After delays due to the COVID 19 pandemic, the 2018 Washington State Energy Code (WSEC) became effective on February 1, 2021. The 2018 Seattle Energy Code (SEC) will take effect on March 15, 2021.

Our Energy & Sustainability team has been helping a number of project teams navigate the 2018 WSEC and SEC changes. Along with authoring this blog post, Jason Acosta and Denali Jones presented their take on the code changes at the Lighting Design Lab’s Cost Effective Code Compliance Building Envelope webinar on January 27. Want to hear how we anticipate the code will affect building design and construction?


This blog post refers to changes to the Commercial portion of the WSEC and SEC, which covers most buildings other than residential-unit-only buildings that are three stories and less. The blog post was updated on February 24, 2021, check back for further updates soon.

Introduction of a New Compliance Path

The 2018 WSEC introduces a new outcome-based energy budget compliance path in addition to the prescriptive compliance and the total building performance paths. The outcome-based energy budget was adopted from the 2015 Seattle Energy Code and sets a maximum Energy Use Intensity (EUI) budget for each building occupancy/use type. Project teams must develop a single whole-building energy model for the proposed building to comply with this new standard. This path limits the building enclosure’s total heat loss coefficient (UA) to be no more than 20% higher than the level stated in the prescriptive code. To ensure compliance is maintained, the building owner or the owner’s representative must submit a year of the building’s actual energy use within the first three years of occupancy, and every five years after the first submission. Penalties will be imposed if the building’s actual EUI exceeds the code-mandated EUI budget. It is important to note that the outcome-based energy budget compliance path is not applicable unless it has been adopted by your local jurisdiction.

While the outcome-based path presents a new option for project teams to explore, teams are still able to use the existing compliance paths as follows:

  • The prescriptive compliance path can be met by either achieving the specified R-value for each assembly or using the envelope component trade-off method.
  • The total building performance path requires developing two whole-building energy models—a baseline building and the proposed building— to demonstrate that the energy performance of the proposed design is better than the baseline building.

Changes to the Total Building Performance Path

The 2018 WSEC and SEC include the following important changes to the total building performance compliance path:

  • For the WSEC, this path now limits the building enclosure’s total UA to no more than 20% higher than the level stated in the prescriptive path; for the SEC, the total UA is limited to no more than 10% higher.
  • The energy modeling requirements are now based on ASHRAE Standard 90.1 Appendix G rather than the International Energy Conservation Code (IECC).
  • The primary performance metric is now carbon emissions rather than energy consumption.
  • The Section C406 extra efficiency credits are no longer required to use this path.

New and Updated Definitions

The 2018 WSEC includes several definition updates that relate to the code changes. The highlighted definitions listed below are important in understanding the new updates.

  • Continuous insulation: Only metal thermal bridges are stipulated in this definition. Insulation is considered continuous only if metal thermal bridges have a cross-sectional area £ 04% of the enclosure surface area; otherwise, the insulation does not meet this definition and metal thermal bridges must be accounted for.
  • Mass transfer deck slab edge: Cantilevered balconies are now excluded from the definition of a mass transfer deck slab edge. This means that cantilevered concrete balconies must meet the insulation requirement for mass walls. The diagram below depicts a mass transfer slab edge at a typical deck condition or the transition from an above-grade structure to a below-grade structure.

Air-leakage Testing

In 2009, Washington State introduced mandatory air-leakage testing for all new buildings and has continued to incrementally increase air tightness targets. The 2018 WSEC requires project teams to aim for a more stringent target in terms of air leakage. Completed buildings will need to achieve a maximum air-leakage rating of 0.25 cfm/ft at 0.3 inches water gauge in order to meet the 2018 WSEC.

If the test result is greater than 0.25 cfm/ft and less than 0.40 cfm/ft, the project team is required to conduct visual inspections, seal the leaks, and document corrective actions.

If the test result exceeds 0.40 cfm/ft, the project team will be required to make the necessary remediations and conduct the test again. Further remedial work and retesting will be required until a leakage rate of 0.40 cfm/ft or less is achieved. A test above 0.40 cfm/ft will not be accepted.

While the change should not affect the design and construction of new buildings, it does reduce the margin for error and increases the consequences of not meeting the required air-leakage target. Project teams will need to focus on well thought-out air barrier detailing, as well as quality assurance and quality control programs during construction. Added attention to all aspects of the air barrier design and construction are necessary to avoid the potentially significant consequences of not meeting the desired air-leakage rating.

RDH Victoria airtightness testing

Understanding the Extra Efficiency Credits

The extra efficiency credits were introduced in the 2015 WSEC for project teams pursuing the prescriptive compliance path. The 2018 WSEC now requires project teams to achieve a minimum of six credits—a four-credit increase from the 2015 code. The 2018 WSEC provides a series of options that ultimately contribute to the energy efficiency of the finished building. Each option is weighted differently depending on the building occupancy type. As shown in Efficiency Package Credits table below, some options allow project teams to earn more credits than others. For example, by achieving enhanced envelope performance per Section C406.10c on a Group R-2 building, project teams will earn all six credits needed. Teams that implement a high-performance, dedicated outdoor air system (DOAS) per Section C406.7 on the same building type will earn just four credits, requiring the team to seek more credits using another option.

Efficiency Package Credits table has been adapted by RDH from the WSEC 2018 edition.

Future Changes and How to Learn More

This blog post was updated on February 24, 2021. As the code continues to evolve including possible amendments and our knowledge of these changes expands, we will update this post for your reference. To stay informed as this post is updated, please provide your contact information below.

The second printing of the WSEC – Commercial 2018 Edition can be viewed here. Seattle’s draft amendments to the 2018 WSEC are available here. The second printing of the Residential Edition can be viewed here.

Our Energy & Sustainability team has the unique ability to provide an integrated approach to enclosure and energy-efficient design, allowing for the development of optimized solutions that provide cost-effective means of achieving design intent and energy performance goals. RDH has extensive expertise in whole-building energy modeling and completing prescriptive energy code compliance calculations for many building types, from small and large residential buildings to commercial offices, and institutional and government buildings. If you have any questions about these changes and would like to learn how they will affect your project, please reach out to us at or call 206-324-2272.

Written by:

Natalie Michaelis