Fibreboard inside above-grade basement walls...how do I insulate over this?
1/6/2021 at 11:12:51 AM
I have an unfinished walkout basement that I'd like to eventually get finished. My home was built in the 1980's and I found something questionable in the basement walls. Looking from the interior, I found one of the walls that is above grade to have fibreboard sheathing and then have the stud wall against it. There is nothing within the studs and the fibreboard is exposed. One of the previous owners then built another stud wall in front of it, attached top plate onto the ceiling joists and not touching the concrete floor. They then placed fibreglass batts (which seems quite loose) inside this second wall and then loosely added vapour barrier sheets on top. I am trying to figure out how I could go about renovating this wall. Do I remove the second wall and add tar paper along the stud cavity of the first stud wall to mitigate or remove the ability of vapour transmission and then add XPS boards with expanded foam within the stud cavity to insulate the stud wall. And then add a vapour barrier on top of the stud walls and finally topping it off with drywall? Does this plan make sense or am I asking for vapour issues down the road? Also, when adding any kind of insulation on top of the fibreboard, is there supposed to be a gap required so as to not have any contact with the fibreboard so that any vapour would evaporate and leave the way it came in? Any answers/insight would be greatly appreciated!
1/6/2021 at 3:10:49 PM
The wall should be able to breath from one side. So if you install a vapour barrier inside, before drywall, the outside should be lined with a breathable material to be able to release the Vapour trapped inside the wall. The insulation does not necessarily need to have a gap to the fiberboard. The space in between studs + insulation and the exterior wall works as a thermal bridge, preventing the heat to be conducted from inside to the exterior wall.
1/6/2021 at 3:33:38 PM
It's difficult to assess with just one photo. The Code requires either 6 mil poly to separate the bottom plate from concrete below grade or else a pressure treated bottom plate. If both walls do not correctly satisfy this requirement I would disassemble and repair if you are unable to slip it under.
I prefer a stud spacing of either 19.2" or 24" to minimize thermal bridging (if the current spacing is 16" that would also be grounds for me to want to disassemble.
It's unusual to see the fiberboard panels and it's not known if they are contacting the concrete walls anywhere. If you can determine they're in contact with the concrete that's one more reason to disassemble.
Note: current Energy Code requires a minimum of R-20 for basement walls (as an assembly). If you were to disassemble you may also want to consider applying dampproofing to the interior surface of the basement walls (if not already present) which is permitted provided it terminates at grade level.
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