ERI / Performance Path (R405) — When Whole-House Modeling Saves Money

Most Washington residential projects comply with the energy code through the prescriptive path or the Total UA Alternative. There is a third path: R405, the Total Building Performance Path, which uses whole-house energy modeling to demonstrate the proposed home performs as well as a code-baseline reference home. It is more work, requires specialized software, and in the right project saves serious money.

This page covers what the path is, what it costs, and when it is worth pursuing.

What does the R405 Performance Path actually do?

Under the prescriptive path, you meet specific R-values and U-factors on every component and pick R406 credits from a menu. Under the UA trade-off, you can shift insulation between components but must still hit the credit menu separately. The R405 Performance Path replaces both with a single test: does your proposed home use less site energy than a reference home built to code minimums?

If yes, you pass. There is no separate R406 credit requirement when you use R405.

The reference home is generated by the modeling software using your project's geometry, climate zone, and a fixed set of code-minimum component values. The proposed home uses your actual values — your real wall assemblies, real windows, real HVAC equipment, real water heater. The software runs an annual energy simulation on both and reports the result.

How much does R405 / ERI modeling cost?

Hiring it out (hiring an energy consultant or RESNET HERS rater):

• $800 – $2,000 per single-family home (typical $1,200 – $1,500)
• Cost depends on project size, complexity, and how much input data you provide
• Most consultants want plans, window schedule, HVAC equipment specs, water heater specs, and lighting schedule

Doing it yourself:

• Software license: $1,000 – $3,000+ per year (Ekotrope, REM/Rate, or similar)
• Initial learning curve: 20 – 40 hours
• Per-house once trained: 4 – 10 hours

The DIY route makes sense if you are doing more than 8 – 10 R405 projects per year. Below that, hiring it out is almost always cheaper.

Cheaper offshore providers: Fiverr and similar platforms have providers offering REScheck/ComCheck/ERI reports for $200 – $500. Quality is uneven. For a permit submittal where the AHJ may push back on the numbers, the savings may not be worth the rework risk.

When does the R405 Performance Path save money?

The Performance Path lets you trade an expensive envelope for a more efficient mechanical system, or vice versa, as long as the total energy use comes out under the reference. Real cases where it pays off:

• Window-heavy designs. Large glazing areas blow the prescriptive U-factor budget. A whole-house model can compensate with better walls, a higher-SEER heat pump, or a heat pump water heater and still pass.
• Custom homes with unusual geometry. Cathedral ceilings, expansive vaulted spaces, or nonstandard floor plans often miss prescriptive limits but pencil out under a model.
• Builders standardizing on heat pumps. A high-efficiency heat pump and heat pump water heater combination often models well below the reference, leaving budget room to skip thicker insulation or upgraded windows.

Builders who run the path successfully typically save $5,000 – $15,000+ on materials and labor compared to brute-force prescriptive compliance — savings that more than cover the modeling fee.

When is R405 not worth pursuing?

• Small, simple projects. A 1,500 sq ft rectangle with code-minimum windows and a standard heat pump pencils fine on the prescriptive path. Adding a $1,500 modeling fee just to confirm what you already know does not pay back.
• You like the prescriptive checklist. If your crew is comfortable hitting standard R-values and you do not need flexibility, prescriptive is cleaner.
• You are racing a deadline. R405 modeling adds a week or two to plan submittal. If the permit is on a critical path, prescriptive or UA trade-off is faster.

Does WSEC.ai support the R405 Performance Path?

Not currently. R405 modeling requires whole-house simulation software (Ekotrope, REM/Rate, EnergyGauge USA, or equivalent) that is licensed annually. WSEC.ai handles the prescriptive path, the Total UA Alternative, and the full R406 credit calculation— together those cover the large majority of Washington residential projects.

For projects that genuinely need R405, we can refer you to a vetted energy consultant. Contact us at hello@wsec.ai with your project location and we will connect you.

Quick decision guide