Residential Code Methodology

 The U.S. Department of Energy (DOE) has established a methodology for evaluating the energy and economic performance of residential energy codes. This method serves to ensure DOE proposals are both energy efficient and cost-effective. In addition, DOE will evaluate published codes, which will help states and local jurisdictions better understand the impacts of updating residential energy codes.

The DOE methodology contains two primary assessments:

1. Energy savings
2. Cost-effectiveness

Energy and economic calculations are performed through a baseline comparison, modeling changes between representative building types. National energy savings are reported, in addition to economic metrics by state and climate zone. In considering cost-effectiveness, longer term energy savings are balanced against additions to initial costs through a life-cycle cost perspective. These first costs are referenced in DOE's Residential Cost Database, which serves as a supporting tool for the analysis.

Energy Savings

The DOE methodology models code changes for single-family and multifamily homes across a range of climates, foundation and fuel types. Energy is modeled using DOE's EnergyPlus™ software for two residential building types:

1. Single-family: A two-story home with a 30-ft by 40-ft rectangular shape, 2,400 ft² of floor area excluding the basement, and windows that cover 15% of the wall area, equally distributed on all sides of the house.
2. Multifamily: A three-story building with 18 units (6 units per floor), each having conditioned floor area of 1,200 ft² and window area equal to approximately 10% of the conditioned floor area, equally distributed on all sides of the building.

In addition, DOE also examines variations by heating system and foundation type. This results in the analysis of multiple unique scenarios across each climate zone. DOE's EnergyPlus™ software covers almost all aspects of residential envelopes, HVAC, water heating, and lighting systems. In some cases, alternative methods are employed to more accurately portray the effects of a given change.

Cost-effectiveness

The DOE methodology accounts for the benefits of energy efficient home construction over a 30 year analysis period, balancing initial costs against longer term energy savings. DOE evaluates energy codes based on three measures of cost-effectiveness:

1. Life-Cycle Cost: Full accounting over a 30-year period of the cost savings, considering energy savings, the initial investment financed through increased mortgage costs, tax impacts, and residual values of energy efficiency measures.
2. Cash Flow: Net annual cost outlay (difference between annual energy cost savings and increased annual costs for mortgage payments, etc.).
3. Simple Payback: Number of years required for energy cost savings to exceed the incremental first costs of a new code.

Life-cycle cost is the primary measure by which DOE assesses cost-effectiveness of residential energy codes. These savings assume: initial costs are mortgaged, homeowners take advantage of mortgage interest deductions, and some components retain a residual value after the analysis period.

For more information on the DOE assessment of residential energy codes, please visit the following:

• DOE Residential Methodology
• BC3 Residential Cost Database