IECC energy code & glazing

The IECC is the reason you cannot just put any glass in any building: it sets the minimum U-factor and SHGC your windows have to hit, and those numbers change depending on where the building sits on the climate map. Get the climate zone right and the glass practically specifies itself; get it wrong and the project fails plan check.

6 min read Foundational Energy compliance

What the IECC covers

The International Energy Conservation Code (IECC) is a model code developed by the International Code Council (ICC) that establishes minimum energy efficiency requirements for new buildings. Most US jurisdictions have adopted some version of the IECC, though states and municipalities often amend it or lag the current edition by one or more code cycles. When a project's specifications reference "energy code requirements," they mean the version of the IECC (or ASHRAE 90.1, which the commercial provisions of the IECC often reference) that is in force in the jurisdiction where the building will be constructed.

For glazing, the IECC sets minimum performance requirements for fenestration, all windows, doors, and curtainwall, based on climate zone.

Climate zone map

The US is divided into eight climate zones based on heating degree days, cooling degree days, and moisture levels. Each zone has different energy priorities:

Very Hot

South Florida, Hawaii

Hot

Houston, Phoenix, Tampa

Warm

Atlanta, Los Angeles, Dallas

Mixed

Baltimore, Seattle, Raleigh

Cool

Chicago, Denver, Boston

Cold

Minneapolis, Burlington

Very Cold

International Falls, MN

Subarctic

Fairbanks, AK

Fenestration requirements by zone

IECC maximum U-factor and SHGC requirements tighten with each code edition. The values below are representative of the 2021 IECC commercial building requirements for vertical fenestration. Always verify against the adopted code version in your jurisdiction:

Climate Zone	Max U-Factor	Max SHGC	Dominant concern
1 (Very Hot)	NR*	0.25	Cooling load; solar control is paramount
2 (Hot)	0.40	0.25	Cooling dominant; some winter heat loss concern
3 (Warm)	0.35	0.25	Balance between solar and thermal control
4 (Mixed)	0.35	0.40	Mixed heating/cooling; higher SHGC allowed
5 (Cool)	0.32	NR*	Heating dominant; solar gain can be beneficial
6 (Cold)	0.32	NR*	Heating dominant; maximize solar gain
7 (Very Cold)	0.28	NR*	Severe heating demand; triple pane often needed
8 (Subarctic)	0.25	NR*	Extreme cold; very high insulation required

* NR = No Requirement in this direction (no maximum). Values are representative of 2021 IECC commercial vertical fenestration; always verify the adopted edition.

Using the code to drive glass selection

The path from code requirement to glass specification runs through NFRC whole-window values:

Determine the project's climate zone (IECC climate zone map or county lookup)
Find the applicable code edition in effect in the jurisdiction
Identify the maximum U-factor and SHGC from the applicable fenestration table
Look up candidate glass products in the NFRC Certified Products Directory, use whole-window values for the specific frame system being specified
Confirm that the candidate product's NFRC whole-window U-factor ≤ code maximum and SHGC ≤ code maximum

Fenestration area limits

The IECC also limits the percentage of wall area that can be fenestration (the "window-to-wall ratio" or WWR). For commercial buildings, exceeding the prescriptive WWR limit requires the designer to use an energy model to demonstrate that the total building energy use meets or exceeds the prescriptive baseline, even if individual windows meet the U-factor and SHGC requirements. High-glazing buildings should engage an energy modeler early.

ASHRAE 90.1 and the IECC

For commercial buildings, many jurisdictions allow compliance with ASHRAE 90.1 as an alternative to the commercial provisions of the IECC. ASHRAE 90.1 has its own fenestration tables and climate zone designations (which align with IECC climate zones). Both pathways require NFRC-certified values for compliance documentation.

Canadian projects, NECB 2020

Canadian projects do not fall under the IECC. Instead, they are governed by the National Energy Code of Canada for Buildings (NECB) 2020, published by the National Research Council of Canada (NRC), or by the equivalent provincial code:

Ontario, Ontario Building Code (OBC), which adopts NECB with amendments
British Columbia, BC Building Code (BCBC) / BC Energy Step Code
Alberta, Alberta Building Code (ABC)
Quebec, Code de construction du Québec
Other provinces, Most adopt NECB 2020 with provincial amendments

NECB 2020 classifies locations by Heating Degree Days (HDD) below 18°C (referenced in NECB Appendix C), rather than the numbered zone system used in the IECC. However, the ASHRAE climate zone map does align geographically, making the following approximate equivalencies useful for planning:

ASHRAE Zone	Approx. NECB HDD Range	Representative Canadian Cities
Zone 4	HDD < 3000	Vancouver BC, Victoria BC, Kelowna BC
Zone 5	HDD 3000–4000	Toronto ON, London ON, Windsor ON, Hamilton ON
Zone 6	HDD 4000–5000	Ottawa ON, Montreal QC, Calgary AB, Halifax NS, Fredericton NB
Zone 7	HDD 5000–7000	Edmonton AB, Winnipeg MB, Quebec City QC, Saskatoon SK, Regina SK, Thunder Bay ON, Sudbury ON
Zone 8	HDD > 7000	Whitehorse YT, Yellowknife NWT, Iqaluit NU

Do not use IECC U-factor tables directly for Canadian code compliance

NECB 2020 fenestration requirements are expressed in SI units (W/m²·K for U-factor, and use slightly different performance thresholds). For Canadian code compliance documentation, obtain NFRC-equivalent certification from the Canadian Standards Association (CSA A440) or confirm with your AHJ which rating system is accepted.

Disclaimer: Code requirements vary by jurisdiction and code edition. Always verify the adopted code version and any local amendments with the authority having jurisdiction (AHJ) for your specific project.