Hi all,

I am renovating an original bathroom on a 1920s house that is brick with a slate roof. Zone 5. The sheathing is drywall surprisingly. A previous homeowner had loose fill cellulose sprayed randomly in stud bays but overall there wasn’t much insulation. Also, the bathroom does have air conditioning.

I plan on using mineral wool insulation behind drywall on the walls. However the roof line cuts the bathtub area in a diagonal and there is an unused/unconditioned space behind half the tub area. The rafters do extend all the way into the attic. The upper part of the wall shown in the picture is sheathed with wood planks and slate is nailed to that. Otherwise the wall sheathing is drywall.

Should I treat this as an attic area and put sheathing behind the tub and perpendicular interior wall so I can insulate that? Or should I insulate the exterior walls and against the rafters up until where the attic starts?

Thanks in advance! I’ve done a lot of reading on proper insulation techniques but can’t wrap my head around how to insulate given the random unused area.