IECC CZ 5A (cold): Chicago, Boston, Denver, Salt Lake City area. HDD: 5500-7000. CDD: 1500-3500. Design temps: summer 91F, winter -4F (Chicago). Residential envelope: ceiling R-49, wall R-20+5ci or R-13+10ci, floor R-30, windows U-0.30. Air leakage: 3 ACH50. Key challenges: ice dams, freeze-thaw cycling (foundation, masonry), frost heave, snow loads (25-40 psf ground). Vapor retarder: Class I or II (on warm-in-winter side). HVAC: heating-dominated, high-efficiency furnace/heat pump. Seismic: SDC A-B (most), SDC D (New Madrid zone). Insulation: continuous exterior insulation critical to prevent thermal bridging.