Ask a California builder what a window "has to be", and the answer comes down to two numbers: U-factor and SHGC. Everything else — the brand, the frame profile, the glass stack — is in service of those two figures, plus the structural performance grade behind them.
This is the practical guide to reading those numbers, what the NFRC certifies, and what Title 24 actually requires — including why a window with a higher U-value can still be legal.
The NFRC label: five numbers, one council
The National Fenestration Rating Council (NFRC) is the independent body that certifies window energy ratings in the United States. An NFRC label is what makes a U-factor or SHGC claim verifiable — without it, a number is just a manufacturer's word.
The label carries five ratings:
| Rating | What it measures | Better is |
|---|---|---|
| U-factor | Rate of heat transfer through the whole window (frame + glass + edge) | Lower |
| SHGC — Solar Heat Gain Coefficient | Fraction of solar heat admitted through the window (0–1) | Lower in hot climates, higher in cold |
| VT — Visible Transmittance | Fraction of visible light admitted (0–1) | Higher |
| AL — Air Leakage | Air passing through the assembly (cfm/ft²) | Lower |
| CR — Condensation Resistance | Resistance to interior condensation (1–100) | Higher |
The single most important distinction to check when sourcing from China:
Whole-window vs center-of-glass. An NFRC U-factor is a whole-window number — it includes the frame, the spacer and the edge of the glass. A factory that quotes you a "U-value" of 0.30 may be quoting the center-of-glass value of the glass itself, which ignores the aluminium frame conducting heat around the perimeter. The whole-window number is always worse (higher). When two quotes differ on "the same" glass, ask whether the number is NFRC whole-window or centre-of-glass — it is the most common source of a pleasant number that fails compliance later.
The structural companion: NAFS and the Performance Grade
Energy codes set the U/SHGC limits, but a window also has to hold up. In North America that is governed by NAFS — AAMA/WDMA/CSA 101/I.S.2/A440 — which defines four Performance Classes (R, LC, CW, AW) and a Performance Grade (PG), from PG15 upward.
Each PG is defined by a Design Pressure (DP), and the test regime hangs off it:
- Structural: ASTM E330, loaded to DP, then a proof load at STP = 1.5 × DP with no permanent damage.
- Water: ASTM E331, sprayed at 3.4 L/m²·min under a pressure of 15% of DP (minimum 140 Pa).
- Air: ASTM E283, at ±75 Pa.
For a house, the relevant classes are R (residential) and LC (low-rise). When you see "PG30" on a datasheet, it means the window was structurally tested to 1440 Pa and proven at 1.5× that — a far more useful number than "hurricane rated" or "high quality".
Title 24: California's energy code
Title 24, Part 6 is the California Energy Code. It divides the state into 16 climate zones, and sets prescriptive limits on window U-factor and SHGC for each. Temecula and the Inland Empire sit in Climate Zone 10 — a hot, inland zone where the point of the window is to keep heat out.
The key prescriptive limits for residential fenestration (the default you build to):
- U-factor: 0.30 — this is the prescriptive U-factor across climate zones.
- SHGC: 0.23 in the hotter zones (including Climate Zone 10), 0.25 in the cooler coastal and northern zones.
So for a Temecula home, the prescriptive target is U-0.30 / SHGC-0.23. A window that meets that, NFRC-certified, is compliant as a matter of course.
Why a higher U-value can still be legal: the performance path
Here is the part most people get wrong, and it matters when you are watching budget.
Title 24 is not only prescriptive. It also offers a performance compliance path: a whole-building energy model that lets you trade off one element against another. If the wall insulation, roof, or mechanical system over-performs, the windows can under-perform the prescriptive number — slightly — and the building still passes overall.
We saw exactly this on a recent Climate Zone 10 project. The client's approved Title 24 baseline was U-0.34 / SHGC-0.28 — a looser window than the prescriptive U-0.30 / SHGC-0.23 — because the package was modelled as a whole and compensated elsewhere.
The practical point for a buyer: the prescriptive number is not always the number you must build to. An energy consultant can tell you whether your project can afford a slightly higher U-value in exchange for a cheaper window. What you cannot do is skip the compliance check entirely, or accept a centre-of-glass number as if it were whole-window.
What this means when sourcing the glass
The U-factor and SHGC are delivered by the glass stack, and the two levers are:
- Low-E coating — cuts SHGC and lowers U. A double-silver Low-E gets SHGC lower (into the 0.22–0.25 range) than a single-silver, for a modest cost premium per square metre.
- Argon fill — lowers U by reducing conduction through the cavity.
For a Title 24 CZ10 home chasing U-0.34 / SHGC-0.28, a double-glazed unit with a double-silver Low-E and argon comfortably clears the SHGC, and the U-value is then a function of the frame and the spacer as much as the glass.
The trap to avoid: specifying "Low-E" without a number. "Low-E" is a category, not a performance. The only figure that counts is the NFRC whole-window U-factor and SHGC on the certified label — everything else is a description of intent, not a compliance value.
The buyer's checklist
- Ask for the NFRC label, not a brochure U-value. Confirm it is whole-window, not centre-of-glass.
- Confirm the climate zone the numbers were checked against — Zone 10 is not Zone 3.
- Know your prescriptive target (U-0.30 / SHGC-0.23 for CZ10), and ask your energy consultant whether the performance path lets you relax it.
- Check the structural PG alongside the thermal numbers — a U-0.30 window that fails NAFS is not a window, it is a liability.
- When a factory quotes the glass, ask what coating (single or double silver), what gas fill, and what the resulting certified U/SHGC is — in that order.
Where this fits in a Foshan buy
A Foshan factory can produce glass to almost any U-factor and SHGC you specify. The failure mode is not capability — it is specification: a buyer who asks for "energy-efficient glass" instead of "NFRC-certified, whole-window U-0.34 / SHGC-0.28" receives whatever the factory finds cheapest to call energy-efficient.
Our drawing review pins the U-factor and SHGC to a certified value before production, and our production monitoring checks the glass spec and the frame at the section — because the difference between a compliant window and a non-compliant one is almost never visible from the outside, and is almost always expensive to discover after the container has landed.