ENERGY STAR and Phius: what they cover, and what they do not.
Two programs architects ask about on every energy-focused job, what each one asks of a window, and which numbers actually govern on a mid-rise building.
Vistaza Engineering · Sep 20, 2026 · 5 min read
Two names come up on nearly every energy-focused project: ENERGY STAR and Phius. They are not the same kind of thing, and neither of them is a building code.
ENERGY STAR is a product label. It puts a maximum U-factor and a maximum or minimum SHGC on a window, by climate zone, and it is written for houses. Phius is a whole-building performance standard. It sets targets for the building, and a window earns its place by helping the building meet them.
On a mid-rise building, neither label is the number that governs. The certified NFRC values and the energy code are.
What ENERGY STAR asks of a window.
The criteria are short. A certified window has to meet a U-factor limit and an SHGC limit for its climate zone, and an air-leakage limit. Version 7.0 is the current window specification.
| Zone | U-factor | SHGC |
|---|---|---|
| Northern | 0.22 or lower | 0.17 or higher |
| North-Central | 0.25 or lower | 0.40 or lower |
| South-Central | 0.28 or lower | 0.23 or lower |
| Southern | 0.32 or lower | 0.23 or lower |
Air leakage has to be 0.3 cfm/ft² or lower for windows and sliding doors. The northern zone also allows a trade-off: a slightly higher U-factor is accepted in return for a higher SHGC, because in a heating climate the sun coming in is worth something.
Two things follow from that table. The requirement is a pair of numbers, not a technology, so nothing about the frame material, the number of panes or the spacer is prescribed. And a window is never certified as a whole product line — it is certified glass option by glass option, and the same unit can qualify in one zone and not in the next.
Where it stops.
The Version 7.0 eligibility criteria define a residential building as one of three stories or less in height, and exclude products intended for installation in buildings that are not residential buildings in that sense. There are no ENERGY STAR window criteria for multifamily mid-rise buildings. The program has never written any.
This matters because the label is often asked for on buildings it was not written for. On a mid-rise, the honest way to use it is as a yardstick rather than as a certificate: "the glass for this elevation meets the ENERGY STAR Version 7.0 U-factor and SHGC values for our zone" is a checkable statement about numbers. It does not become a certification of the building.
What governs instead is the pair of certified NFRC values for the exact type and glass, and whatever the energy code asks for — prescriptive limits by assembly, or a whole-building model. The energy guide covers both, and certified or simulated U-factor covers the difference between a number on a certificate and a number out of a calculation.
Where Vistaza stands.
Vistaza is a partner in the ENERGY STAR program. Selected glass options of the Vi5 fixed and Vi5 casement windows are certified to ENERGY STAR Version 7.0. Which options qualify depends on the climate zone, so ask for the list for yours.
What Phius asks of a building.
Phius certifies buildings, not brands. A project is modeled and then measured against targets for annual heating and cooling demand, peak heating and cooling load, source energy and airtightness. The targets are adjusted for climate and for building type, which is why a Phius project in a cold northern city and one in the south do not carry the same numbers.
The certification is earned by the building as a whole: the enclosure, the ventilation, the mechanical systems and the workmanship together. No single component certifies a building, and no component can be said to make a building Passive House.
What Phius asks of a window.
Windows are handled as components, listed with verified performance data so that a modeller uses consistent numbers rather than a manufacturer's own. Four things about a window carry the weight in that model.
- Whole-window U-factor. Not center-of-glass. Frame, edge of glass and spacer are all in it, and they are where most of the loss is on a small unit.
- SHGC, in both directions. In a heating climate the window is a source of free heat; in a cooling climate the same glass is a load. Phius projects usually end up with different glass on different elevations.
- The installed detail. A window is modeled with the thermal bridge at its perimeter. Where the frame sits in the wall, and how it is insulated over, changes the result.
- Airtightness. The building has a whole-building airtightness target, and every operable unit and every perimeter joint counts against it.
What neither program is.
Neither one is a code, and neither one replaces the certified values on the label. Both are voluntary, both are renewed rather than granted once, and both describe a scope: ENERGY STAR describes a product in a climate zone, Phius describes a building. A claim that crosses those boundaries — a certified building because of a certified window, or a certified window because it went into a certified building — is not a claim either program makes.
What to put in the specification.
- State the required U-factor and SHGC for each elevation as numbers, with the test standard, not as a program name.
- Require certified NFRC values for the exact type and glass, with the certificate number.
- If a program's values are the target, name the version and the climate zone, and say that the requirement is the numbers.
- If the project is pursuing a whole-building certification, give the window requirements as the energy modeller wrote them, including the perimeter detail.
- Put the glass make-up on the window schedule by elevation, so the requirement survives to the order.
To get the certified values and the glass options for your zone, send your plans. The full picture of how the numbers are earned is in the energy guide.