ENERGY STAR Window Criteria by Climate Zone

Four zones, not one national standard

ENERGY STAR doesn't set one U-factor for the whole country. Version 7.0 of its window specification, effective since October 23, 2023, defines four climate zones, Northern, North-Central, South-Central and Southern, each with its own certification criteria.^[1],[2]

The four zones' numbers

Northern requires a U-factor of 0.22 or lower and an SHGC of 0.17 or higher. North-Central requires a U-factor of 0.25 or lower and an SHGC of 0.40 or lower. South-Central requires a U-factor of 0.28 or lower and an SHGC of 0.23 or lower. Southern requires a U-factor of 0.32 or lower and an SHGC of 0.23 or lower.^[1]

Every zone also has to meet a 0.3 cfm per square foot Air Leakage ceiling to qualify at all.^[1]

Why the U-factor requirement gets looser moving south

Colder zones demand a lower U-factor because heat loss through the glass is the dominant energy cost there. As you move toward warmer zones, U-factor still matters, but less severely, which is why the ceiling relaxes from 0.22 in the Northern zone to 0.32 in the Southern zone.

Why the Northern zone's SHGC rule runs backward

North-Central, South-Central and Southern all cap SHGC, limiting how much solar heat a window can let in. The Northern zone instead sets a floor, requiring SHGC of at least 0.17, because in a cold enough climate, solar heat gain through south-facing glass heats the room at no fuel cost at all, and a window that blocks too much of it forces the furnace to work harder to make up the difference.^[1]

Where Washington, California and Florida homes tend to land

Washington, California and Florida each cover a real range of IECC climate zones rather than sitting entirely in one, so the same state can straddle more than one ENERGY STAR zone depending on the county. That's exactly why this site reports climate and ENERGY STAR zone at the city level on each city page, instead of asserting one number for an entire state.^[1]

Certification is a floor, not a ceiling

A product that just clears its zone's requirement is meeting the minimum, not necessarily the best available option in that zone. Many manufacturers publish U-factor and SHGC numbers well past the certification floor, and comparing the specific number on a quote against the zone requirement, rather than just checking for the ENERGY STAR mark, is how you tell the two apart.

How to use this when a quote lands in front of you

Find your climate zone on this site's city pages, then ask for the U-factor and SHGC of the specific product you're being quoted rather than trusting a general ENERGY STAR claim on the sales sheet. If the number offered exactly matches the zone floor, ask what it would cost to beat it; the gap is often smaller than the gap between window styles or frame materials.

Questions people ask

Do these criteria change over time?

Yes. Version 7.0 took effect October 23, 2023 and tightened several zone requirements from the prior version, so a product certified years ago under an older version may not meet the current numbers.^[1],[2]

Can a window certified for one zone be sold in another?

Manufacturers often sell the same physical product across zones, but ENERGY STAR certification for a specific zone requires meeting that zone's numbers, so a Southern-zone-certified product isn't automatically certified for the Northern zone.

What happens to skylights and doors under this same specification?

They're covered by their own tables in the same specification, with different numbers than windows; a door's glazing level (opaque, half-lite or more) changes its own U-factor and SHGC targets separately from the window table.^[1]

Is there any way to qualify without meeting a zone's exact numbers?

For the Northern zone specifically, the specification also allows an equivalent-performance path, where a slightly higher U-factor can qualify if it's paired with a higher SHGC that offsets it, rather than requiring the exact prescriptive numbers.^[1]