Second-Story Addition: Ducts, Stairwell and Solar Under Title 24
A second story is an addition, and its Title 24 questions come from building upward: where the stairwell sits, where ducts run under the new ceiling, how much cooling capacity the system may have, and what the new floor and walls must include. Solar PV is not required, whatever the roof area.
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The stairwell and the zoning credit
A new floor means a stair opening, and the manual puts a number on it. A living and sleeping two-zone system can earn a performance-method credit only if noncloseable openings between the zones, such as halls and stairwells, total less than 40 square feet. Each zone needs its own return register, the credit is for furnaces only, and bypass ducts are barred on the prescriptive path by Section 150.1(c)13.
Ducts below the new ceiling
Over 700 square feet the attic has two routes. Option B keeps ducts in the attic with the roof-deck measures of Table 150.1-A. Option C puts the air handler and ducts in conditioned space, below the ceiling that separates living space from the attic, verified under RA3.1.4.3.8, so chases get drawn at framing. Without field-verified airflow of 350 cfm per ton, an air conditioner may not exceed the calculated cooling load by more than 6 kBtu/h. The load calculation may not assume more leakage than Table 150.2-C, whose two-story column allows 0.41 heating and 0.21 cooling air changes at 2,001 to 3,000 square feet, against 0.32 and 0.16 for one.
Framing and the floor below
- Above 700 square feet every new wall must meet all of QII, headers included. The wall budget is U-0.048 in zones 1 to 5 and 8 to 16, and U-0.065 in zones 6 and 7.
- A floor over an unconditioned garage is a raised floor, R-19 or U-0.037.
Roof area and solar
A new home sizes its PV by the smaller of Equation 150.1-C or the solar access roof area times 18 watts per square foot on steep roofs. An addition skips both: Exception 6 to Section 150.2(a) exempts it under any compliance approach. Roofing still applies above 300 square feet in zones 10 to 15 (low-sloped in 13 and 15) unless the roof weighs 25 pounds per square foot or more, which tile at 10 does not.
See second-story additions, HVAC load calculations, the 700 square foot addition page, quality insulation installation, residential solar and our residential reports.
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
Only for the zoning credit. Noncloseable openings such as stairwells between living and sleeping zones must total less than 40 square feet for a two-zone system to qualify.
No. Photovoltaic systems are not required for additions under Exception 6 to Section 150.2(a), including a full second floor.
Explore Ducts
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- Can You Use Duct Tape on HVAC Ducts? What Title 24 Allows
- Do I Have to Replace My Old Ducts When I Replace My AC or Furnace?
- Title 24 Duct Insulation Requirements
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