Window U-Factor 0.27 in New York: What It Means for Replacement Windows in NYC and Long Island

If you are replacing windows or building an addition in New York City or on Long Island, you will likely hear the number 0.27. That is the maximum window U-factor allowed under the current residential energy codes for our climate zone. Here is what it means and how to choose windows that meet it.

What U-factor measures

U-factor measures how much heat passes through a window. The lower the number, the better the window insulates. It is expressed in Btu per hour per square foot per degree Fahrenheit, but you only need to remember that lower is better. U-factor ratings for windows are for the whole unit, including frame and glass, when rated by the National Fenestration Rating Council (NFRC).

U-factor vs. SHGC

Solar Heat Gain Coefficient (SHGC) measures how much of the sun's heat passes through the window, on a scale from 0 to 1. Lower SHGC means less solar heat gets in, which helps in summer; higher SHGC allows more passive solar heat in winter. U-factor is about insulation; SHGC is about sunlight.

The verified requirement: 0.27 in climate zone 4

New York City is assigned to climate zone 4A for energy code purposes, and most of Long Island is in climate zone 4A as well.

• NYC: The 2020 NYC Energy Conservation Code, Table R402.1.2, sets a maximum fenestration U-factor of 0.27 for climate zone 4, with a maximum glazed fenestration SHGC of 0.40. The 2025 NYC Energy Conservation Code keeps the vertical fenestration U-factor at 0.27 for climate zone 4.
• Long Island (Nassau and Suffolk): The 2025 Energy Conservation Construction Code of New York State, based on the 2024 International Energy Conservation Code with New York amendments, became effective December 31, 2025. Its residential provisions set a maximum vertical fenestration U-factor of 0.27 for climate zone 4.

These are residential prescriptive requirements. Projects can also comply through other paths in the code, such as performance modeling, and commercial buildings follow separate provisions. The code's provisions for additions and alterations generally require replacement windows to meet the applicable U-factor and SHGC requirements. Your design professional or building department can confirm exactly which provisions apply to your project.

ENERGY STAR is a different standard

ENERGY STAR is a voluntary program, not a code. ENERGY STAR criteria for windows in northern climates are stricter than 0.27, so an ENERGY STAR-certified window for our region will typically also meet the code maximum. Check the ENERGY STAR climate zone map and the product's certification for your area.

Reading the NFRC label

Look for the NFRC label on new windows. It lists:

• U-factor
• Solar Heat Gain Coefficient
• Visible Transmittance, how much light comes through
• Air Leakage, where reported
• Condensation Resistance, where reported

Make sure the U-factor on the label is 0.27 or lower if you are aiming for code compliance.

What helps a window reach 0.27 or lower

• Double or triple glazing. Most compliant windows have at least two panes. Triple-pane units offer lower U-factors and can reduce outside noise.
• Low-E coatings. Thin metallic coatings reflect heat back into the room.
• Gas fills. Argon or krypton between panes insulates better than air.
• Warm-edge spacers between panes reduce heat loss at the glass edge and help limit condensation.
• Frame materials. Vinyl, fiberglass, wood, and composite frames generally insulate better than standard aluminum. Thermally broken aluminum performs better than non-broken frames.

Replacement considerations

• Permits. In NYC, window replacement permit requirements depend on the building and scope. On Long Island, many towns and villages require permits for window replacement, especially when openings change size.
• Landmarks. In NYC, windows on landmarked buildings and in historic districts require Landmarks Preservation Commission approval, which often addresses materials, profiles, and divided light patterns.
• Egress. Bedroom windows may need to meet emergency escape opening requirements. Replacement windows should not reduce existing egress openings below what is required.
• Safety glazing. Tempered glass is required in certain locations, such as near doors, in bathrooms near tubs and showers, and close to floors.
• Installation quality. A high-performance window installed poorly will leak air and water. Proper flashing, air sealing around the frame, and insulation of the gap between the window and the rough opening matter as much as the window rating.
• Insert vs. full-frame replacement. Insert windows fit inside existing frames; full-frame replacement allows inspection and repair of the rough opening and better air sealing.
• Co-ops and condos often have window specifications and approval processes.

Summary

For residential projects in the five boroughs and in Nassau and Suffolk, the current energy codes set a maximum window U-factor of 0.27 in climate zone 4. Choose NFRC-labeled windows that meet it, confirm permit and landmark requirements, and make sure installation is done right.

This article is general information for property owners, not legal or engineering advice. Rules and procedures change; confirm current requirements with the agency or utility involved.