Why energy ratings matter more than the sales pitch
Every window replacement salesperson in San Diego will tell you their windows are “energy efficient.” What that actually means, and what the California Title 24 energy code requires, is a different conversation.
Two numbers determine how a window actually performs: the U-factor and the Solar Heat Gain Coefficient (SHGC). Understanding those numbers, and knowing which ones Title 24 requires for San Diego’s specific climate zones, lets you compare quotes on real terms instead of marketing claims.
U-factor: how well the window holds heat
U-factor measures how quickly a window transfers heat. Lower is better: a lower U-factor means less heat escapes in winter and less heat enters in summer through the glass itself.
Single-pane glass has a U-factor around 1.0. Standard double-pane without low-E coating runs about 0.50. Double-pane with a low-E coating runs 0.25-0.35. Triple-pane with two low-E coatings can reach 0.15-0.20.
For San Diego’s mild coastal climate, triple-pane is generally not cost-justified. The incremental heat loss reduction does not pay back the higher window cost in a climate that rarely sees freezing temperatures. A quality double-pane low-E window at U-0.25 to U-0.30 is the right call for almost all San Diego residential replacement projects.
Solar Heat Gain Coefficient: how much sun heat gets through
SHGC measures how much solar radiation passes through the glass as heat. A rating of 1.0 means all solar heat passes through; 0.0 means none does. Lower SHGC means less heat gain from direct sunlight, which matters most on west and south-facing windows in San Diego’s sun-heavy climate.
Here is the tension: low SHGC reduces cooling load in summer but also reduces passive solar heat gain in winter. In a San Diego winter that rarely drops below 45°F, this is generally not a concern. Ask the installer for the labeled SHGC on west-facing glass and match the permitted schedule for Climate Zones 7 and 10.
For north-facing windows that see little direct sun, SHGC is not a meaningful factor. For south-facing windows in homes that want passive solar in winter, a moderate SHGC of 0.30-0.40 may be preferable to a very low one.
What Title 24 requires for San Diego
California’s Title 24 energy code sets U-factor and SHGC by climate zone. San Diego County is Climate Zones 7 (coast) and 10 (inland). It is not Zone 9. The 2025 Title 24 package, in force January 1, 2026, uses a 0.30 prescriptive U-factor for fenestration in those zones, with a 0.40 mandatory maximum. SHGC is on the NFRC label and on the permitted window schedule. This page does not invent a west-facing SHGC cap from memory.
The inspector checks the NFRC label on the unit, not a brochure. If a supplier cannot provide NFRC-rated windows, that is a problem.
The NFRC (National Fenestration Rating Council) certification is the third-party rating system that Title 24 references. Every window you buy for a permitted replacement should have an NFRC label attached. If a supplier cannot provide NFRC-rated windows, that is a problem.
Low-E coatings: what they actually do
Low-E (low-emissivity) coatings are thin metallic layers applied to the glass surface that reflect infrared radiation while allowing visible light to pass through. In practice, they keep more heat on the side it started on: they keep warmth inside in winter and reflect solar heat away in summer.
There are two families of low-E coatings:
Hard-coat (pyrolytic) low-E is applied to the glass during manufacturing while the glass is still molten. It is more durable but less effective as an emissivity reducer than soft-coat.
Soft-coat (sputtered) low-E is applied after manufacturing in a vacuum chamber and provides better thermal performance. It must be protected within an insulated glass unit (sealed between panes) because it degrades if exposed to air and humidity. Most quality replacement windows use soft-coat low-E.
For San Diego applications, a dual-zone low-E coating optimized for both U-factor and SHGC control gives you the best balance of heat rejection in summer and heat retention on the rare cold nights. Milgard’s SunCoat, Andersen’s High-Performance Low-E4, and Simonton’s Stormbreaker Plus are examples of products designed for western U.S. climates.
Climate zones within San Diego County
San Diego County is not a single climate. The CEC maps it as Climate Zone 7 on the coast and Climate Zone 10 inland:
Climate Zone 7 (San Diego, La Jolla, Coronado, Del Mar, Encinitas, Carlsbad, Oceanside): Marine layer and milder summers. Spec dual-pane low-E that meets the 0.30 prescriptive U-factor. Read SHGC off the label for west glass.
Climate Zone 10 (Escondido, San Marcos, El Cajon, Santee, La Mesa, Ramona, Alpine, Fallbrook, Descanso): Inland heat. West-facing rooms are the cooling load. Same 0.30 U-factor. Ask for the SHGC the plans require. This is not a triple-pane climate for most houses.
For guidance on how window selection integrates with a full energy-efficient window replacement project, the service page walks through the selection process in more detail.
Rebates and tax credits
Energy rebate programs change and can run out. The federal 25C window credit ended for windows placed in service after December 31, 2025. Confirm SDG&E or manufacturer offers in writing. Do not sign a contract that depends on a rebate that is not documented.
The Energy Star certification (which requires meeting specific U-factor and SHGC thresholds) is a reasonable proxy for rebate eligibility in most programs, but program requirements shift independently of the certification. Ask your installer which specific program they are referencing and confirm eligibility in writing.
What to ask your installer
Before signing a window replacement quote, verify three things related to energy performance:
- What is the NFRC-certified U-factor and SHGC on the specific window unit being quoted? (Not the glass only, the whole-window rating.)
- Does this window meet Title 24 requirements for my climate zone? (The installer’s responsibility, not yours, but you should confirm it.)
- Is the permit pulled in my name or the company’s name? (It should be in the company’s name, and it should include a field inspection that verifies the NFRC compliance.)
For the full explanation of retrofit vs. full-frame installation and when each method applies to your specific Title 24 situation, see the installation guide.
The bottom line
Title 24 for replacement windows in this county means meeting the 2025 package for Climate Zones 7 and 10: 0.30 prescriptive U-factor, 0.40 mandatory maximum, plus the SHGC on the permitted schedule. Get NFRC-rated product and a real permit where the department requires one.
Call (858) 400-6418 to connect with a local window crew that pulls permits, uses NFRC-certified products, and installs to Title 24 compliance across San Diego County.