What is the Washington State Energy Code?

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?

The R406 path has two components:

• R406.2 Fuel Normalization Credits: You select your primary heating system type (gas furnace, heat pump, electric resistance, etc.) and receive 0–3.0 credits based on fuel type.
• R406.3 Energy Credits: You select options across 7 categories — building envelope, air leakage, HVAC equipment, HVAC distribution, water heating, renewable energy, and appliances — earning credits for each selection.

Your total (R406.2 + R406.3) must meet the requirement for your building size tier: 5.0 credits for small homes, 8.0 for medium, 9.0 for large. See our credit requirements guide for details.

In short: pick your heating system (that’s your biggest single decision), then choose efficient options across the other categories until your total hits the target for your building size. The wsec.ai wizard walks you through each choice and shows your running total in real time.

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?

Yes. The energy code applies to the building, not the builder. Whether you’re a licensed contractor or an owner-builder under RCW 18.27.090, your permit will require 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.