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CRES: Energy Modeling, Codes, and Program Standards: Baselines, Indices, and Rules

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Energy Modeling, Codes, and Program Standards: Baselines, Indices, and Rules

Energy Modeling, Codes, and Program Standards tests whether you can build a defensible baseline, run a model that matches the house you actually inspected, and report results in the currency a program uses - HERS Index, percent savings, or a code compliance path.

The HERS Index

A HERS Index of 100 equals the reference home: same geometry, size, and bedroom count, built to the reference specifications. Zero means a zero-net-energy home. Lower is better, and the index is built on annual source energy, so fuel type and site-to-source factors matter.

Inputs that swing results

  • Insulation values at installed Grade, not bag labels.
  • Framing fraction and thermal bridging.
  • Infiltration: measured ACH50 converted with an n-factor, or a documented default.
  • Duct location and leakage, split between inside and outside the envelope.
  • Equipment efficiency as installed, derated for duct loss and age.
  • Window U-factor and SHGC, foundation type, and slab or crawlspace assumptions.

Codes and programs

IECC offers a prescriptive path and a performance path (ERI or simulated performance) that must meet the same code-minimum baseline. Later editions tighten envelope and lighting assumptions, so comparing a 2012-based model to a 2021-based model is not apples to apples. ENERGY STAR Certified Homes adds a HERS Index target plus mandatory checklists: thermal enclosure, HVAC, water management, and indoor air quality. DOE Zero Energy Ready Home layers additional requirements on top.

Worked example. The reference home models 95 MMBtu of annual source energy; the proposed design models 66.5 MMBtu. HERS Index = (66.5 ÷ 95) × 100 = 70. If a package of measures costing $4,200 reduces annual energy cost by $350, simple payback is 12 years - and that is before accounting for measure life, maintenance, or interactive effects such as reduced cooling load from added insulation.

Traps that cost marks

  • Comparing outputs from different code vintages or different reference homes.
  • Changing the baseline mid-project to make savings look larger.
  • Double counting interactive effects when stacking measures.
  • Entering nameplate AFUE or SEER2 instead of installed, derated performance.
  • Presenting modeled savings as measured savings without calibration to utility data.