ENERGY STAR splits the country into four zones and sets two numbers for each: a U-factor ceiling and a SHGC limit. Version 7.0 took effect on 23 October 2023 and lowered the U-factor in all four zones.
The current table
| Zone | U-factor | SHGC |
|---|---|---|
| Northern | ≤ 0.22 | ≥ 0.17 |
| North-Central | ≤ 0.25 | ≤ 0.40 |
| South-Central | ≤ 0.28 | ≤ 0.23 |
| Southern | ≤ 0.32 | ≤ 0.23 |
Two things in that table surprise people.
The Northern zone has a minimum, not a maximum, on SHGC
Every other zone caps solar heat gain because the problem is summer. The Northern zone sets a floor of 0.17 because the problem is winter — free solar heat through a south-facing window offsets heating load, and a coating tuned to block it throws that away.
This was new in Version 7.0. A window that blocked almost all solar gain could previously qualify in the north; it cannot now.
The U-factor tightened everywhere
North-Central went from 0.30 to 0.25 and South-Central from 0.30 to 0.28. That sounds small and is not: 0.30 to 0.25 is a 17% reduction in heat flow through the glass, and for most product lines it is the difference between a double-pane with one low-E coating and a double-pane with a better coating, a warm-edge spacer and a full argon fill — or a triple.
The practical consequence is that a window model which carried the ENERGY STAR label in 2022 may not carry it today, in the same configuration. The label is per glazing package, not per product family.
Which zone you are in
The zones follow county lines and do not match the IECC climate zones used by the building code, which is a recurring source of confusion. Roughly:
- Northern — the northern tier and most of New England, the upper Midwest, and the mountain west
- North-Central — a band across the mid-Atlantic, Ohio Valley and Pacific Northwest
- South-Central — the lower Midwest, mid-South and much of California
- Southern — the Gulf states, Florida, south Texas, southern Arizona
Check the county rather than the state. Several states span two zones, and a few span three.
ENERGY STAR is not the code
Two separate requirements apply to a window, and they are not the same number.
The building code sets a prescriptive maximum U-factor in IECC Table R402.1.2 by IECC climate zone. It is the legal minimum, and it is looser than ENERGY STAR in most zones.
ENERGY STAR is a voluntary performance mark. It is what utility rebates and the federal efficiency tax credit generally point at.
A window can meet code and miss ENERGY STAR. If a rebate or a tax credit is part of the plan, the label on the glass is what the paperwork will ask for.
What to check on the actual quote
The NFRC label is per configuration, and a quote often lists one performance number for the whole job while the house has several different windows on it.
- A fixed picture window almost always has a lower U-factor than an operating window of the same line, because there is no sash meeting rail and no weatherstripping gap.
- A grid between the glass raises U-factor slightly and lowers visible transmittance.
- Triple glazing is frequently what gets a line to 0.22 in the Northern zone, and it changes the weight, the hardware and the price.
Ask for the NFRC number for each window type on the order, not the best number in the brochure.
The honest summary
Version 7.0 is a real tightening, not a relabel. If the plan depends on a rebate or the tax credit, confirm the specific configuration is listed — the same window in a different glass package is a different product as far as the label is concerned.