Georgia's leakage limit: 6 cfm per 100 sq ft
Under the Georgia-amended 2015 IECC R403.3.3, duct leakage testing is mandatory. Total leakage must be 6 cfm or less per 100 sq ft of conditioned floor area, at both the rough-in test (air handler installed) and the post-construction test, tested at 0.1 in w.g. (25 Pa).
The trap is citing the unamended model 2015 IECC: its 4 cfm figure, or its 3 cfm rough-in figure without the air handler, are not Georgia's limit. 8 cfm is not in the code.
Computing the allowance
Maximum leakage = conditioned floor area ÷ 100 × 6 cfm.
Worked example: a 2,400 sq ft house. 2,400 ÷ 100 = 24, and 24 × 6 = 144 cfm.
Wrong answers come from the wrong rate:
- 24 × 4 = 96 cfm uses the model code figure.
- 24 × 3 = 72 cfm uses the model rough-in figure.
- 240 cfm would be 10 cfm per 100 sq ft, which is not a code figure.
Who is exempt from the test
Georgia's R403.3.3 exempts:
- systems where the ducts and air handler are entirely within the building thermal envelope;
- existing systems unless more than 50% is modified.
So ducts in a vented attic are outside the envelope and must be tested. Replacing 75% of an existing system is more than 50%, so it triggers the test. A rough-in test with the air handler installed is one of the required tests, not an exemption.
Duct insulation in attics: R-8 and R-6
2015 IECC R403.3.1, which Georgia does not amend, sets attic supply and return duct insulation by diameter:
- 3 in diameter and larger: at least R-8
- Below 3 in: at least R-6
A 6 in supply duct needs R-8; a 2 in duct needs R-6. R-6 for both undersizes the larger duct, R-8 for both overstates the requirement for the small one, and R-13 is a wall value, not a duct value.