Floor insulation: R-19 in Zones 3 and 4
Georgia amends 2015 IECC Table R402.1.2. For floors:
- Climate Zone 2: R-13
- Climate Zones 3 and 4: R-19
So a house in Climate Zone 4 needs R-19 floor insulation. The traps: R-13 is the floor value for Climate Zone 2 only, R-30 is the cantilevered-floor value, and R-38 is the ceiling value, not a floor value.
Cantilevered floors: R-30 plus blocking
Footnote j to Georgia's Table R402.1.2 covers a cantilevered floor over outside air, such as a second-floor bedroom projecting past the wall below. It requires:
- R-30 insulation (equivalent to U-0.035 under footnote d of Table R402.1.4);
- blocking of the band area above the supporting wall;
- air sealing of any penetrations in that blocking.
R-19 alone is the ordinary floor value, not the cantilever value. A vapor retarder is not what the footnote requires, and R-38 with no blocking is wrong on both counts.
R-30 can satisfy an R-38 ceiling
Under Georgia's amended R402.2.1, where R-38 is required, R-30 over 100% of the ceiling area satisfies it wherever full-height, uncompressed R-30 extends completely over the wall top plate at the eaves.
- Partial coverage such as 80% does not qualify; the text says 100%.
- A radiant barrier is not the condition.
- The trade-off does not apply under the U-factor alternative (R402.1.4) or the total UA alternative (R402.1.5).
HVAC platforms in the attic: R-19
The same Georgia R402.2.1 deems R-19 to meet R-38 beneath an attic HVAC platform, for up to 32 sq ft of decking per HVAC system, plus a passage up to 32 in wide to the equipment, per IRC M1305.1.2.
The traps: the allowance is 32 sq ft per system, not 100 sq ft per house; R-30 over 50 sq ft mixes in the eave trade-off value; and R-13 is the wall value, with no basis for a whole platform.