Energy: U-factor, SHGC and the energy code.

Windows are usually the weakest part of a building's thermal envelope. A wall can be insulated to R-20 or more. A good window is about R-5. So the window numbers carry a lot of weight in the energy model, and small differences between products matter. This guide covers the numbers on the NFRC label, the parts of the glass that move them, and how they meet the energy code.

U-factor.

U-factor is the rate at which heat passes through the window. In the US it is given in Btu per hour, per square foot, per degree Fahrenheit. Lower is better. A lower U-factor means less heat lost in winter and less gained in summer. It also means a warmer inside glass surface. That reduces drafts near the window and lowers the risk of condensation.

Two points are easy to miss.

• It is a whole-window number. The NFRC U-factor covers the glass, the edge of the glass and the frame together. A center-of-glass value from a glass supplier will always look better than the rating of the window it goes into. Do not compare the two.
• It belongs to one product type with one glass. A fixed window and a casement from the same series, with the same glass, get different ratings. Change the glass and the rating changes again.

R-value and U-factor.

Walls and roofs are described by R-value, which is resistance to heat flow. Windows are described by U-factor. One is the inverse of the other.

The relationship is U = 1 / R. A window rated U 0.20 is R-5. A window described as R-3.5 has a U-factor of about 0.29. Older European literature gives U in W/m²K, which is a different scale. Do not mix the two in one schedule.

SHGC.

The solar heat gain coefficient is the share of the sun's heat that gets through the window. It runs from 0 to 1. An SHGC of 0.30 means 30 percent of the available solar heat comes in. Like U-factor, it is a whole-window number. The frame lets in no sun, so an operable window with more frame and less glass has a lower SHGC than a fixed window with the same glass.

Whether a high or a low SHGC is right depends on climate and orientation.

• Cooling-dominated climates want a low SHGC. Less sun comes in, so the cooling plant is smaller and runs less.
• Heating-dominated climates can use a higher SHGC, because winter sun is free heat.
• Mixed climates, which covers most of the US, need a balance. Apartments with large south and west glass overheat in spring and fall as well as in summer, so mid-rise residential work usually leans toward the low side.

Rules of thumb exist for each climate, but they are not the design target. The energy code sets the SHGC limit for the project's climate zone, and that is the number to design to.

Clear glass has a high SHGC by nature. Getting it down takes work, and there are four tools.

• Low-E coatings. A low-emissivity coating is a microscopically thin metallic layer on one glass surface. It reflects infrared and ultraviolet energy and lets most visible light through. Solar-control versions are tuned to reject more of the sun's heat.
• Tinted and reflective glass. These cut heat, but they cut daylight as well.
• Orientation and shading. Overhangs, fins and exterior shades stop the sun before it reaches the glass.
• Films. A retrofit measure for existing glass, not a specification item for new windows.

Visible transmittance.

Visible transmittance, VT, is the share of visible light that gets through, also from 0 to 1. It falls as SHGC falls, because both are being filtered by the same coatings. The design question is how much daylight to give up for how much solar control. Check VT whenever a very low SHGC is specified. A dark window can lead to more electric lighting and unhappy residents.

Air leakage and condensation.

The NFRC label can also carry an air leakage figure and a condensation rating. Air leakage on the label is a laboratory value. The installed result depends on how the window is set and sealed into the wall. For the envelope specification, use the air leakage figure from the NAFS test report.

How NFRC arrives at the numbers.

NFRC, the National Fenestration Rating Council, is an independent non-profit. It does not set limits. It sets the method, so that every manufacturer's numbers can be compared.

• NFRC 100 determines U-factor. The rating comes from computer simulation. The frame, the edge of the glass and the center of the glass are each modeled with software developed at Lawrence Berkeley National Laboratory, and the results are combined for the whole product.
• NFRC 102 is the physical test that backs up the simulation. The window is built into a wall between a warm room and a cold room. Conditions are held steady at 70°F on one side and 0°F on the other, with a 15 mph wind on the cold side, and the heat flow is measured.
• NFRC 200 determines SHGC and VT, again by simulation. It uses summer conditions of 75°F inside and 90°F outside. NFRC 201 is the matching physical test, using a solar calorimeter that tracks the sun.
• NFRC 500 determines condensation resistance. A higher number is better. It comes from controlled laboratory conditions and does not account for air movement, sun or thermal bridging at the opening.

Each certified product is listed in NFRC's Certified Products Directory under a CPD number. That number is how a reviewer confirms that the values on a window schedule belong to a real certified product, with that frame, that glass and that spacer.

A U-factor without a CPD number is an estimate. Ask for the number.

NFRC and ENERGY STAR are different things. NFRC measures. ENERGY STAR is a government label aimed mainly at the residential market, and it uses NFRC numbers as its input. A mid-rise project is governed by the energy code, not by that label.

What moves the numbers.

Two panes or three.

A double-pane unit has two lites of glass and one gas-filled space. A triple has three lites and two spaces. The extra space adds insulation, and the extra glass gives the manufacturer more surfaces to coat with low-E.

Certified values for one window show the size of the step. A Vi6 casement with standard insulated double-pane glass is rated U 0.27. With triple glazing it reaches 0.17.

All four rows are NFRC-certified values. Note how VT falls along with U. The step from 0.23 to 0.18 is two changes, not one: the warm-edge row also carries a second low-E coating. Certified rows in between show the split. The warm-edge spacer alone takes the triple from 0.23 to 0.21, and the second coating alone from 0.23 to 0.20.

Triples cost more and weigh more. The premium is roughly 10 to 15 percent over insulated double-pane glass. The reasons to choose a triple are an energy code or energy model that needs it, a cold climate, or a comfort target next to large areas of glass. A triple is not automatically the quietest glass. The sound page shows what is.

Gas fill.

The space between the lites is filled with an inert gas, not air. Argon is the usual choice. It is clear, non-toxic and inexpensive, and it conducts about two-thirds as much heat as air. It works best in wider gaps. Krypton insulates better still and does its best work in the narrower gaps of a triple. It costs a good deal more, so it is used where the last few hundredths of U-factor matter.

Sealed units lose a little gas over time. The US standard for insulating glass units, ASTM E2190, asks for an average of at least 90 percent argon when the units are made, and at least 80 percent after its accelerated weathering test. Fogging inside a unit means the seal has failed and the unit should be replaced.

The spacer.

The spacer is the bar that holds the lites apart around the edge of the unit. A conventional aluminum spacer is a thermal bridge. It conducts heat straight past the gas space, so the edge of the glass runs cold. That raises the whole-window U-factor, and it is why condensation and mold show up first at the bottom edge of the glass.

A warm-edge spacer is made from low-conductivity material such as a plastic composite or stainless steel. On our certified ratings, swapping the aluminum spacer for a warm-edge one, with nothing else changed, lowers the U-factor by 0.02 to 0.03: from 0.27 to 0.25 on a Vi6 casement with standard insulated double-pane glass, and from 0.30 to 0.27 on a Vi5. The second benefit is a warmer glass edge and less condensation. A dual-seal unit adds a second sealant line, which holds the gas in longer and copes better with thermal movement than a single seal.

The frame.

Frame material counts because the frame is part of the rated area. uPVC profiles are multi-chambered and do not conduct heat the way aluminum does. That is a large part of how High Performance uPVC windows reach low whole-window U-factors without exotic glass.

The energy code.

Most US jurisdictions adopt a version of the IECC or ASHRAE 90.1 and amend it locally. New York City's version is the NYCECC. The points below describe the 2020 NYCECC. It still governs jobs filed before March 30, 2026. Jobs filed from that date fall under the 2025 NYCECC, which is based on the 2024 IECC; its window limits are noted where they differ.

• Climate zone. New York City is in climate zone 4. The maximum U-factor and SHGC are set by zone.
• Two compliance paths. The prescriptive path holds each window to the tabulated maximum U-factor and SHGC. The performance path uses approved energy-modeling software to show that the whole building complies, which allows trade-offs between components.
• Stricter than before. The 2020 cycle tightened fenestration U-factor and SHGC limits compared with the 2016 code.
• Height matters for metal frames. The 2020 code sets different U-factor limits for metal-framed fenestration below 95 feet above grade and at 95 feet and higher: 0.30 and 0.36 for fixed metal windows, 0.40 and 0.42 for operable ones. Non-metal frames, uPVC included, have one limit at any height: 0.28. The 2025 code keeps 0.28 for non-metal frames and tightens fixed metal windows above 95 feet to 0.34.
• Alterations. Replacing windows in an existing building brings the new windows under the code. Under the 2025 code each replacement unit, sash and glass included, must meet the table's U-factor and SHGC, though the U-factor may be averaged by area across the replacement windows of one type. Storm windows fitted over existing windows are exempt.
• Documentation. The window schedule on the drawings must give the U-factor and SHGC of each window, skylight and glazed door, along with the manufacturer's information for the assembly that achieves them. Energy-modeling reports are required on the performance path, and air leakage test reports where the code calls for them.

Use the certified whole-window value for the exact type and glass on the schedule. A simulated or center-of-glass number will not survive plan review.

Where Vistaza fits.

Vistaza's ratings are NFRC-certified, and each one is tied to a product type, a glass makeup and a CPD number. The lowest certified U-factor for a window is 0.17, on the Vi6 casement with a krypton-filled triple and warm-edge spacer (CPD VST-K-4-00221-00001). The lowest for any product is 0.16, on the Vi6 in-swing terrace door with the same glass. The Vi5 casement reaches 0.20 with a triple and warm-edge spacer. Certified SHGC runs from 0.10 to 0.23 across the Vi6 casement glass options, and the glass page lists each option with its own numbers.