I built my home in 1995 and I insulated the walls with R22 glass batts between 2×6 studs and then 1 1/2″ codebord around the perimeter outside. The house was tightly sealed with 6 mil poly and acoustical sealant on the interior. At the time I was advised not to tape the joints or use sheeting on the outside of the codebord and to just strap it and apply my siding. The walls were braced with metal T Straps for wind shear resistance.

After 25 years I need to take the siding off the house as the strapping has split in many places and is allowing the siding to come loose. As well, I have had a few mice get up between the siding and the foam and burrow through the foam. At the same time, I am thinking of adding another 1″ or so of foam to the exterior. I have read that I should have R12 on the exterior to keep the dew point inside the foam on the coldest nights rather than having it inside the wall cavity where condensation could form. I am in Southern Ontario at the West end of Lake Ontario and we can get down to 4 Deg. F occasionally in the winter. In the summer we can have weeks of 85 Deg or higher with high humidity.

Should I use sheeting and an air-water barrier over the new foam I am planning to add? I have had occasion to open up a few places of the exterior walls and I have not seen any signs of water, but I do not want to suffer any unintended consequences.