Split insulation seems much more common and I am trying to understand the relative importance of the various factors that contribute to its greater popularity. These factors might include (but aren’t limited to): cost, ease of installation, setback requirements, tax considerations, moisture management, etc 

Since the factors are likely going to vary depending on the climate, let’s say the effective R-value desired is R-40. (Picked arbitrarily because it is on the high end, but not all the way to Climate Zone 8+ where they are probably thinking about exterior-only for durability reasons anyways).

I priced out the two wall types using Comfortboard 80 and dense pack cellulose, not including labor or fasteners, and for a 2×6 wall, exterior-only insulation was about 20-25% more expensive than the split insulated.

Some questions that come to mind:

1. If you’re already installing exterior insulation, how much more difficult is installing 10″ (2×5″) vs 6″ (2×3″) continuous exterior insulation? Is it so much more painful that most contractors won’t even consider it?

2. Would going exterior-only skip a significant amount of labor on the inside, and would it make sequencing easier? Or is fitting batts or putting up netting and blowing cellulose significantly easier and cheaper than increasing the thickness of the exterior insulation layers by 2″ each, and the sequencing with things like electrical isn’t really a big deal?

3. Is it a fastener thing? Do you have to special order  fasteners for 6″ of exterior insulation anyways? Are there fasteners even available that would work for 10″ of exterior insulation?

4. With mineral wool board specifically, is the compressive strength a factor? 

5. Does increasing the exterior insulation by 4″ make windows, doors, or other penetrations significantly more difficult to install?

6. Are these considerations an artifact of the desired R-value chosen? i.e. does it only even begin to make sense to consider exterior-only for certain climate zones or performance requirements?

I’m sure there are other poignant questions, but due to my experience these are the only ones I can think of for now.

I didn’t really consider things like taxes or setback requirements because they vary so much depending on location, but I’m guessing setbacks matter considerably on tighter lots.