The prescriptive tables in the 2021 IRC/IECC specify minimum acceptable insulation and fenestration performance levels based on geographic area.
If a builder chooses the prescriptive path, these requirements must simply be met. If a performance path is chosen, the overall energy use of the building must be equal to or better than if the house had been built to these requirements.
Here is a comparison of the prior (2018 IECC) requirements to the current IRC/IECC requirements. 2018 IRC/IECC changes are highlighted in Yellow. There are significant changes:
The 2021 IECC requires R-8 duct insulation, plus ceiling insulation above and below the ducts that totals at least R-19, excluding the value of the duct insulation.
The 2018 IRC/IECC requirement that the R-value of insulated siding be reduced by R-0.6 from the manufacturer's lableled value has been retained.
