Fiberboard Sheathing for Knee Wall Air Barrier
We live in a cape cod style home and I’d like to install an air barrier on the second floor knee walls. There is bare insulation just hanging between the 2×4’s so an air barrier looks to be needed. On one wall that is partially exposed to the outside but mainly in the attic, the original builder used fiberboard. Will fiberboard provide enough of an air barrier or do I need to go to something like OSB? The one area is easily accessible, but the other is through a hatch so the lightweight and easy to cut fiberboard would be preferred.
There is also a return duct exposed directly to the hot attic in my knee wall, going to build it out with some 2x’s so I can add insulation to that area as well. Talk about a giant hole to the room inside!
I’m toying with adding some rigid foam insulation, but the thermal barrier is going to be a pain to deal with. Having to haul 1/2″ drywall through that tiny hatch and hanging it will be a pain and the added cost for an extra say R5 doesn’t seem worth it.
As some background. We had new gutters and a continuous soffit installed and blown in insulation for the attic next week. They insulation company will be installing air barriers to allow for the soffits to work properly as well. I’ll also be searching out for any air leaks in the attic prior to the insulation.
We have no ridge vent and only a few passive top vents. The roof is due for replacement in the next month, so we will have a ridge vent installed at that time and the broken power attic fan removed.
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