Understanding the footnote for R13+5 wall insulation

[BillS]

I'm designing a home in Climate Zone 5, IRC2009. The options for wood-framed walls are R-20 or R-13 cavity+R-5 insulated sheathing. I don't fully understand what footnote h is saying...

I take it to read that, if the structural sheathing (e.g. plywood) covers <25% of the walls, then insulating sheathing is not required... so it'd be just R-13; if the structural sheathing covers >25% of the walls, then the insulated sheathing only needs to be R-2.

If this is the case, why is it called 13+5 and not 13+2? If it's not, what am I reading incorrectly?

Thanks

Bill

 

[JBI]

At 25% or less you do not need the insulated sheathing over the structural sheathing panels.
At more than 25% a minimum of R-2 is required over the structural panels with R-5 over the rest.
The reduction over the structural sheathing allows a uniform surface for wall covering/siding attachment.
Not sure why it is not required at less than 25% structural but, the footnote (and the requirement) is changed in the 2015 and makes more sense. Also the cavity only goes away in climate zone 5 in the 2015.

Can an Admin move this to Residential Energy?

 

[steveray]

Bill must be from the northern half...not everyone uses structural sheathing, especially in the South...