Window U-Factor and SHGC Requirements Under IRC 2024

Quick Answer

IRC 2024 Table N1102.1.3 tightened fenestration U-factor requirements across climate zones 3 through 8. Zone 4 now requires U-0.30 (down from U-0.35 in IRC 2021). Zones 5 and 6 now require U-0.27 (down from U-0.30).

Under IRC 2024, zones 7 and 8 require U-0.22. In the hottest zones (1 through 3), SHGC limits are tightened to reduce solar heat gain. Triple-pane windows are now the practical standard in zones 7 and 8. An NFRC label documenting the certified U-factor and SHGC is required for compliance documentation at all inspections.

What IRC 2024 Actually Requires

IRC 2024 Table N1102.1.3 organizes fenestration requirements by climate zone. “Fenestration” includes windows, glazed doors, and skylights. The U-factor measures how much heat flows through the assembly per unit of temperature difference; lower is better. SHGC (Solar Heat Gain Coefficient) measures how much solar energy passes through the glazing; in hot climates a lower SHGC is desired to reduce cooling loads.

U-factor requirements by zone: Zone 1: U-0.50; Zone 2: U-0.40; Zone 3: U-0.35; Zone 4: U-0.30 (down from U-0.35 in IRC 2021); Zone 5: U-0.27 (down from U-0.30 in IRC 2021); Zone 6: U-0.27 (down from U-0.30 in IRC 2021); Zone 7: U-0.22 (unchanged from 2021 but now applies more broadly); Zone 8: U-0.22.

SHGC requirements by zone: Zone 1: SHGC 0.25; Zone 2: SHGC 0.25; Zone 3: SHGC 0.25 (tightened from 0.30 in IRC 2021); Zones 4 through 8: No SHGC requirement but credit is available for low-SHGC windows under the performance path.

Skylights: Table N1102.1.3 sets separate, higher U-factor limits for skylights: U-0.75 maximum in zone 1, stepping down to U-0.55 in zones 4 through 8. Skylight SHGC in zones 1 through 3 is 0.30 maximum.

The NFRC (National Fenestration Rating Council) label is the standard documentation for U-factor and SHGC compliance. Products without an NFRC label must use the default values from Table N1102.1.4, which are assumed to be poor performers and will typically fail the prescriptive requirements.

Why This Rule Exists

Windows are the weakest link in the building envelope from a thermal standpoint. A standard double-pane low-e window in zone 5 has an effective R-value of about R-3.6, compared to R-20 or more for the insulated wall around it. Fenestration typically represents 15 to 20 percent of the wall area in a home but accounts for a disproportionately large fraction of heat loss. The Department of Energy estimates that heat loss and gain through windows accounts for 25 to 30 percent of residential heating and cooling energy use. Tightening U-factor requirements pushes the market toward better glazing packages that have become cost-competitive as triple-pane manufacturing has scaled up, particularly in European and domestic manufacturers serving the northern U.S. and Canadian markets.

How to Read an NFRC Label

Every code-compliant window ships with a small sticker produced by the National Fenestration Rating Council. Understanding each field on that label is essential for homeowners comparing products, contractors verifying orders, and inspectors confirming compliance. The label lists five ratings, each measured under standardized conditions.

U-factor appears at the top and is the value IRC 2024 enforces. It is the whole-unit rating, not the center-of-glass value. It accounts for the frame, the spacer between panes, and the glazing itself. A lower number means better insulating performance. For a Zone 5 project, this number must be 0.27 or lower.

Solar Heat Gain Coefficient (SHGC) is listed directly below U-factor. It runs from 0 to 1. In hot climates (Zones 1 through 3), IRC 2024 requires SHGC 0.25 or lower. In northern climates you may actually want a higher SHGC on south-facing windows to capture passive solar heat in winter, though there is no code minimum in those zones.

Visible Transmittance (VT) measures the fraction of visible light that passes through the glass, also from 0 to 1. Higher VT means a brighter interior. There is no IRC code minimum, but windows with very low-e coatings sometimes sacrifice daylight to achieve low U-factors, so compare VT when evaluating products at similar U-factor levels.

Air Leakage (AL) is measured in cubic feet per minute per square foot of frame area. The NFRC reports this voluntarily; IRC 2024 requires AL no greater than 0.3 cfm/ft² under Section N1102.4.3. This is a separate requirement from U-factor compliance and is verified by the label or by test documentation.

Condensation Resistance (CR) rates the window’s resistance to condensation on a scale from 1 to 100; higher is better. This is a voluntary rating not enforced by IRC, but it is a useful proxy for frame quality. Windows with warm-edge spacers and thermally broken frames tend to score CR 50 or higher and are better suited to cold climates.

When standing in a home improvement store or reviewing a manufacturer’s spec sheet, confirm you are looking at the whole-unit NFRC value, not the center-of-glass value that manufacturers sometimes display more prominently because it looks better. The whole-unit value is always labeled “NFRC” and includes the frame contribution.

What the Inspector Checks at Rough and Final

At rough framing inspection, the inspector typically notes window rough openings and may review the window schedule on the approved plans. At final inspection, the critical verification is the NFRC label. Inspectors look for the NFRC sticker still attached to the glass or frame, which lists the certified U-factor, SHGC, visible transmittance, and condensation resistance rating. The listed U-factor must meet or exceed (numerically be equal to or lower than) the table requirement for the climate zone. The inspector compares the labels on installed windows against the approved window schedule.

Inspectors also check that the window schedule in the permitted plans matches the installed products. If the plan set calls for a specific manufacturer and model, a field substitution to a different product must be documented with an approved change order. Inspectors in jurisdictions that use the online REScheck compliance report will cross-reference the fenestration entries on that report against what is labeled on the windows. A discrepancy between the REScheck submission and the installed products is a common reason for a failed final inspection.

If labels were removed during installation, the contractor must provide the window manufacturer’s certification documents showing compliance. The energy certificate posted near the panel must list the fenestration U-factor values. Skylights are inspected separately, and their NFRC labels must also be present or documented.

What Contractors Need to Know

The Zone 4 shift from U-0.35 to U-0.30 eliminates many standard double-pane products that previously met code. A typical double-pane low-e window rates U-0.30 to U-0.35; the better end of that range is now required. For Zone 5 and 6 at U-0.27, you are into higher-performance double-pane territory with warm-edge spacers and improved frame conductance, or entry-level triple-pane. For Zones 7 and 8 at U-0.22, triple-pane windows with two low-e coatings are standard.

Spec writing: When preparing the window specification for plans or a subcontractor bid, always state the required NFRC whole-unit U-factor as the governing criterion rather than listing a specific product as the only option. Write it as “U-0.27 maximum (NFRC whole-unit rating) or approved equal.” This gives the window supplier flexibility to propose alternatives while locking in the performance floor. Include the SHGC requirement if the project is in Zones 1 through 3. Listing both U-factor and SHGC limits in the spec prevents suppliers from substituting a window that meets U-factor by sacrificing solar performance.

Window schedule review: Before signing off on the window order, compare every line of the manufacturer’s order confirmation against the window schedule in the approved plans. Verify model numbers, rough opening sizes, and NFRC ratings. A supplier error caught at this stage costs nothing; the same error caught at final inspection means pulling and replacing installed windows. Note that some manufacturers offer the same frame line in multiple glazing packages with different U-factors; confirm the order reflects the high-performance glazing option, not the builder-grade default.

Field verification: Before windows are installed, spot-check NFRC labels on a sample of units as they come off the delivery truck. Confirm the labeled U-factor matches the order. After installation, photograph the NFRC labels on each window while they are still intact. Store these photos by room or window ID in the job folder. If labels are removed during trim-out, you have documentation. This is especially important for large projects where the building department may request label evidence for multiple units. Some inspectors will ask for photos if labels are not visible at final.

Coordinate with the window supplier early. U-factor compliance must be established at the product selection stage; substituting a cheaper window in the field to save money will cause a failed final inspection. When windows with exterior continuous insulation are installed, the frame’s thermal performance can be degraded if not properly detailed. Follow manufacturer instructions for installation at jamb extensions. Also note that the code measures the whole-window U-factor from the NFRC label, not just the center-of-glass value; make sure you are ordering based on the NFRC whole-unit rating.

What Homeowners Get Wrong

A frequent question: “The salesperson said these windows are energy efficient — are they code compliant?” Marketing language like “energy efficient” or “double-pane low-e” does not guarantee IRC 2024 compliance. The only reliable indicator is the NFRC label showing a U-factor at or below the zone requirement. In Zone 5, a window labeled U-0.30 is no longer compliant under IRC 2024; it must be U-0.27 or lower. Another misconception: “Triple-pane windows are always overkill.” In zones 7 and 8, they are now effectively required. In zones 5 and 6, high-performance double-pane products can meet U-0.27, but the margin is slim and triple-pane provides a buffer. Homeowners also sometimes confuse R-value and U-factor for windows. U-factor is the reciprocal of R-value; a window with U-0.25 has an effective R-value of 4. Unlike wall insulation, windows are always rated by U-factor, not R-value.

State and Local Amendments

States frequently adopt the IRC with amendments that tighten or modify the base fenestration requirements. Because state adoption cycles lag IRC publication, the applicable standard in your jurisdiction may not match the 2024 edition exactly.

California’s Title 24 Part 6 sets fenestration requirements based on climate zone and orientation, with some zones requiring U-0.30 or lower for windows facing north versus SHGC limits for south-facing glass. The state’s 16 climate zones produce a more granular matrix than the IRC’s 8-zone system, and prescriptive compliance often requires orientation-specific values that go beyond a single U-factor number.

Washington State has required U-0.30 or better in most zones since its 2018 code update and is among the states that most closely align with IRC 2024 fenestration values. Washington’s state amendments also require tested air leakage documentation separately from U-factor compliance.

New York’s stretch energy code, which applies in New York City and many downstate municipalities, requires U-0.27 windows, aligning with IRC 2024 Zones 5 and 6 requirements. Upstate New York projects in Zone 6 fall under the same U-0.27 threshold through the base state energy code.

Massachusetts requires U-0.27 or better in its stretch code, which applies to most new construction in the state because nearly all Massachusetts communities have adopted the stretch code. The base code requires U-0.30 in Zone 5 areas, but the stretch code tightens this to U-0.27 statewide.

Minnesota, which sits in climate zones 6 and 7, imposes U-0.22 requirements in its coldest counties and follows the IRC 2024 table closely for the rest of the state. Minnesota also mandates air leakage testing documentation at final inspection.

Michigan’s northern Lower Peninsula and Upper Peninsula span zones 6 and 7, where U-0.22 is required. The state adopted the 2021 IECC with amendments, and local jurisdictions in the U.P. sometimes apply additional cold-climate requirements.

In states that have not adopted IRC 2024, the IRC 2021 values may still apply, but checking with the local building department before ordering windows is essential because state amendments often exceed the base code.

Common Violations Found at Inspection

• Windows installed with U-0.30 or U-0.35 ratings in climate zones that now require U-0.27, due to ordering against the previous IRC 2021 specification.
• NFRC labels removed before final inspection, leaving no documentation of compliance; inspectors require manufacturer certification paperwork as a substitute.
• Skylights installed without verification of their U-factor; skylights have higher U-factor limits than windows but must still meet the table requirements and bear NFRC labels.
• Window schedule on the approved plans lists compliant U-factors but different windows were substituted in the field without an approved change order reviewed by the building department.
• Energy certificate near the panel lists the window U-factor incorrectly (using center-of-glass value rather than whole-unit NFRC value), causing a discrepancy with the actual installed product.
• Zone 3 windows ordered with SHGC 0.30 rather than the tightened 0.25 required under IRC 2024, failing the solar heat gain requirement in hot-humid climates.
• Frame conductance not accounted for in product selection; a glazing unit with a good center-of-glass U-factor can have a whole-unit NFRC rating that fails due to a thermally poor frame.