The best time to insulate exterior walls is when renovation or demolition is required. Exposing the wall studs from the interior side of the wall to repair fire damage offers you the best of opportunities.
- Wall assemblies need to be designed so they can dry either to the interior or exterior in the event of water penetration.
- The air gap between the brick and the 1x8 board sheathing is designed as a drainage plane prevents water from wicking into the wall assembly.
- The space also allows moisture drive from interior to exterior to vent to atmosphere without damaging the wall assembly.
I recommend closed cell spray foam insulation
If this were my home I would have the wall insulated with closed-cell, spray-applied foam insulation.
- Closed-cell foam is a superior insulation, an effective air barrier and moisture barrier.
- It will prevent moisture drive from the interior (as well as heat loss) and prevent exterior moisture/water from driving into the wall assembly from the exterior.
Closed-cell insulation has an aged R-value of approximately 6 per inch. Three inches of material would qualify for any code jurisdiction requiring an R-19 cavity (IECC 2009).
But consider using open cell foam
I recommend closed-cell foam because you mentioned that you need a vapor barrier. If a vapor barrier is not required in a rehab situation you might consider using open-cell, spray-applied foam.
- The R-value is lower per inch (3.88 - 4) but it performs nearly as well and it has elasticity and memory so it moves with the structure, maintaining contact with the wood framing members.
- Since the cell structure is open water and/or vapor can pass through the material.
This isn't a big concern unless you have very high humidity levels in the home. In this case the humidity can drive through the open-cell foam and potentially condense on the surfaces of the 1x8 planking with possible mold or decay issues.
I would caution that high humidity in the home is highly unlikely in winter months unless you have a humidifier and don't use the bathroom exhaust vent fans properly.
In spite of everything this is my preferred insulation product if code doesn't require a vapor barrier.
- I am not a fan of vapor barrier paints applied to the interior surfaces.
- Moisture driven from the outdoors could become trapped in the wall cavity creating potential structural problems.
- Another advantage of open-cell (half-pound foam) insulation when compared to closed-cell (two-pound foam) insulation is lower cost.
Good luck on your renovation project.