Vapor barrier with rock wool insulation in southeastern PA
I’m renovating a 100+ year old house that has plaster on lath on rough cut 2 1/2” x 4” studs at 18” oc. Exterior sheathing is 1×12 pine boards. Lap siding that has been covered with 3/8” foil faced accordion board and vinyl siding.
I’ve removed the plaster and lath, aligned the interior wall surface using 2 1/2” metal studs, foamed the gaps between the sheathing boards and at the top and bottom plates and insulated the cavities with rockwool insulation. I’m wondering what sort of vapor barrier should be used if any.
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
In southeastern Pennsylvania, you are either in Climate Zone 4 or Climate Zone 5.
Here is a link to the climate zone map.
If you live in Climate Zone 5, most building codes require that exterior walls have an interior vapor retarder (not a vapor barrier). Technically, the code calls for a Class I or Class II vapor retarder in Zone 5. Among the vapor retarders that could work in this zone are kraft facing, vapor retarder paint applied to drywall, or a so-called "smart" retarder like MemBrain.
In Climate Zone 4, building codes do not require an interior vapor retarder on walls.
In all cases, you want to pay attention to airtightness, especially at electrical boxes. Consider buying airtight electrical boxes for this job.
For more information, see this article: "Vapor Retarders and Vapor Barriers."