What is the Washington State Energy Code?

First time dealing with the energy code? Start with the whole thing in plain English — what the terms mean, what applies to your project, and what you hand the county. No code sections.

If you're building a new home, addition, or major renovation in Washington State, your local building department will require you to demonstrate compliance with the Washington State Energy Code (WSEC). For residential buildings, this is formally known as WSEC-R 2021, codified in WAC 51-11R.

Who does it apply to?

The WSEC-R applies to all new residential construction in Washington State, including single-family homes, duplexes, townhouses, and additions over 150 square feet. It applies whether you're a licensed general contractor or an owner-builder doing the work yourself.

Your permit application will be reviewed against the energy code, and your inspector will verify compliance before issuing a certificate of occupancy.

What does it require?

The WSEC-R sets minimum requirements for your building's thermal envelope (insulation, windows, air sealing), mechanical systems (heating, cooling, ventilation), water heating, and lighting. The goal is to reduce energy consumption in new buildings.

There are several compliance paths, but the most common for single-family residential is the R406 Energy Credits path. This is what wsec.ai implements. Under R406, you earn credits by selecting energy-efficient options across several categories, and your total must meet a threshold based on your building size.

How does the R406 credit system work?

Your credit total comes from two tables, and you need both.

R406.2 — Fuel Normalization

Pick exactly one option: your primary heating system's fuel type. Gas furnace, heat pump, electric resistance, and so on. This is your single biggest decision, because it can move your total by several credits before you've chosen anything else.

R406.3 — Energy Credits

Pick options across seven categories, taking as many or as few as fit your project:

• Building envelope — insulation and window performance
• Air leakage & ventilation — air sealing, plus heat recovery ventilation
• HVAC equipment — heating and cooling efficiency
• HVAC distribution — where the ducts run and how tight they are
• Water heating — water heater type and efficiency
• Renewable energy — solar generation
• Appliance package — ENERGY STAR appliances

Add the two together, and the total has to meet the requirement for your building's size tier. Small homes need the least, large homes the most — see the tier table and the per-category credit caps.

The order that works: settle the heating system first, then fill the gap from the categories that suit your build. The wizard walks each choice and keeps a running total.

Why does it exist?

Washington State has some of the most progressive energy codes in the country. The legislature has set targets to reduce building energy consumption, and the State Building Code Council (SBCC) updates the energy code on a regular cycle. The 2021 WSEC-R is the currently enforceable version.

How does wsec.ai help?

Traditionally, builders use the WSU C3 spreadsheet to demonstrate R406 compliance. It's a complex Excel workbook that's hard to navigate if you're not a code specialist. wsec.ai replaces that spreadsheet with a guided wizard that walks you through every selection in plain English, calculates your credits in real time, and tells you the cheapest ways to close any gap.

Frequently Asked Questions

Do I need to comply if I'm an owner-builder?

The energy code applies to the building, not the builder — the 2021 WSEC-R applies to all residential buildings, regardless of who constructs them. Even building your own home as an owner-builder — exempt from contractor registration under RCW 18.27.090 — your permit still requires energy code compliance documentation.

What is the WSU C3 spreadsheet?

The C3 spreadsheet is the official compliance tool developed by WSU Extension Energy Program. It's a detailed Excel workbook that calculates R406 credits. It's thorough but complex —wsec.ai provides the same calculation in a more accessible format.

Does REScheck work for Washington State?

No. REScheck does not support Washington State. Washington has its own energy code (WSEC-R) that differs significantly from the base IECC. You must use either the WSU C3 spreadsheet or a tool like wsec.ai that implements the WSEC-R R406 credit path. For the full breakdown, see our guide on whether REScheck works in Washington.

Still feel like you're missing something? The energy code in plain English → — every term on this page, defined without jargon.