How Do House Size, Stories and Bedroom Count Affect Title 24 Calculations?

The inputs, in order

Conditioned floor area is the floor area of enclosed conditioned space on all floors, measured at the floor level of the exterior surfaces of the exterior walls. The software needs the number of stories, the occupancy type and the climate zone. Reference house floor area and volume match yours, so a larger house gets a larger budget.

Where size changes the equipment

• The total whole-dwelling ventilation rate is 0.03 times floor area plus 7.5 times the sum of bedrooms and one, in cfm, so both area and bedroom count change the fan.
• Required solar size follows floor area and number of dwelling units through Equation 150.1-C.
• Window limits are percentages of floor area, so a bigger house earns more glass; the window area page has the arithmetic.

Thresholds that change the rules

New dwellings of 500 square feet or less need no whole-house fan, may use an electric water heater with point-of-use piping, and in zone 5 may keep a window U-factor of 0.30. A unit with one bedroom or fewer may use a 120 volt heat pump water heater.

Additions have tiers. An addition of 400 square feet or less may have the greater of 75 square feet of glass or 30 percent of its floor area when it is modeled with the existing house; modeled alone it is held to 20 percent. Between 400 and 700 it is the greater of 120 square feet or 25 percent, and above 700, 175 square feet or 20 percent. Quality insulation installation does not apply at 700 or less. Additions that add 1,000 square feet or less to a dwelling skip the whole-dwelling ventilation airflow rule. Solar is not required on additions.

Common input mistakes

Counting a garage as conditioned or measuring to the inside of the walls skews floor area, and the story count must match the elevations. Pricing steps are on the cost page, and the 500 sq ft addition page covers that one line. Ventilation sizing is in fan sizing. Start a residential report or an ADU report with the plan numbers.

Common Title 24 Questions

1. When is Title 24 required for a home?
2. Does a remodel need Title 24?
3. Does a room addition need Title 24?
4. What plans are needed for a residential Title 24 report?
5. How much does a residential Title 24 report cost?
6. How long does a residential Title 24 report take?

Frequently Asked Questions

Does a bigger house make Title 24 harder to pass?

Do bedrooms matter to the energy report?

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