Georgia's table, not the model code
The prescriptive envelope values come from the Georgia-amended 2015 IECC Table R402.1.2, covering Climate Zones 2, 3 and 4. The common errors all come from quoting the unamended 2015 IECC instead of Georgia's figures.
Values that are the same in zones 2, 3 and 4
- Fenestration U-factor: U-0.35.
- Glazed fenestration SHGC: 0.27. Swapping them into U-0.27 and SHGC 0.35 is a trap, and SHGC 0.40 is not in the table.
- Ceiling: R-38. Not R-30, not R-49.
- Wood-frame wall: R-13. R-20 and R-13+5 are the unamended IECC values for zones 3 and 4, and using them is the listed common error.
- Attic kneewall: R-18. Georgia added this row. Leaving it out is a listed common error, and giving the kneewall R-13 treats it like an ordinary wall.
- Slab: 0. Georgia's table requires no slab insulation; R-5 is not in it.
Values that change by zone
- Skylight U-factor: U-0.65 in zone 2; U-0.55 in zones 3 and 4. Do not confuse U-0.55 with a window value.
- Mass wall: 4/6 in zone 2; 8/13 in zones 3 and 4.
- Floor: R-13 in zone 2; R-19 in zones 3 and 4. R-19 is a floor value, not a wall value.
- Basement wall: 0 in zone 2, 5/13 in zone 3, 10/13 in zone 4.
- Crawl space wall: 0 in zone 2, 5/13 in zone 3, 10/13 in zone 4.
How to keep the zone values straight
Basement and crawl space walls step up together: 0, then 5/13, then 10/13 as you move from zone 2 to zone 4. The 8/13 figure belongs to mass walls, not basements, so a basement answer of 8/13 is wrong in every zone. In zone 3, the correct pairing is slab 0 with crawl space wall 5/13.
When a question gives a U-factor, check what it describes: 0.35 is the window, 0.65 or 0.55 is the skylight, and 0.27 is never a U-factor at all; it is the SHGC.