Window U-factor and SHGC requirements for homes, by climate zone

The residential International Energy Conservation Code (IECC) sets a maximum U-factor for windows in every climate zone. In the warmer zones it also sets a maximum SHGC (solar heat gain coefficient).

How to use the limits

1. Find the home’s IECC climate zone (by county).
2. Confirm which edition of the IECC your jurisdiction has adopted. Values change between editions, and some states (California, Washington) use their own codes.
3. Compare the NFRC-certified whole-window U-factor and SHGC for the exact configuration on the label. Do not use center-of-glass numbers.

The code lookup tool shows the limits by state, province and climate zone, and which lines can meet them.

Reading an NFRC label

The NFRC label on a US window lists the U-factor, SHGC and visible transmittance for that exact configuration, and sometimes air leakage and condensation resistance. The ratings change with the glass package, the spacer, the gas fill and the frame, so one product line can carry many ratings. Ask for the rating of the configuration you are ordering, not the best one in the brochure. The label also carries an NFRC product ID that you can look up in the NFRC Certified Products Directory.

U-factor in Canada

Canadian codes write the limit as a U-value in W/(m²·K) rather than a U-factor in Btu/(h·ft²·°F). To convert, divide the metric value by 5.678: a limit of 1.60 W/(m²·K) is about U-0.28. Canadian ratings come from CSA A440.2 and are usually shown with the NAFS rating on the label. National Building Code section 9.36 sets the limits by NBC climate zone, based on heating degree-days, and provinces may adopt it, change it, or replace it with their own path (BC’s Energy Step Code, Ontario’s SB-12).

Why European and Passive House values don’t match

European windows are rated to EN ISO 10077, and the Passive House Institute certifies a window’s Uw at a standard size under its own conditions. Those numbers are real but are calculated differently from NFRC ratings, so a Uw cannot be converted into a US code value. A window with an excellent PHI certificate still needs NFRC-certified ratings, or the code’s default values, to pass inspection in the US. RFV shows PHI values labelled as PHI and never uses them to check code compliance.

What to ask the manufacturer

• The NFRC product ID (or CSA report in Canada) for each glass package you are considering.
• The U-factor and SHGC for your actual configuration, including any grilles or divided lites.
• Whether the line meets ENERGY STAR for your region, if a rebate depends on it.

Code minimum vs ENERGY STAR

ENERGY STAR sets its own, usually stricter, limits by region. Utility rebates and some tax credits are tied to ENERGY STAR or its “Most Efficient” tier rather than to the code minimum. Check the rebate rules before choosing glass.

The values in RFV’s lookup are marked draft until verified against the published code tables.