How Does Roof Type Affect Title 24 Calculations?
In a Title 24 calculation the roof settles three things: which insulation option applies, whether a reflectance rule applies, and where ducts and the air handler may sit. Vented, unvented and cathedral roofs follow different paths, and roof shape also decides whether solar is in the comparison.
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Vented attic: Option B or Option C
Option B is a high-performance vented attic. It pairs ceiling insulation with insulation under the roof deck in climate zones 4 and 8 through 16, and a radiant barrier in zones 2, 3 and 5 through 7. It is also the roof the performance model uses as its reference. Option C puts the ducts and air handler inside conditioned space with ECC verification, takes ceiling insulation of R-38 or R-30 depending on zone, and requires a radiant barrier in zones 2 through 15.
Cathedral and unvented roofs
Cathedral ceilings under Option C need R-38 roof deck insulation in every zone. In zones 4 and 8 through 16 a new attic above conditioned space must have roof-deck insulation to U-0.184, unless ducts and air handler sit entirely in conditioned space, the air handler has 12 linear feet or less of supply duct outside, the system is ductless, or ducts are buried in attic insulation in a performance-method design and verified. Unvented attics get a duct bonus: ducts may be R-4.2 instead of R-6 if the rafters hold R-30, the gable ends meet wall insulation levels and the house tests at 3.0 ACH50 or less. The unvented attic page gives the rest.
Cool roofs
Steep-sloped roofs in zones 10 through 15 need an aged solar reflectance of 0.20 and thermal emittance of 0.75, or SRI 16. Low-sloped roofs in zones 13 and 15 need 0.63 and 0.75, or SRI 75. Roofs weighing 25 pounds per square foot or more are exempt, and building-integrated solar panels are exempt too.
The link to solar
The reference house includes a solar array. No panels are required when the solar access roof area is under 80 contiguous square feet, so a small, shaded or complex roof can remove solar from the comparison, as the solar sizing page shows.
What to decide early
Pick the attic type before the duct layout is drawn. A reroof on an existing home raises different questions; see cool roof rules for reroofing and how insulation enters the model. Our residential report compares roof options.
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
Tile usually leaves an air space under the roofing, which the below-deck insulation values assume. The 25 pounds per square foot weight exemption applies only to roofs that heavy.
No. In the performance approach a radiant barrier is modeled apart from the U-factor, and it also affects duct efficiency.
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