BattWiseR-value targets by climate zone, and what it takes to hit them.

basement wall, climate zone 4, mineral wool

Required R-value
R-10= 2.4″ of mineral wool
IECC 2021 minimum for a basement wall in climate zone 4
Climate zone4
Assemblybasement wall
Code minimumR-10
mineral wool — R per inchholds shape, fire and sound resistant4.2R/in
Thickness needed2.4″
One R-value betterabove code — usually cheap in an attic, expensive in a wallR-20= 4.8″
Climate zone
4
Assembly
basement wall
Material
mineral wool

The number and the thickness

IECC 2021 sets R-10 for a basement wall in climate zone 4. That is a resistance target, not a thickness — how deep it has to be depends entirely on what you fill it with. In mineral wool, it comes to about 2.4 inches.

Check your local code first

IECC 2021 is the reference here, but it is not automatically the law where you live. Plenty of states still enforce the 2009, 2015 or 2018 edition, and some cities require more. The building department decides.

Installation beats the number on the bag

A batt compressed around wiring, or blown insulation that has been walked on, loses a large part of its rated R-value. And air sealing comes first regardless: moving air carries far more heat than conduction does, so an air-sealed assembly at R-10 outperforms a leaky one rated well above it.

Notes

  • IECC 2021 is the reference here. Many states still enforce the 2009, 2015 or 2018 edition, and some cities go above it. Your building department decides, not this table.
  • R-value assumes the insulation is installed without gaps, compression or wind-washing. A batt squeezed around wiring can lose a third of its rated value — installation quality matters more than the last R-5 on paper.
  • Air sealing comes first. An attic at R-60 with unsealed penetrations underperforms an air-sealed R-38, because moving air carries far more heat than conduction through the assembly.

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