Installing Mineral Wool over FSK Batts on Foundation Walls
Basement wall insulation- mineral wool over Foil-Scrim Kraft-Faced batts? We’re finishing off a basement space zone 4 mid-atlantic. There is FSK fiberglass batt against the concrete foundation wall, the ground slopes to a walkout so the wall is 90 to 20 percent below grade depending on where in the slope. Above grade the exterior concrete is painted, below grade it has the standard bituminous coating and dimple drainage matt, no insulation.
The FSK is in very good shape and studs seem somewhat tight to the insulation. Compressed to maybe 1.5″ at the studs.
We don’t want to remove/replace the fiberglass though I know no one likes it on GBA (ie condensation risk behind the fiberglass on the concrete). It’s just not in the budget. Since the 2×4’s have space we were thinking of filling the cavities with roxul batt before sheetrock for improved sound isolation, fire resistance and extra R-value.
Assuming FSK is somewhat of a vapor barrier, does having a layer of it in the middle of an assembly like that create problems?
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
Replies
I don't think adding mineral wool here really changes your risk profile any. The moisture from a typical basement wall is coming FROM the wall TOWARDS the living space, not the more typical "from the living space towards the exterior wall" direction you see in above-grade walls during the heating season. This means that foil facer is already trapping any moisture between it and the wall, and adding additional interior-side insulation won't change things much. I I would avoid using an interior vapor barrier or retarder though, which would only act to limit any drying ability of your assembly here.
My preference would be to replace that foil faced fiberglass and replace it with polyiso, which I do see you expected would be the GBA community's preferred approach :-D
Bill
Thanks Bill! I think the fiberglass is compressed down a bit by the framing so the R13 batt may be more like R6 (just a guess). So my concern is if we add mineral wool, in front of that might the FSK roomside surface end up cool enough to condense roomside moisture? I suspect the FSK generally prevents roomside moisture from getting to the concrete though not perfectly since the tops and bottoms are "open".
I’m in this exact same situation. What did you wind up doing? I’m very tempted to do build a 2x4 wall in front of the FSK and fill with Rockwool. Seems like a good idea but was slightly worried about room side vapor condensing on the foil if the Rockwool were to result in that surface being cold.
Curious to know how this worked out. Thanks!
User...358,
The typical stem-wall insulation here is completely encapsulated fiberglass batts. It usually works, but sometimes it doesn't - and that's in the temperate PNW climate. The only method that you can guarantee not to have problems is using impermeable insulation against the concrete - either spray foam, or foam board.
https://www.greenbuildingadvisor.com/article/three-ways-to-insulate-a-basement-wall