This is the 2015 IECC with commentary. The IECC is a model code that regulates minimum energy conservation requirements for all aspects of energy uses in both commercial and residential construction, including heating and ventilating, lighting, water heating, and power usage for appliances and building systems. The IECC is a design document. For example, before one constructs a building, the designer must determine the minimum insulation R-values and fenestration U-factors for the building exterior envelope. Depending on whether the building is for residential use of for commercial use, the IECC sets forth minimum requirements for exterior envelope insulation, window and door U-factors and SHGC ratings, duct insulation, lighting and power efficiency, and water distribution insulation.