What U-factor do I need for my climate zone?
ENERGY STAR's residential window thresholds are a U-factor of 0.22 or lower in the Northern zone, 0.25 or lower in North-Central, 0.28 or lower in South-Central and 0.32 or lower in the Southern zone, in Btu/h·ft²·°F. Air leakage must be 0.3 cfm/ft² or lower in every zone. Your local energy code may be stricter.
The criteria table
These are the thresholds a window has to achieve to be certified in a zone. They are not our product ratings — those live on the product pages, each next to the NFRC directory number that proves it.
| Climate zone | U-Factor (Btu/h·ft²·°F) | SHGC | Basis |
|---|---|---|---|
| Northern | ≤ 0.22 | ≥ 0.17 | Prescriptive |
| Northern | = 0.23 | ≥ 0.35 | Equivalent energy performance |
| Northern | = 0.24 | ≥ 0.35 | Equivalent energy performance |
| Northern | = 0.25 | ≥ 0.40 | Equivalent energy performance |
| Northern | = 0.26 | ≥ 0.40 | Equivalent energy performance |
| North-Central | ≤ 0.25 | ≤ 0.40 | Prescriptive |
| South-Central | ≤ 0.28 | ≤ 0.23 | Prescriptive |
| Southern | ≤ 0.32 | ≤ 0.23 | Prescriptive |
Source: US EPA / ENERGY STAR, ES_Windows_Table_Oct2023.pdf (October 2023). Air leakage, all zones: ≤ 0.3 cfm/ft². These are the government's thresholds, not NEXWD product ratings.
The Northern zone has two routes, and most summaries only give one
A window can qualify in the Northern zone prescriptively, at a U-factor of 0.22 or lower with an SHGC of 0.17 or higher. It can also qualify by equivalent energy performance at a higher U-factor, provided it lets more solar heat in: at U-factor 0.25 the SHGC has to be 0.40 or higher.
That is not a loophole. In a heating-dominated climate, admitted winter sun is free heating, and the criteria recognise a window that trades a little conduction for a lot of gain.
Two things the number does not settle
First, U-factor is a whole-window figure or it is not comparable. A centre-of-glass number omits the frame, which on an aluminum window is precisely the part under question. Ask for the whole-product rating and the CPD number behind it.
Second, the zone tells you the U-factor target but not the coating. Coating choice is decided by whether you are fighting heat loss or heat gain, and on a mixed-climate building it can differ by orientation on the same elevation.
We do not compute a pass or fail badge against this table. ENERGY STAR certification is a formal determination made by EPA-recognised certification bodies against tested ratings, model by model and zone by zone. Tell us your zone and we will confirm in writing which models qualify.
Source: US EPA / ENERGY STAR, ES_Windows_Table_Oct2023.pdf, October 2023. A US federal work.
Other questions
- How long do imported aluminum windows take to arrive in the US?
- What is a thermal break, and does it matter?
- Tilt and turn or casement: which should I specify for a US project?
- What does NFRC certification actually tell me?
- Can imported windows meet Florida HVHZ requirements?
- What documentation should a window supplier give me?
- Can a large window opening ship as one assembly?
Still the wrong question for your project?
Send us the opening schedule →