NFRC vs. ISO Window Ratings: How to Read U-Value and SHGC

Same window. Two rating systems. Two different sets of numbers.

If you’ve compared a European window’s spec sheet with an American one, you’ve probably seen it: one document lists an NFRC U-value, the other an ISO U-value, and the numbers don’t line up. It’s easy to assume one of them is inflated. Neither is. NFRC and ISO answer different questions, and once you know what each one measures, you can use both to make better decisions on a project.

Here’s how to read them.

What NFRC actually measures

One number. One size. One very cold night.

The National Fenestration Rating Council (NFRC) rating is the one most people in the US know. It reports whole-window values: the frame and the glass averaged together into a single U-value (heat loss), SHGC (solar heat gain) and VT (visible light).

To keep the results comparable, every manufacturer is rated at the same standard size for each operation type. That’s roughly 4′ × 5′ for fixed and operable windows and 6′8″ × 6′8″ for sliding doors. Every unit is rated against an exterior temperature of −0.4°F.

Using this model makes brands directly comparable, and it’s what most US building code is written to.

Why your window isn’t the test window

Bigger windows usually perform better than their label suggests.

The NFRC test size is a fair yardstick, but very few windows on a real project are exactly 4′ × 5′. And the size matters, because a window’s frame and its glass don’t perform the same.

On most Zola windows, the glass outperforms the frame. So the more glass there is in a unit, the better its whole-window U-value will be. A large fixed window or a lift-and-slide door, where glass makes up most of the area, will typically beat the U-value printed on its NFRC label.

Solar heat gain works the same way. The frame lets through 0% solar gain, while Zola glass gains roughly 25–50%, depending on the glass package. Glass rated at 0.50 SHGC lands at around 0.38 for the whole window at the NFRC test size. On a larger unit, with less frame relative to glass, that number goes up; on a smaller one, it goes down.

What ISO adds

Components, not just a total.

ISO, the international standard used across Europe, also reports an overall window value. But it breaks that value down into its parts:

• Frame U-value: heat loss through the frame alone.
• Center-of-glass U-value: heat loss through the middle of the glass, away from the edges.
• Psi-spacer: the extra heat transfer along the edge where the frame meets the glass.

ISO values are calculated at around 32°F outside, which is closer to an average US outdoor temperature than NFRC’s −0.4°F. They’re reported in metric units, W/m²K, which convert easily to the BTU/hr·ft²·°F used in the US (1 W/m²K ≈ 0.176 BTU/hr·ft²·°F).

Having those three components separately is what makes ISO so useful in design. With them, you can calculate the performance of any size and configuration: the 10-foot fixed unit, the corner window, the full glass wall. That’s why Passive House and Net Zero designers reach for ISO data.

Reading them side by side

Neither is wrong. They answer different questions.

At identical size and configuration, an NFRC U-value will look about 15% worse than the ISO value for the same window. That gap comes from the test temperature (−0.4°F vs. 32°F), not from the window.

SHGC differs too. NFRC SHGC reads lower because the frame, which blocks all solar gain, is included in the calculation. ISO’s solar value describes the glass only.

So which numbers should you use?

Both, for different jobs.

Use NFRC ratings to compare manufacturers on equal terms and to show code compliance. That’s what they were built for.

Use project-specific ISO calculations when the design moves away from the standard test size: an 8′ × 10′ fixed window, a wall of lift-and-slide doors, a curtain wall. At those sizes, the NFRC label stops describing the window you’re specifying. ISO numbers show how high-performance glazing and the actual glass-to-frame ratio drive whole-window performance on the real unit.

At Zola, we provide both. Every project comes with a dedicated project manager who can run ISO calculations for your specific sizes and configurations, alongside NFRC ratings for code.

Planning a project? Start the conversation with our team.