The numbers that matter here
| Rating | What it measures | What we specify for St. Louis |
|---|---|---|
| U-factor | Heat loss through the whole unit (lower is better) | 0.25–0.28 on double-pane low-e argon; 0.20–0.22 if you choose triple-pane |
| SHGC | Fraction of solar heat admitted (lower blocks more) | 0.25–0.40 — lower on unshaded west glass, higher under a south porch |
| Visible transmittance | How much light gets through | ≥ 0.50, so the low-e coating does not dim the room |
| Air leakage | cfm per square foot of frame | ≤ 0.3, and ask how the meeting rail seals |
ENERGY STAR Version 7.0, in effect since October 2023, sets the North-Central zone at U ≤ 0.25 and SHGC ≤ 0.40. The Northern zone is U ≤ 0.22 with a minimum solar gain of 0.17; the South-Central zone is U ≤ 0.28 with SHGC ≤ 0.23. National window brands sell all three here. The label on the unit tells you which one you are being quoted; the performance ratings guide tells you how to read it.
Why the climate is the way it is
July's normal high at Lambert is 89.6°F with a dew-point normal of 66.8°F — humid, not just hot — and the normal heat index across July hours peaks near 93°F (NOAA hourly normals). January's normal low is 23.8°F. The first fall freeze lands around November 1 at the airport and two weeks earlier in St. Charles and Chesterfield; the last spring freeze is early April. That is seven months when the window is holding heat in and four when it is holding heat out, with a swing season in between where the right SHGC is what keeps the thermostat off.
Where the money actually goes
- Double-pane low-e argon is the baseline we quote. It is what gets a vinyl, fiberglass or clad-wood unit to a U-factor in the mid-0.20s.
- Triple-pane drops the U-factor to about 0.20 and cuts street noise noticeably. In a mixed climate the energy payback over double-pane low-e is long; the comfort case — no cold draft off a north-facing picture window, a quieter bedroom on Manchester Road — is real. We quote it where you ask and we say so when the payback does not pencil.
- Frame material matters as much as glass. A 1960s steel casement or a hollow aluminum slider conducts cold straight through; replacing the frame is the efficiency upgrade in those houses, whatever glass goes in. Vinyl, fiberglass and clad-wood frames all insulate well; see styles and materials.
- Installation is the difference between the label and the room. A unit foamed and sealed properly at the perimeter performs; a unit stuffed with fiberglass and caulked over does not. In brick openings this is most of the labor — see full-frame replacement.
What is left of incentives in 2026
Nothing federal. The Section 25C credit — up to $600 on ENERGY STAR Most Efficient windows — ended for property placed in service after December 31, 2025. The IRS says so directly. Ameren Missouri's residential rebate chart lists HVAC, thermostats, insulation and air sealing, not windows; Ameren Illinois and Spire likewise. Residential PACE through the Missouri Clean Energy District is suspended. If a 2026 quote still carries a "tax credit" line, it is wrong. The full accounting is in tax credits and rebates in 2026.
What does reduce the cost of an efficient window: choosing the right zone product instead of an over-specified Northern unit, choosing full-frame only where the frame needs it, and buying without the in-home sales markup — which is the point of starting online.
What is included
Units specified to the NFRC label values written on the quote (North-Central thresholds or better where you choose them); foam-and-sealant perimeter insulation; removal and disposal; lead-safe practices on pre-1978 houses; permit handling where it applies; an operation and lock walkthrough. See window replacement for the full inclusions.
Not ready to talk yet?
Start with the planning layer: the complete St. Louis replacement window guide, then window performance ratings for St. Louis for the label-reading detail.
Frequently asked questions
Is low-e glass worth it in St. Louis? Yes — it is the baseline, not an upgrade. Low-e is what gets a double-pane unit to a U-factor in the mid-0.20s and an SHGC under 0.40.
Should I get different glass on different sides of the house? Often. A lower SHGC on an unshaded west elevation and a standard one elsewhere is a common, sensible spec.
Will new windows cut my Ameren bill in half? No, and be wary of anyone who says so. Windows are a portion of a house's envelope; the gain is real but depends on what you are replacing — biggest when replacing single-pane or metal-frame windows, smaller when replacing 1990s double-pane vinyl with failed seals.
Does ENERGY STAR Most Efficient still matter? As a product tier, yes; as a tax-credit qualifier, no longer.