Yet another ‘Devil’s Triangle’ Quandry
Hi, name’s Frank and first time posting a question. I live in the middle of the country, in Zone 5.
I hunted through many ‘knee wall’ questions, but couldn’t quite find my particular situation. I have a bonus room over my garage with a sloped roof that goes from the eaves to the sidewall of the house (see picture). Of course, I have a 4′ knee wall in there. The room has always been uncomfortable and, after I added a closet, it would sometimes get ice on the base trim in a couple spots, meaning the fiberglass job behind the drywall was terrible. So my plan was originally to replace the fiberglass with mineral wool. I also am now planning to build a frame into the wall to slide a dresser into to make ‘built in’ drawers (frame would have insulation).
What has been tripping me up on how to handle this space is that this area has soffit vents, but no exit vents up top, so it’s a sorta-but-not-really-vented area. Once I opened up the wall to get in the space, I could see it looks like the floor of the room has a 2×12 insulated space, walls are 2×4 with fiberglass, and the rafters above are 2×6 fiberglass batts. As you can see from the photos, they didn’t air seal the space with the ceiling batts at all, which probably lets hot/cold air infiltrate along those spaces. So as I see it, I have 2 potential paths: 1) work to fully seal up the knee wall with the mineral wool, seal off the ceiling rafter space, and add batts to the floor to help insulate the garage more (it gets extremely cold/hot in there, I think a lot due to the lack in insulation there), or 2) insulate the floor, walls, and rafters of the triangle to pull that space into the envelope.
Which leads to my questions: Can I simply block the soffit vents off and put mineral wool right against the roof deck? Or would I need some sort of foam/rigid insulation against the decking first or a vapor barrier on the inside of the framing? And if I go with the mineral wool on the knee wall, do I need any other vapor barrier or is the mineral wool barrier enough? Thanks!
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