Certified or simulated? How to check a U-factor.
Where to find the NFRC certificate number, and what it means when there isn't one.
Vistaza Engineering · Mar 30, 2023 · Updated Sep 15, 2026 · 4 min read
Every window U-factor starts life as a computer simulation. What separates a number you can put in an energy model from one you cannot is whether it was certified afterwards. The check takes a few minutes.
What the number is.
U-factor is the rate at which heat passes through a window. Lower is better. The National Fenestration Rating Council (NFRC) sets how it is worked out, so that one maker's number can be compared with another's.
An NFRC U-factor is a whole-window number. It combines three zones: the frame, the edge of the glass, and the center of the glass. The center of the glass is always the best of the three. A U-factor quoted for the glass alone will look better than the window it goes into. So the first question is simple: is this the whole window, or only the glass?
How NFRC gets there.
Two procedures do the work.
- NFRC 100, the simulation. Software models the frame, the glass edge and the center of glass in cross-section, and combines them into one U-factor for the whole product.
- NFRC 102, the physical test. A real window is built into a wall between a warm room and a cold room. The warm side is held at 70°F. The cold side is held at 0°F with a 15 mph wind. The heat passing through the window is measured.
The test is there to confirm the simulation. Once the model is confirmed, it rates every glass option in the series. That is how one window series can carry hundreds of ratings without hundreds of tests.
Certified versus simulated.
Simulated is not an insult. Every rating is simulated. The difference is what happened next.
A certified rating has been through the NFRC program. The simulation was checked against a physical test, an independent inspection agency licensed by NFRC reviewed the reports and inspects the plant, and NFRC certified the product and listed it in its Certified Products Directory under its own number, the CPD number. The window ships with an NFRC label.
A simulated-only value means someone ran the software. Nobody outside the company checked it, and it is not in the directory. It may be right. It may also describe a window you are not getting: a different spacer, a different gas fill, or no grilles.
No CPD number, no certified rating.
Vistaza's glass library marks each glass as certified or simulation only. This site states only the certified values. For a Vi6 casement, without grilles:
| Glass on a Vi6 casement | Certified U-factor |
|---|---|
| Standard insulated double-pane | 0.27 |
| Insulated triple-pane | 0.23 |
| Triple with warm-edge spacer | 0.18 |
| Krypton triple with warm-edge spacer | 0.17 |
The best certified values are 0.17 for a Vi6 window and 0.16 for a Vi6 in-swing terrace door, both with the krypton triple, and 0.20 for a Vi5 casement with the warm-edge triple.
How to check one.
- Ask for the CPD number that goes with the U-factor on the submittal.
- Look it up in the NFRC Certified Products Directory. The listing should match the series, the operating type, the glass makeup, the gas fill and the spacer.
- Confirm it is a whole-window value, not center of glass.
- Check the options you are actually buying. A change of spacer moves the number.
- Read the rest of the same line. The other values on it belong to that exact glass, and they trade against each other.
The rest of the label.
- Solar Heat Gain Coefficient (SHGC). The share of the sun's heat that gets through. 0.30 means 30 percent. Lower suits buildings that mostly cool. A higher value can help buildings that mostly heat.
- Visible Transmittance (VT). A number between 0 and 1. Higher means more daylight. Coatings that cut solar heat usually cut some light as well, so VT and SHGC are chosen together, by orientation.
- Air leakage. Lower is tighter. The label value comes from a lab. What the building gets depends on how the window is installed.
- Condensation resistance. Optional. Higher is better. It is calculated for lab conditions and does not account for the surrounding wall.
What counts as a good U-factor.
The energy code for the project sets the maximum, by climate zone. The limits change from one code edition to the next: the 2021 IECC, for example, cut the residential limit for climate zones 3 and 4 from 0.32 to 0.30. Take the number from the code edition adopted for the project, not from a general article. A lower U-factor than the code asks for gives the energy model room elsewhere. It also keeps the inside surface of the glass warmer in winter, which means fewer cold spots near the window and less condensation.
Certified values for every Vistaza type are on the energy page. The glass finder shows the U-factor, SHGC and VT for each glass side by side.