Hello,

I will be renovating the exterior of my late 1950’s home in Climate Zone 4a (Southeast NJ).  Part of the renovation will include replacing the home’s siding, roofing and homasote sheathing, so I plan to upgrade the home’s insulation as well.  Because I am replacing the old sheathing, I will take the opportunity to replace the existing wall cavity insulation (currently thin, kraft-faced fiberglass batt).  I have couple of questions that I cannot seem to find straightforward answers to, so I thought I would pose them here.

The current plan is to cut out the old fiberglass batt and kraft paper, and replace with R-15 mineral wool batt (unfaced), and install 7/16 Zip sheathing (with seams taped per manufacturer) with 2″ of XPS (R-10, also taped seams) on the exterior.  New siding would then be installed on 1×4 furring over the XPS. 

I believe I have enough exterior insulation to keep the sheathing warm enough in my climate zone (4a – Southern NJ) to prevent moisture issues at that layer.  Any and all input is appreciated, but my main question concerns the interior side of the wall.  The walls in the home are plaster over metal lath, with an 75 years worth of coats of paint of unknown number and type, with the exception of last coat, which is latex paint which I added.

As I read it, vapor retarders of Class III would be “permitted” in this assembly, by NJ IRC 2021 Table R702.2, as would Class II retarders because my exterior insulation (greater than R-3 over a 2×4 wall) meets the criteria in Table R702.4.   My questions are:

1) If the existing paint layers on the plaster constitute a Class III vapor barrier, and Class II retarders are “permitted”, would there be an actual advantage or positive reason to try and retrofit in Class II vapor retarder in this case?  If so, what product would be recommended for this?  Or is unfaced mineral wool OK?

2) Is there anything I’m missing, or should consider for a retrofit for a house with plaster walls as opposed to gyp wallboard?

Thanks for the help,
Jason