7 min read · Cost
Every window has a U-factor and SHGC number on its NFRC label. These two numbers determine California Title 24 compliance and tell you most of what you need to know about energy performance. Here's how to read them correctly.
U-factor — heat conducted through the window
U-factor measures how much heat conducts through the entire window assembly (frame + glass) at a given temperature differential. Lower is better — lower U-factor means less heat conducted through (less heat loss in winter, less heat gain in summer through conduction). Standard ranges: high-performance double-pane 0.25-0.30; triple-pane 0.18-0.25; older single-pane and basic dual-pane 0.40-0.60.
SHGC — solar heat gain coefficient
SHGC measures the fraction of solar heat that passes through the window. Lower SHGC = less solar heat gain. This is the cooling-load number — critical in valley and southern California where summer cooling dominates the energy bill. Modern low-SHGC glass: 0.15-0.30. Standard low-e glass: 0.35-0.45. Clear glass without low-e coating: 0.65-0.75.
California climate zone targets — Title 24
California's Climate Zones determine target U-factor and SHGC. Sacramento (Zone 12): U≤0.30 and SHGC≤0.25 for compliance on many configurations. San Jose (Zone 4): similar but slightly less restrictive. Truckee (Zone 16): lower U-factor target (heating-load critical), SHGC less critical. The exact targets vary by orientation, area, and other factors; a Title 24 calc determines specifics.
The marketing trap — emphasis on U-factor vs. SHGC
Window marketing emphasizes U-factor (heat conduction) because the numbers sound impressive and the industry is more focused on it. In Sacramento and most California climates, SHGC is at least as important — the summer cooling load dwarfs the winter heating load. A window with great U-factor but poor SHGC misses what California homes actually need.
Reading the NFRC label
Every replacement window must show an NFRC label with U-factor, SHGC, visible transmittance, and air leakage. Insist on seeing labels at install — that's how you verify what was specified is what was delivered. Don't accept windows without NFRC labels.
Orientation matters
South- and west-facing windows take the biggest solar heat gain; low-SHGC glass is most valuable here. North-facing windows take essentially no direct solar gain; SHGC is less critical (you can use higher-SHGC for visible transmittance without summer penalty). Some specifications use different glass on different orientations.
Practical Sacramento spec
Standard valley home: U-factor ≤0.30, SHGC ≤0.25. This typically means dual-pane low-e with appropriate glass selection. Premium spec: U-factor ≤0.25, SHGC ≤0.20 — better but with diminishing returns. Triple-pane for Sacramento is usually overkill on energy economics; for noise reduction it can make sense on freeway-adjacent homes.
Practical Tahoe spec
Heating-load climate: U-factor ≤0.25 is the priority, SHGC less critical (winter solar gain is welcome). Triple-pane for Tahoe is often justified on both energy and comfort grounds. Modern dual-pane with very low U-factor coatings is the lower-cost path to similar performance.
California window spec targets by climate
| Climate / location | U-factor target | SHGC target |
|---|---|---|
| Sacramento Valley (Title 24 Zone 12) | ≤0.30 | ≤0.25 |
| Bay Area (Zone 4) | ≤0.32 | ≤0.25 |
| Foothill (Zone 11-12) | ≤0.30 | ≤0.25 |
| Tahoe (Zone 16, heating-load) | ≤0.25 | ≤0.35 (less critical) |
| Coastal Marin (Zone 3) | ≤0.32 | ≤0.30 |
Key takeaways
- U-factor measures conduction; SHGC measures solar heat gain
- California cooling climates need low SHGC
- California heating climates (Tahoe) prioritize low U-factor
- Read the NFRC label; don't accept windows without one
FAQ
Quick Answers
≤0.30 for code compliance; ≤0.25 for premium performance.
On pure energy economics, usually not — the cooling-load benefit doesn't justify the premium. On noise reduction near busy roads, it can.
Yes — west-facing takes the most afternoon heat; low-SHGC most valuable there.
Sources
Authoritative references
- ENERGY STAR — Residential Windows, Doors & Skylights
- National Fenestration Rating Council (NFRC) — window performance ratings
- California Energy Commission — Title 24 Building Energy Efficiency Standards
- Contractors State License Board (CSLB) — verify a California contractor
External links to government, code, and manufacturer sources. Sierra Siding is not affiliated with these organizations; references are provided for verification.