Glass Block Windows Under Title 24: Defaults, Labels and Averaging
Glass block is allowed, but it is rated from default tables and cannot reach the 0.40 U-factor limit by itself. Use the default for the frame and product type, post a temporary default label, caulk and weatherstrip the unit, then fit the area inside the 10 square foot exemption or average it with better windows.
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Which default value applies
The compliance manual treats glass block as a field-fabricated product, so there is no maker's label to read. For U-factor, take the Table 110.6-A figure for the frame and product type that holds the blocks; an operable, metal-framed unit reads 0.87. For SHGC, use the Table 110.6-B glass block value. That table has no tinted entry, so the clear block value stands in for tinted block. The Standards also send unrated translucent or transparent panels to the glass block values.
Label and sealing
A field-fabricated product may be installed only if the compliance documents show compliance using the default values. The manual asks for a temporary label carrying those values so the inspector can compare them. Under Section 110.6(b) the unit is caulked between product and building, then weatherstripped; only unframed glass doors and fire doors are excused.
Why the 0.40 limit bites
The manual states that no product in the default tables meets the mandatory 0.40 U-factor individually. Section 150.0(q) is a weighted average, so block panels need company. Either they fall inside the exempt area, which is the first 10 square feet of glazing or 0.5 percent of floor area if larger (15 square feet in a 3,000 square foot house), or rated windows elsewhere pull the average down. Buildings under the California Wildland-Urban Interface Code, Title 24 Part 7, in designated fire areas are excepted from the cap.
On the prescriptive path and on the plans
The prescriptive limit of 0.27 or 0.30 is also an area-weighted average, and a miss moves the project to the performance method. List each block opening on the window schedule with its default values, because the plans examiner compares the schedule with the report. Swapping a block opening for a rated window replaces the default with its NFRC figures.
Related reading: what happens when a window has no label, windows and Title 24, mixing window types and NFRC labels. Our residential reports run the average 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
No. The manual treats it as field-fabricated, so it uses default values and a temporary default label, with the same values in the compliance report.
Not in the default table. The manual tells you to use the clear glass block SHGC for both clear and tinted block.
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