How to insulate prefab chimney interior wall
I have had a roof leak around the chimney masonry which was apparently “fixed” by the previous owner while continuing to leak small amount of water into the wall by the side of the fireplace. This wall is now open to allow for replacement of the rotted studs, and I’d like to improve on the original insulation from the inside. On the other side of the wall is the open prefab chimney cavity – apparently just one layer of brick lined on the interior with some plywood and fiberboard.
My first question is how to insulate the wall itself. The original build (30 yr old house) has 2×4 stud cavities lined with unfaced fiberglass in this area, followed by a layer of plain plastic on the interior, then drywall. To the exterior, there is only masonite siding (no house wrap). Directly above the fireplace insert, they omitted the plastic and used felt paper instead (why?)
I am planning to spray all available studs with boracare, then use canned spray foam insulation around the window casing (there was some moldy fiberglass stuffed in here originally). I’m not sure that I should replace the plastic sheet again (and tape every seam). For insulation I could use either new fiberglass (backed or plain), or a couple of 2 inch thick XPS panels, perhaps sealed at the stud junction with caulk or foam. How do I create a higher R value while minimizing moisture problems (in humid NC) and keeping things relatively breathable should future leaks occur (esp with aging masonite siding)?
For the interior of the fireplace, I have removed a small strip of swollen and mildewy fiberboard, only to reveal the brick immediately under. I wonder if I need to replace this at all since it’s certainly not serving as any kind of insulation. I would be tempted to just take out any other section with water damage, do a breathable waterproofing on the outside of the fireplace, and caulk around pipes.
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