Energy Code Requirements for Additions and Remodels

Chapter 5: Existing Buildings

The 2021 WSEC-R doesn’t only apply to new construction. Chapter 5 (Section R501–R503) covers existing buildings — additions, alterations, renovations, and component replacements. The scope of what you’re changing determines how much of the energy code applies to your project.

Addition Size Thresholds

The credit requirement for additions depends on the size of the addition:

• Under 150 square feet: Exempt from R406 credit requirements entirely.
• 150–500 square feet: Requires 2.0 credits under the R406 path.
• Over 500 square feet: Full tier requirements apply, based on the total conditioned area of the home (existing plus addition).

What Triggers Compliance

Specific replacement and renovation activities trigger energy code requirements for that component:

• Replacing windows: New fenestration must meet U-0.30 area-weighted average.
• Replacing HVAC equipment: New equipment must meet current code efficiency requirements.
• Replacing water heater: New unit must meet current code efficiency requirements.
• Replacing more than 10% of light fixtures: 100% of replaced fixtures must be high efficacy (typically LED). This 10% rule applies specifically to alterations/remodels under Section R503.1.4. If you’re changing fewer than 10%, the unchanged fixtures can stay as-is. For new construction, 100% high-efficacy lighting is already required regardless.
• Exposing wall cavities: Must insulate to R-15 for 2x4 walls or R-21 for 2x6 walls.
• Exposing ceiling cavities: Must insulate to R-60, or to the maximum depth the attic space can accommodate based on the roof pitch — whichever is less. In low-slope roof situations, you install as much as physically fits.
• Exposing floor cavities: Must insulate to R-30.

What Does NOT Trigger Compliance

• Storm windows: Adding storm windows over existing windows does not trigger fenestration replacement requirements.
• Roof recover without exposing cavity: Re-roofing over existing sheathing without removing the existing roof covering does not trigger insulation requirements.

Replacement Fenestration

Section R503.1.1.1 explicitly permits an area-weighted average for replacement fenestration. This means you don’t need every individual window to hit U-0.30 — your total replacement fenestration package just needs to average U-0.30 or better when weighted by area. This provides useful flexibility when replacing windows of different sizes or types. See the WAC 51-11R for the full section text.

Frequently Asked Questions

I’m just replacing windows — do I need to comply with the whole energy code?

No. Only the window replacement itself needs to comply. Your replacement fenestration must meet a U-0.30 area-weighted average. You do not need to upgrade insulation, HVAC, or other components unless you are also altering those systems.

I’m adding a 400 square foot sunroom — how many credits do I need?

A 400 square foot addition falls in the 150–500 square foot range, which requires 2.0 credits under the R406 path. This is a reduced threshold compared to new construction, making compliance straightforward for most moderate-sized additions.

Are there rebates for energy-efficient upgrades?

Many Washington utilities offer rebates for high-performance windows, insulation upgrades, heat pumps, and heat pump water heaters. These rebates can sometimes cover the cost difference between meeting minimum code (U-0.30 windows) and exceeding it (U-0.22 triple-pane). Check your electric utility’s website for current rebate programs.

Additionally, the federal Energy Efficient Home Improvement Credit (25C) provides tax credits for qualifying windows, insulation, heat pumps, and water heaters meeting ENERGY STAR criteria. Consult a tax professional for details on your specific situation.