One wall option is to scale back the double stud wall to a single stud with a layer of exterior insulation. This approach should end up at an R25 wall and still allow it to dry to both sides.
The diagram below from Building Science Corporation shows where we are going -- except replacing the 2x6 wall with a 2x4 wall so we can use a class III vapor retarder (latex paint)
The diagram below from Building Science Corporation shows where we are going -- except replacing the 2x6 wall with a 2x4 wall so we can use a class III vapor retarder (latex paint)
The table below outlines the required exterior insulation thickness to eliminate the layer of poly -- and just use latex paint as a vapor retarder. Mineral wool board has an R value of about 4. So, we will need about 3 inches on a 2x4 wall.
Some other good reading on Vapor Retarders and what is required:
http://www.greenbuildingadvisor.com/blogs/dept/musings/vapor-retarders-and-vapor-barriers
http://www.greenbuildingadvisor.com/blogs/dept/musings/do-i-need-vapor-retarder