How Do Duct Location and Leakage Affect Title 24 Calculations?
Where ducts sit changes the score as much as the equipment brand. The reference house has sealed, insulated ducts, so ducts in an attic are charged against you, while ducts inside the conditioned envelope can earn credit. Several of those credits only count after an ECC-Rater tests the system.
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The baseline you are scored against
The standard design assumes ducts sealed as Section 150.0(m) requires, the minimum Option B duct insulation and correct refrigerant charge. Mandatory rules apply under both methods: at least R-6.0 insulation, and total leakage no greater than 5 percent of air handler airflow, confirmed by field verification. At rough-in, before the air handler is installed, the ceiling is 4 percent.
Three layouts
- Attic with Option B. Ducts and air handler stay in a ventilated attic that has a high-performance roof and ceiling, and duct insulation follows Table 150.1-A.
- Conditioned space. Ducts and air handler lie entirely inside the envelope, below the ceiling that separates living space from the attic, and are verified to RA3.1.4.3.8.
- Buried ducts. Ducts covered by attic insulation, verified to RA3.1.4.1, which can also excuse the roof-deck insulation rule in climate zones 4 and 8 through 16.
Credits that bring a test
A low-leakage air handler is verified to RA3.1.4.3.9. Conditioned-space and buried-duct credits are verified as described above. Each is a visit from an ECC-Rater, so the credit comes with a cost and a scheduling step. In an unvented attic, ducts may fall to R-4.2 if the rafters carry R-30, the gable ends meet wall insulation and the home tests at 3.0 ACH50 or less.
New homes and additions
A new home with central HVAC has duct leakage, airflow and fan watt draw verified. On an addition, extending an existing duct system by any length brings the combined system under a leakage test: 10 percent of airflow or less, 7 percent to outside, or a rater's smoke test where neither can be met. See extending ducts and, for alterations, the 25-foot rule.
Decide at design stage
The layout is set when framing is drawn, so decide before the calculation. The insulation page and leakage testing page list the field rules, and roof type affects which layouts are open. Our residential report models each layout.
Common Title 24 Questions
- When is Title 24 required for a home?
- Does a remodel need Title 24?
- Does a room addition need Title 24?
- What plans are needed for a residential Title 24 report?
- How much does a residential Title 24 report cost?
- How long does a residential Title 24 report take?
Frequently Asked Questions
It is the mandatory ceiling for total leakage on single-family homes, confirmed by a field test. Going over means the ducts are resealed and retested.
No. They are allowed with an Option B roof and ceiling, but they cost margin compared with ducts inside the envelope.
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