Best U-Factor and SHGC for Windows in Littleton, Colorado
Data updated October 12, 2026.
ENERGY STAR Version 7.0 places Littleton, CO in its Northern climate zone. ENERGY STAR windows for that zone need a U-factor of 0.22 or lower and an SHGC of 0.17 or higher. Both ratings appear on the window's NFRC label. In the GlassBase heating and cooling model, replacing a single-pane window with one at these limits saves 595 kWh per window per year in Littleton. Upgrading a double-pane window to the same limits saves 216 kWh.
In the Northern zone, the SHGC limit is a minimum, not a maximum. ENERGY STAR also lets a window with a slightly higher U-factor qualify if its SHGC is high enough to collect more winter sun. See window energy ratings in the US for the full table.
A window with a lower U-factor than the limit loses less heat than one at the limit. Before you buy, confirm the product's certified ratings on its NFRC label or in the ENERGY STAR Product Finder.
What matters most for windows in Littleton
In Littleton, most of the modeled saving comes from heating: 400 kWh of heating load against 195 kWh of cooling load per window per year, for a single-pane to ENERGY STAR upgrade. U-factor measures how fast heat passes through the window. A lower U-factor means less heat lost in winter.
ENERGY STAR assigns climate zones by state and county, and in some places by ZIP code. Building energy codes use a separate map, the IECC climate zones, with their own window requirements. Your local building department can tell you which code applies to your project.
Modeled savings per window per year in Littleton
One window of 39 × 59 in (1 × 1.5 m), averaged over eight facing directions. Weather: 2 TMY3 weather stations.
| Upgrade | Heating saved, kWh/yr | Cooling saved, kWh/yr | Total, kWh/yr | Natural gas saved, therms/yr gas furnace |
Heat pump + AC, $/yr | Gas furnace + AC, $/yr |
|---|---|---|---|---|---|---|
| Single-pane → double-pane | 340 | 39 | 380 | 13.7 | 19 | 17 |
| Single-pane → ENERGY STAR (Northern) | 400 | 195 | 595 | 16.1 | 32 | 29 |
| Double-pane → ENERGY STAR (Northern) | 60 | 156 | 216 | 2.4 | 13 | 12 |
Dollar figures use Colorado average prices, not your utility's rate. Electricity $0.1585/kWh: Colorado residential average, 2025. Source: U.S. EIA, Electric Power Monthly, Table 5.6.B, residential sector, year-to-date through December 2025, retrieved 2026-10-06. Natural gas $1.05/therm: Colorado residential average, 2025. Source: U.S. EIA, Natural Gas Prices: price delivered to residential consumers, and heat content of natural gas delivered to consumers, 2025, retrieved 2026-10-06. Heat pump COP 3.2, air conditioner COP 2.5, gas furnace efficiency 85%.
The single-pane and double-pane windows use values from the calculator's reference-window table; the ENERGY STAR window is at the Northern zone limits. kWh here measures heat, not electricity on a bill. Air leakage and the rest of the house are not modeled. Model version tmy3-hourly-iso13790/f4822a26309b, data run October 12, 2026. How the model works.
Savings by window facing in Littleton
For a single-pane to ENERGY STAR upgrade, the model saves 655 kWh per window per year on NW-facing walls. On S-facing walls it saves 537 kWh.
| Facing | Heating saved, kWh/yr | Cooling saved, kWh/yr | Total, kWh/yr |
|---|---|---|---|
| N | 498 | 132 | 630 |
| NE | 453 | 154 | 606 |
| E | 374 | 197 | 570 |
| SE | 327 | 218 | 545 |
| S | 317 | 221 | 537 |
| SW | 349 | 238 | 587 |
| W | 407 | 227 | 634 |
| NW | 478 | 176 | 655 |
Payback of each upgrade in Littleton
Calculate savings for your windows in Littleton
Looking for a company to supply or install them? GlassBase lists 9 companies in Littleton.
Compare states: window energy savings by state. All of Colorado: U-factor and SHGC for Colorado.
Frequently asked questions
What U-factor and SHGC should windows in Littleton have?
For ENERGY STAR Version 7.0 in the Northern zone, which covers Littleton, the U-factor must be 0.22 or lower and the SHGC 0.17 or higher. A window with a lower U-factor than the limit loses less heat. Check the numbers on the NFRC label of the exact product you are quoted.
How much energy can new windows save in Littleton?
In the GlassBase heating and cooling model, one window of 39 × 59 in (1 × 1.5 m) saves 380 kWh per year going from single-pane to double-pane, and saves 595 kWh going from single-pane to ENERGY STAR. At Colorado's 2025 average prices from the U.S. Energy Information Administration (EIA), the single-pane to ENERGY STAR upgrade is worth about $32 per window per year with a heat pump. For your own windows, sizes and facings, use the calculator.
Is the ENERGY STAR zone the same as the building code climate zone?
No. ENERGY STAR uses four zones of its own, assigned by state and county. Building energy codes use the IECC climate zones, a separate map with its own window requirements. A window can meet one and not the other, so check both if your project needs a permit.