The International Energy Conservation Code (IECC) establishes minimum energy efficiency requirements for buildings. Building envelope performance is established through R-values (thermal resistance) for walls, roofs, and foundations. R-values are specified by climate zone, with colder climates requiring higher R-values for insulation. A typical climate zone specifies R-13 or R-15 for above-grade walls (cavity insulation), R-30 to R-38 for roofs, and R-10 to R-20 for basement walls. Continuous insulation (outside the framing) is often required in addition to cavity insulation in certain climate zones. Wall assembly performance can also be demonstrated using U-factor (overall heat transfer rate) with maximum values specified for climate zones. Air barrier performance is critical to energy efficiency, with sealed drywall, foam sheathing, or house wrap materials creating continuous barriers against air leakage. Windows and doors must meet performance specifications including U-factor and Solar Heat Gain Coefficient (SHGC). Window U-factors range from 0.23 to 0.62 depending on climate zone, with lower values in colder climates. Basement and crawlspace foundations must meet insulation requirements with R-values ranging from R-5 to R-30 depending on below-grade depth and climate. Thermal bridging (heat flow through framing and fasteners) reduces overall assembly R-value, so specifications for cavity insulation fill must be verified. Performance testing of the building envelope using blower door testing verifies air leakage rates do not exceed specified limits.