Every project I quote starts with the same question, whether the architect says it out loud or not: what spec actually fits this climate? The honest answer is "it depends on the zone" - but that's not useful on its own, so here's the zone-by-zone version, with the code targets and which of my other write-ups covers the deep dive for your project.
This is a reference page, not a full explainer. If you want the reasoning behind U-factor and SHGC, start with what a good U-factor actually is - this page assumes that background and gets straight to "so what do I spec."
IECC climate zones, in one table
The US is split into eight IECC climate zones, numbered 1 (Miami, hottest) through 8 (interior Alaska, coldest), with humid/dry/marine sub-zones (A/B/C) layered on top. Residential prescriptive code sets a U-factor and SHGC ceiling per zone - these are typical 2021 IECC baseline numbers, not a substitute for your local jurisdiction's actual amendment:
| Zone | Representative areas | Typical U-factor max | Typical SHGC max | What drives the spec | |---|---|---|---| | 1-2 | Miami, Houston, South Texas | 0.40-0.50 | 0.25 | Cooling load - SHGC leads | | 3 | Dallas-Fort Worth, Austin, Atlanta, Charlotte | 0.30-0.32 | 0.25 | Mostly cooling, code tightens on U-factor | | 4 | NYC, DC, Nashville, Denver (marine 4C: Seattle, Portland) | 0.30 | 0.40 (not required in marine 4C) | Mixed load - both numbers matter | | 5 | Chicago, Boston, Minneapolis-adjacent | 0.30 | not required | Heat loss leads, SHGC becomes an orientation choice | | 6-7 | Minneapolis, Burlington, Duluth | 0.30 (high-performance targets run lower) | not required | Heat loss, comfort, condensation control | | 8 | Interior Alaska | 0.30 or tighter by local amendment | not required | Extreme heat loss, always verify local code |
Two things to flag before you read this as gospel: first, zones 1-3 cap SHGC and largely ignore U-factor as the binding constraint, while zones 5-8 do the opposite - the code target actually inverts depending on where you are, which is why a single national "good window" number doesn't exist. Second, plenty of jurisdictions run their own overlay on top of IECC (California Title 24, several Texas municipalities, some Northeast states) - always check the local plan reviewer's copy before locking a spec.
Hot zones (1-3): SHGC leads, U-factor is the easy part
In zones 1 through 3, cooling load dominates. A window with a mediocre U-factor but well-controlled SHGC will usually clear code without drama - the reverse combination fails. Double glazing with a strong low-E coating is typically the right call here; triple glazing adds cost and weight without much cooling-load payoff.
I've written the full breakdown for Texas specifically - the SHGC-first logic, the code table by sub-zone, and how it plays out on large lift-slide openings - in European windows in Texas. The same reasoning applies broadly across zones 1-3.
Mixed zones (4-5): both numbers matter, and orientation starts to count
Zone 4 and 5 projects (NYC, DC, Chicago, Denver) sit in the transition band where you can't lead with just one metric. U-factor becomes the primary constraint, but SHGC still matters - particularly on south and west glazing, where too much solar gain drives up cooling cost even in a heating-dominated climate. This is also where triple glazing starts to make financial sense on a case-by-case basis rather than as a default.
Commercial projects in this band - office, multifamily, mixed-use - run a different compliance path (ASHRAE 90.1 and COMcheck rather than residential IECC); see commercial energy code compliance for how that math works.
Cold zones (6-8): U-factor leads, and the details around it decide comfort
North of zone 5, U-factor is the binding constraint and triple glazing typically earns its cost. But the code minimum isn't the whole comfort story - warm-edge spacers, genuine thermal breaks, and a real compression seal against air infiltration matter as much as the headline U-factor number once you're fighting real cold. I go through all of that, plus the SHGC-by-orientation nuance that's specific to cold climates, in the best windows for cold US climates.
The one spec mistake that crosses every zone
Regardless of climate zone, the most common spec error I see is comparing a glass-only U-factor to a whole-unit U-factor. A glass-only number is always better than reality - it ignores the frame and spacer, which is exactly where performance is lost. Whatever zone you're in, make sure the number on your schedule is the tested whole-unit value. That single check catches more spec problems than any zone-specific detail on this page.
Where to go next
Run your actual window schedule - sizes, material, glazing, zone - through the project estimator for a landed budget number, or send me your drawings directly and I'll have a first pass back the same or next working day.
FAQ
What climate zone am I in? The Department of Energy's IECC climate zone map covers every US county. Most jurisdictions also print their assigned zone directly in the local building code amendment - worth confirming with your plan reviewer, since a handful of counties sit right on a zone boundary.
Do I need triple glazing for my zone? Generally only in zones 5 and up, or for a specific acoustic or Passive House target regardless of zone. In zones 1-3 it rarely pays for itself on thermal performance alone - see triple vs. double glazing by climate zone for the full cost-versus-performance case.
Does SHGC matter in a cold climate? Yes, but differently than in a hot one. In zones 6-8, a moderate-to-higher SHGC on south-facing glass helps harvest free winter solar heat, while north and west glazing usually stays lower. It's an orientation decision, not a single spec-wide number.
Is IECC the same everywhere, or do states change it? States and even individual municipalities can amend IECC or run their own code entirely - California's Title 24 is the best-known example. Always verify the local adopted code and cycle before finalizing a spec; the table above is a starting reference, not the plan reviewer's final word.
Kai, your window guy!