I’m in the planning phase of adding external insulation, new windows and siding to a 1906 craftsman style 3-story house in ND. Making the trade-offs of cost of materials, simplicity of install (doing it ourselves with help from neighbor who is a contractor) and long term durability. 

Most of the articles recommend 2 layers so the seams can be offset. Foam can shrink over time and the gap could allow air in/out as well as some heat loss. We will be taping the joints. Undecided if the foam will be the WRB or if there will be housewrap over the old sheathing.  With ZIP panels, just one layer with seams taped, why can’t other sheet products be 1 layer?

For my goal of R15:
3″ of XPS at nearby Menards is ~ $60 per sheet.
2 layers of 1.5″ the cost is $84 for the same overall thickness. 

The cost to go to 2 layers is $24 per 32 sq ft (4×8 sheet). Assuming I need 65 sheets, the added cost for 2 layers vs 1 is $1560.  Plus the added labor to install the 2nd layer. 

I’m sure the overall envelope will be improved but is it enough to offset the added cost? Looking for the rationale to do 2 layers when the payback seems small. Likely responses are it’s a drop in the bucket compared to the overall project. Do it right, don’t skimp. 

I don’t recall seeing an article that addressed this aspect of added external insulation.