For a St. Louis house the right window has a U-factor around 0.25 to 0.28 and a solar heat gain coefficient between roughly 0.25 and 0.40 — a balance, because the climate is a balance: 4,433 heating degree days and 1,713 cooling degree days a year, a January normal low of 23.8°F and a July normal high of 89.6°F with a dew point near 67°F. EPA's ENERGY STAR map places the region in the North-Central zone for exactly that reason. The national window brands sell Northern-zone and Southern-zone products here too, and the label is how you tell them apart.
What is on the NFRC label?
| Rating | Measures | Better is | St. Louis target |
|---|---|---|---|
| U-factor | Rate of heat flow through the whole unit, frame and glass, in Btu/hr·ft²·°F | Lower | 0.25–0.28 double-pane low-e argon; 0.20–0.22 triple-pane |
| SHGC | Fraction of solar heat that passes through, 0–1 | Depends on orientation | 0.25–0.40 |
| Visible transmittance (VT) | Fraction of visible light passing, 0–1 | Higher for daylight | ≥ 0.50 |
| Air leakage (AL) | cfm per square foot of frame at a test pressure | Lower | ≤ 0.3 |
| Condensation resistance | 1–100 index, optional | Higher | ≥ 50 if shown |
The label is for the whole unit as tested — frame included — which is why a thin aluminum frame with excellent glass can post a worse U-factor than a vinyl frame with ordinary glass.
Which ENERGY STAR zone is St. Louis?
North-Central. Version 7.0 sets the zones as:
| Zone | U-factor | SHGC |
|---|---|---|
| Northern | ≤ 0.22 (or 0.23–0.26 with higher SHGC equivalents) | ≥ 0.17 (gain wanted) |
| North-Central | ≤ 0.25 | ≤ 0.40 |
| South-Central | ≤ 0.28 | ≤ 0.23 |
| Southern | ≤ 0.32 | ≤ 0.23 |
The Northern spec wants solar gain for winter heating; the Southern spec blocks it. St. Louis wants neither extreme, and a window built for Minneapolis (high SHGC) will warm a west-facing St. Louis room in August, while a window built for Dallas (SHGC 0.23) will cost you free heat on a south face in January. Check your county in EPA's climate-zone lookup if you are at the edge of the region; the St. Louis-area counties are expected to fall in North-Central.
U-factor: how low is worth paying for?
Double-pane low-e argon gets a good vinyl, fiberglass or clad-wood unit to about 0.25–0.28, and that is the sweet spot for this climate. Going to triple-pane takes it to about 0.20–0.22. In a cold climate with 7,000+ heating degree days the extra pane pays back; at 4,433 it rarely does on energy alone. Where triple-pane earns its $120–$250 per opening in St. Louis:
- North-facing glass in rooms you sit in — no cold draft off the pane in January.
- Bedrooms on busy roads: Manchester, Kingshighway, Lindbergh, Highway K. The third pane and the thicker unit cut noise noticeably.
- Large fixed picture windows, where the glass is most of the wall.
Where it does not: a south-facing kitchen window with a porch roof, a bathroom, a basement hopper.
SHGC: by orientation
- South: 0.30–0.40 is fine and in winter useful; a porch roof or overhang shades the high summer sun anyway.
- West: the problem side in St. Louis. Low afternoon sun in July on a wall of glass with no shade — specify the low end, 0.25–0.30, or shade it.
- East: morning sun, less heat; 0.30–0.40.
- North: gain is irrelevant; choose for U-factor and VT.
Mixing SHGC by elevation is normal and sensible; the quote should show it.
Frame material and the label
The frame is roughly a quarter of a double-hung's area, and it is in the label:
| Frame | Conducts? | Typical whole-unit U with low-e argon |
|---|---|---|
| Vinyl (multi-chamber) | Low | 0.26–0.30 |
| Fiberglass | Low | 0.25–0.29 |
| Composite | Low | 0.25–0.29 |
| Aluminum-clad wood | Low (wood core) | 0.26–0.30 |
| Thermally broken aluminum | Medium | 0.30–0.40 |
| Non-thermally-broken aluminum or steel (1950s–60s originals) | High | 0.60–1.0+ |
That last row is why the biggest efficiency gain in St. Louis is often not the glass at all: it is getting the steel casement out of a Florissant ranch. See full-frame replacement.
Air leakage and the meeting rail
An old double-hung loses as much through its gaps as through its glass. The AL figure on a new unit should be 0.3 or lower; most quality units test at 0.1 or less. On a double-hung, ask how the meeting rail seals — interlocking rails with a compression bulb are what you want — and on a casement, note that a compression-sealed casement is the tightest common window type.
Condensation
With a July dew point near 67°F, exterior condensation on cold glass is normal on humid summer mornings and means the glass is doing its job. Interior condensation in winter means indoor humidity is high or the unit's inside surface is cold; a lower U-factor and a warm-edge spacer raise the inside glass temperature and reduce it.
Frequently asked questions
What U-factor should I ask for in St. Louis? 0.28 or lower; 0.25 or lower if you are buying for ENERGY STAR North-Central.
Is a 0.30 window bad? It clears the Southern threshold (0.32) but not South-Central (0.28) or North-Central (0.25); it is fine in a frame wall, it is not the best available, and the price difference to 0.26 is small.
Does low-e dim the room? Modern low-e coatings keep VT at 0.50–0.60 on a double-pane unit. Older or heavy coatings, and some triple-pane packages, go lower; check the VT.
Is there a tax credit for a better rating? Not in 2026. See tax credits and rebates.
Next step
Energy-efficient windows is the service that specifies to these numbers. Related: window styles and materials · the complete St. Louis guide.