2×4 framing (actual depth: 3.5″) is commonly used for interior partition walls where insulation is not required. 2×6 framing (actual depth: 5.5″) is required for Utah exterior walls to accommodate deeper insulation — achieving R-20 or better per Climate Zone 5 energy code. It is also used for plumbing walls where 3″ or 4″ drain lines need to be concealed. RainFire Builders uses 2×6 exterior framing as a default on all new residential construction.