Solar Tubes (Tubular Daylighting Devices): Title 24 Rules
A solar tube is a tubular daylighting device and is treated as a skylight. For each dwelling unit, up to 3 square feet of tubes with dual-pane diffusers need not meet the Table 150.1-A U-factor and SHGC. The area still counts toward the 20 percent glazing limit, and the manual caps its U-factor at 0.40.
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What earns the 3 square foot allowance
The adopted exception to Section 150.1(c)3A allows up to 3 square feet of tubular skylight area with dual-pane diffusers, per dwelling unit, to skip the table values. That sits beside a separate allowance of 3 square feet of glazing in doors. Dual pane has a definition here: two panes with an air space under 2 inches (50 mm) between them at the plane of the ceiling insulation. A single acrylic diffuser at the ceiling does not qualify, however good the dome on the roof looks.
Where the numbers come from
A device with a single-pane diffuser has three routes to a U-factor. Use the NFRC 100 label if the product is rated. Otherwise take the default for a metal-frame, fixed, single-pane skylight, 1.19, shown as "CEC Default U-factor". The third route is Nonresidential Appendix NA6, Equation NA6-1; the manual's worked case with a center-of-glass value of 1.20 comes to 1.25. SHGC works the same way: label, a 0.83 default, or Equation NA6-2, which turns a center-of-glass 0.85 into 0.81. Visible transmittance of these devices is rated under NFRC 203.
What the allowance does not waive
- The tube area still counts toward the 20 percent total fenestration area.
- The manual states that its U-factor shall not exceed 0.40.
- Section 150.0(q) sets a weighted-average U-factor of 0.40 for skylights too, with 10 square feet or 0.5 percent of floor area exempt.
Adding one to an existing roof
An alteration that adds up to 16 square feet of fenestration or skylight area does not have to meet the total and west-facing area limits, but the added glazing still faces the U-factor and SHGC values. Whether a single tube is judged under the 3 square foot exception or the standard values is a question for the plan checker, so put the diffuser type and label on the drawings.
For glass skylights, see skylight requirements, skylight replacement and the ADU skylight page. How labels are read is on NFRC labels, and our residential reports carry the entry for you.
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
Yes. The manual includes tubular device area in the 20 percent maximum fenestration area, even when its U-factor and SHGC are exempt from the table.
Use the default for a single-pane metal-frame skylight, 1.19 for U-factor and 0.83 for SHGC, or calculate with Equations NA6-1 and NA6-2. Each default must be labeled as a CEC default.
Explore Skylights
- Title 24 Skylight Requirements for Homes
- How Many Square Feet of Skylight Can I Add Under Title 24?
- Skylight Replacement: Title 24 Requirements
- Skylight Shaft Insulation: What Title 24 Expects Around the Light Well
View all Skylights guides → · Windows and Building Envelope · Title 24 guide