IECC CZ 4C (marine): Seattle, Portland, coastal Pacific NW. HDD: 4500-5000. CDD: 200-800. Design temps: summer 82F, winter 22-27F. Residential: ceiling R-49, wall R-20 or R-13+10ci, floor R-30. Windows: U-0.30. Air leakage: 3 ACH50. Key challenge: rain management. 60+ inches annual rainfall. Walls: rainscreen assembly recommended (drainage gap behind cladding). No vapor barrier on interior — wall must dry to both sides. Roof: steep slopes or proper flat-roof drainage. Mold/moisture: biggest risk. HVAC: heat pump dominant market (mild winters). Cooling: minimal (warming trend increasing need). Seismic: SDC D (Cascadia subduction zone). Foundation: moisture barrier critical.