John Gilmer wrote:
Frame the wall inside the foam, install wiring in the framed wall,
fill cavity with rock wool and cover with drywall.

Huh?

Inside what "foam?" Should the new framing be put over the existing "foil"
based insulaton material? Or should that stuff be taken down when the
framing in installed?

The other poster was assuming the foil is a foil-faced solid foam
insulating panel. W/O knowing for sure what it actually is and how much
insulation value it has, etc., hard to say for absolute certain, but I'd
concur w/ the post--simply leave what's there where it is, frame the new
wall inside it and go on as conventional construction.

I certainly wouldn't use loose rockwool, however, as that implied if do
add any additional insulation; whether it would be needed or even of any
value depends on what the existing actually is as noted above...

BTW, if there's sufficient open floor space, quickest way to proceed is
to frame wall sections flat (w/ enough clearance to stand them up and
then put in place by using the second top plate mounted to the joists
and nail into it rather than framing in place.

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