -is that OK ?

so to recap I should do these
-put back the back paper (if you guys think I should)
-put vapor barrier
-install furring strips (planning to use tapcon screws here)
-pub back the white polystyrene
-mold resistant drywall, green facing the inside of the bathroom

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  • Basement walls below frost level do not need much insulation. R-3/R-5 usually plenty with vapour barrier. Good insulation needed from frost line to bottom of first floor. Plastic vapour barrier is probably better than the paper, quite sure the white polystyrene is not good and probably against most codes, if it is the packing stuff.
    – crip659
    Commented Sep 27, 2021 at 0:18
  • where do I put the barrier? Next on the wall or on the furring strips and under the drywall? Not sure if there is any 3/4" insulation that I can put there to replace the white stuff
    – MiniMe
    Commented Sep 27, 2021 at 0:28
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    If you really needed more space between the toilet and the wall you could replace the existing toilet with one that has a10" rough-in. This would give you 2 more inches of space.
    – d.george
    Commented Sep 27, 2021 at 9:39
  • @d.george is right. That's the wrong size toilet.
    – gnicko
    Commented Sep 27, 2021 at 11:16
  • I think that what was there behind the toilet was over built, I would hate to have to return this toilet, it is this one ovedecors.com/pub/media/amasty/amfile/attach/… and the cost was good (700CAD)
    – MiniMe
    Commented Sep 27, 2021 at 14:13

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