Colorado Window Energy Code: U-Factor, Low-E & Altitude Explained

The numbers on the NFRC label

Every rated window carries an NFRC label. The two figures that matter most in Denver:

• U-factor — how much heat escapes. Lower is better. Codes based on the 2021 IECC call for about 0.30 or lower in climate zone 5.
• Solar Heat Gain Coefficient (SHGC) — how much sun heat passes through. West- and south-facing glass in Denver often benefits from a lower value to cut summer heat.
• ENERGY STAR's Northern-zone criteria are stricter than code (around 0.22 U-factor), which is a useful target if lower bills are the goal.

How Colorado codes apply to replacement windows

Colorado lets each city and county adopt its energy code, and many Denver-metro jurisdictions have moved to the 2021 IECC or newer. Replacement windows generally must meet the adopted U-factor, and some cities require a permit for window replacement. Requirements differ between Denver, Aurora, Lakewood, and others, so confirm with the local building department before work starts.

Why altitude changes the glass

Sealed insulated glass made at low elevation can bulge or lose its seal when shipped to 5,280 ft and above. Ask whether units are built or adjusted for altitude (for example with capillary or breather tubes) — especially for gas-filled glass, since some gas fills are not used on altitude-adjusted units.

Choosing the right low-E package

Low-E coatings reflect heat and UV. In Denver, a coating that keeps winter heat in while limiting summer gain on sun-heavy elevations is usually the best balance. Fading of floors and furniture from intense UV is a real local concern, and low-E glass helps.

Questions to ask your installer

Any reputable Denver installer should answer these without hesitation. If answers are vague, treat it as a warning sign.

• What is the exact U-factor and SHGC of the window you're quoting, and is it on the NFRC label?
• Does this window meet my city's adopted energy code, and will you confirm which code version applies?
• Is the insulated glass built or adjusted for Denver's altitude?
• Which low-E coating is on this glass, and is it right for my sun-exposed elevations?
• Do you pull the permit where one is required, and is the fee included in the quote?
• Does the quoted window qualify for ENERGY STAR's Northern zone, and are there current utility rebates?

Common questions

What U-factor do I need for windows in Denver?

Codes based on the 2021 IECC typically require about 0.30 or lower in climate zone 5. ENERGY STAR Northern-zone windows go further, around 0.22. Check your city's adopted code.

Do I need a permit to replace windows in Denver?

It depends on the jurisdiction and scope. Many Denver-metro cities require permits, especially when the opening size changes. Confirm with your local building department.