500 Sq Ft Addition Title 24 Requirements
A 500 square foot addition falls in the 401 to 700 square foot tier, where glazing is capped at the greater of 120 square feet or 25 percent of floor area, which at 500 is 125 square feet. Quality insulation installation does not apply. The only 500 square foot line inside the addition rules concerns water heaters.
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Where 500 puts the glazing cap
In this tier the flat 120 square foot figure and the 25 percent figure meet at 480 square feet. Below 480 the flat figure governs; at 500 the percentage does, so total glazing is capped at 125 square feet. West-facing glass stays at 60 square feet in zones 2, 4 and 6 to 15, 12 percent of the floor, so the west wall usually binds first. Glazed doors count toward both limits.
The one 500 square foot line in the addition rules
It sits in Section 150.2(a)1D and applies only when the addition installs an extra water heater. At 500 square feet or less an electric water heater with point-of-use distribution, as specified in Reference Appendix RA4.4.5, is allowed. Above 500 you need a single heat pump water heater, its tank not outdoors and set on an R-10 rigid insulated surface, with a communication interface, or one meeting NEEA Tier 3. Other 500 square foot lines, such as the whole-house fan exemption in Section 150.1(c)12, are written for new dwelling units, not additions.
A worked case from the manual
Example 9-50 adds a 500 square foot room with 120 square feet of new windows to a 1,600 square foot house in zone 12 and extends the existing ducts. Under the performance route the addition's standard design follows Section 150.1(c) and Table 150.1-A, and a duct extension of 25 linear feet or more is assumed altered, with 10 percent leakage. Those 120 square feet of glass are 24 percent of the floor, under the 125 square foot cap.
What does not apply at 500
Quality insulation installation, whole-dwelling ventilation airflow and solar PV do not apply; a new bathroom or kitchen still needs local exhaust. The band ends at the line on the 700 square foot addition page, and the tier below is on the 300 square foot addition page. See also water heater requirements, room additions and our residential reports.
Common Title 24 Questions
- When is Title 24 required for a home?
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Frequently Asked Questions
Prescriptively 125 square feet in total, the greater of 120 square feet or 25 percent of floor area, with no more than 60 square feet facing west in zones 2, 4 and 6 to 15.
Only if the addition adds a water heater and uses an electric water heater with point-of-use distribution, an option Section 150.2(a)1D allows at 500 square feet or less.
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