double clear windows in the Northern climate zone

double pane, clear in the Northern zone

0.48U-factor

does not meet the 0.3 code minimum for this zone

This windowtwo sheets of glass and air; no coating at all	0.48U-factor
IECC 2021 code minimumNOT met — this cannot legally be installed as a replacement in some jurisdictions	0.3U-factor
ENERGY STAR 7.0not met	0.22U-factor
SHGC, code	not regulated
SHGC, ENERGY STAR	any
Cheapest build that clears ENERGY STAR hereU-0.2	triple pane, low-E, argon
ZoneIECC zones 6–8 · Minnesota, the Dakotas, upstate New York, Alaska	Northern

Notes

Code minimum and ENERGY STAR are different numbers, and salespeople blur them. In the Northern zone the code lets you install U-0.3; ENERGY STAR wants U-0.22. A window described as "meets code" can be well short of the level that qualifies for rebates and tax credits — and in the Northern zone the gap between those two is the difference between double and triple glazing.
Lower U-factor is better; it is the inverse of R-value. U-0.48 is about R-2.1. That is worth holding on to when comparing a window against the R-19 wall it sits in: even an excellent window is a hole in the insulation, which is why window area matters as much as window quality.
In the north, do not chase the lowest SHGC. Solar gain through south-facing glass is free winter heat, and ENERGY STAR deliberately sets no SHGC limit in this zone. A salesman offering the same low-SHGC glass he sells in Texas is costing you heat you have to pay to replace.
The numbers must be NFRC-certified. The label on the window carries U-factor, SHGC, visible transmittance and air leakage, tested to a common standard for the whole window — frame included. A manufacturer quoting "centre of glass" performance is quoting the best part of the window and omitting the frame, which is where most of the loss happens.
Installation can undo the rating. A U-0.48 window fitted without air sealing around the frame, or shimmed out of square so the sash does not compress its seals, performs like a far worse window. Air leakage around windows is usually a larger loss than conduction through them.
Replacing working windows rarely pays back on energy alone. The saving is real but small against the cost, and simple payback commonly runs past twenty years. The defensible reasons are rot, failed seals, single glazing, comfort next to the glass, and noise — not the utility bill.

U-0.48, against two different targets

A double clear window has a whole-window U-factor of about 0.48.

In the Northern zone the code minimum is U-0.3 and ENERGY STAR 7.0 wants U-0.22. Those are not the same bar, and a window sold as "meets code" can be well short of the level that qualifies for rebates.

Lower U-factor is better

It is the inverse of R-value. U-0.48 is roughly R-, which is worth holding next to the R-19 wall the window sits in — even an excellent window is a hole in the insulation. That is why window area matters as much as window quality.

SHGC is the other half, and it is not "lower is better"

Solar heat gain coefficient is how much of the sun's heat comes through. In the south you want it low, to keep the sun out. In the north, gain through south-facing glass is free winter heat, and ENERGY STAR sets no SHGC limit in the Northern zone at all.

A supplier offering the same low-SHGC glass everywhere is selling a southern product into a northern house.

Insist on the NFRC label

U-factor, SHGC, visible transmittance and air leakage, tested to a common standard, for the whole window including the frame. A manufacturer quoting centre-of-glass numbers is quoting the best part of the assembly and leaving out where most of the loss happens.

The installation can undo the rating

A U-0.48 window fitted without air sealing around the frame, or shimmed out of square so the sash cannot compress its seals, performs like a much worse window. Air leakage around a window is usually a bigger loss than conduction through it — and it is invisible once the trim goes back on.

Nearby sizes

Condensation on New Windows — Usually the House, Not the WindowThe windows did not cause it. They stopped hiding it, and the glass is simply the coldest surface in the room.
Fog Between the Panes — The Seal Failed, and Only One Fix LastsCondensation on the glass wipes off. Condensation inside it cannot be reached, and that distinction is the whole diagnosis.
Can You Replace a Window With a Bigger One?Going wider is structural. Going taller is often not. That single distinction separates a weekend job from an engineered one.