Historically, the residential International Energy Conservation Code (IECC) has offered two main ways (or paths) for builders to demonstrate code compliance: Prescriptive and Performance. The 2015 IECC includes a third and new compliance path: the Energy Rating Index (ERI).
The new code provides a required ERI score for each climate zone to ensure that the efficiency of homes built using this path to demonstrate code compliance are at least as efficient as homes using other compliance paths in 2015. The ERI provides more flexibility for builders to meet code requirements.
Like a miles-per-gallon sticker on cars, an ERI allows homebuyers to compare the energy-efficiency (and estimated annual energy costs) of different homes by providing a score for each house, based on a scale of zero to 100. A HERS Rating is the most commonly used and practical Energy Rating Index in the USA.
A home that scores 100 points is about as efficient as a home built to 2006 standards, and a home that scores zero is considered "net zero".
The ERI Path is different from other compliance paths because it gives builders credit for installing some high-efficiency items not otherwise covered in the code. A home gets ERI credit (i.e. a lower score) for features such as solar panels, high-efficiency mechanical equipment such as heating, cooling, and water heating, and appliances (refrigerators, dryers, etc). This is important because while other compliance paths require 2015-specific building envelope components, the ERI path only requires a building envelope that meets the 2009 requirements.
This is particularly important for builders and contractors who install spray foam insulation. Without the ERI/HERS path, the 2015 prescriptive insulation levels will dramatically increase construction costs for the additional foam required to comply.