Wall Assemblies & Envelope — Design Architect Knowledge Base

Exterior Wall Assembly (Typical — Outside to Inside)

Layer Stack

| # | Layer | Thickness | R-Value | Material | Notes | |---|-------|-----------|---------|----------|-------| | 1 | Cladding | 5/16"–3/4" | — | Fiber cement, metal panel, cedar | Bolted clips (DfD removable) | | 2 | Rainscreen air gap | 3/4"–1" | — | Open cavity, vented top/bottom | Drainage + drying plane | | 3 | WRB (weather-resistant barrier) | — | — | Housewrap or fluid-applied | Continuous, lapped shingle-style | | 4 | Continuous insulation | 1"–2" | R-5 to R-10 | Polyiso or XPS rigid foam | CRITICAL at steel columns (thermal break) | | 5 | Sheathing | 1/2" | — | OSB or plywood | Structural + air barrier (taped seams) | | 6 | Stud cavity | 5.5" (2×6) | R-21 | Mineral wool or fiberglass batt | Between wood studs @ 16" o.c. | | 7 | Vapor retarder | — | — | Kraft-faced batt or smart membrane | Class II for CZ5A (interior side) | | 8 | Interior finish | 1/2"–5/8" | — | Gypsum board (Type X at steel) | Paint finish |

Total Assembly Performance

| Metric | Value | IECC 2021 CZ5A Requirement | |--------|-------|---------------------------| | R-value (cavity + continuous) | R-26 to R-31 | R-20 + R-5ci OR R-13 + R-10ci | | U-factor | 0.038–0.032 | ≤ 0.064 | | Air barrier | Taped OSB + spray foam at penetrations | Required per IECC C402.5 | | Blower door target | ≤ 3 ACH50 | ≤ 5 ACH50 (residential) |

Steel Column Thermal Bridge

Problem: W10×26 column has 10" of solid steel spanning the entire wall cavity — thermal conductivity of steel = 345 BTU·in/(hr·ft²·°F) vs wood at 0.8.

Solution: Continuous exterior insulation wraps AROUND the steel column, maintaining the thermal break. The stud cavity insulation is interrupted at the column, but the exterior rigid foam is NOT.

`` OUTSIDE INSIDE ┌──────┬───┬──────┬───┬────────┬───┬──────┐ │Siding│Air│ R-5 │OSB│ R-21 │VB │ GWB │ ← At wood stud │ │gap│rigid │ │ batt │ │ │ ├──────┼───┼──────┼───┼────────┼───┼──────┤ │Siding│Air│ R-5 │OSB│ STEEL │ │ GWB │ ← At steel column │ │gap│rigid │ │ W10×26 │ │(TypeX│ │ │ │CONTIN│ │(no batt│ │ │ │ │ │UOUS │ │ here) │ │ │ └──────┴───┴──────┴───┴────────┴───┴──────┘

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The R-5 continuous insulation provides the thermal break at the column. Without it, the steel column creates a "cold stripe" visible on IR camera.

Interior Partition Wall (Non-Bearing)

Standard Bedroom/Living Partition

| Layer | Material | Notes | |-------|----------|-------| | 1/2" GWB | Type X where required | Both sides | | 2×4 studs @ 16" o.c. | SPF #2 or better | Non-bearing — single top plate OK | | R-13 acoustic batt | Mineral wool preferred | STC 45+ between bedrooms | | 1/2" GWB | Painted finish | Both sides |

Total thickness: 4-1/2" DfD detail: Bottom plate screwed (not nailed) to precast with reversible powder-actuated pins. Top plate NOT attached to bottom chord of truss (floating clip).

Wet Wall (MEP Spine)

| Layer | Material | Notes | |-------|----------|-------| | 1/2" GWB (moisture-resistant) | Green board or DensArmor | Wet side | | 2×6 studs @ 16" o.c. | SPF #2 | Wider cavity for 3" DWV pipe | | Plumbing: 3" PVC DWV stack | Main drain | Centered in wall | | Plumbing: 1/2" PEX-A (H/C) | Supply lines | Routed with hangers | | 2" PVC vent stack | Through roof | RI requires hard vent (no AAV) | | 3/4" EMT conduit | 200A riser | Electrical service to panel | | 1/2" GWB | Painted finish | Dry side |

Total thickness: 5-1/2" Permanent: This wall NEVER moves between cycles.

Fire-Rated Partition (Garage Separation)

| Layer | Material | Notes | |-------|----------|-------| | 5/8" Type X GWB | Fire-rated | Garage side | | 2×4 studs @ 16" o.c. | SPF #2 | | | R-13 batt | Mineral wool (non-combustible) | | | 5/8" Type X GWB | Fire-rated | Living side |

Rating: 1-hour fire separation (IRC R302.6)

Floor Assembly

Precast Hollow-Core on Steel

| Layer | Material | Notes | |-------|----------|-------| | Finish floor | 3/4" recovered oak (or tile, carpet) | Floated, not adhered | | Precast HC plank | 8" hollow-core | Crane-set, dry bearing on steel | | Steel beam | W12×26 ML Steel | Exposed underside (shop primer + clear) |

Total depth: ~14" (8" plank + 6" beam below) DfD detail: Plank bears on beam flange with neoprene pad — NOT grouted. Crane lifts plank straight up for removal.

Wood Joist on Steel (Secondary Areas)

| Layer | Material | Notes | |-------|----------|-------| | 3/4" subfloor | OSB or plywood | Glued + screwed | | 2×10 joists @ 16" o.c. | SPF #2 or LVL | Simpson LUS hanger to steel beam | | R-30 batt (if over unconditioned) | Mineral wool | Only at cantilevers or overhangs |

Roof Assembly

Truss Roof (Typical)

| Layer | Material | Notes | |-------|----------|-------| | Asphalt shingles | 30-yr architectural | Replaceable each cycle | | Underlayment | Synthetic or #30 felt | Ice & water at eaves (RI: 24" min) | | Roof sheathing | 1/2" OSB or plywood | H-clips at unsupported edges | | Trusses | Fink @ 24" o.c. | Spanning 20' between steel beams | | Ceiling insulation | R-49 blown cellulose or mineral wool | At ceiling plane, not roof plane | | Vapor retarder | Kraft or smart membrane | Below insulation | | 1/2" GWB | Ceiling finish | Attached to bottom chord |

IECC 2021 CZ5A Ceiling Requirement

| Metric | Value | |--------|-------| | R-value | R-49 (or R-38 with advanced framing) | | Air barrier | Continuous at ceiling plane | | Attic ventilation | 1:150 net free area (or 1:300 with balanced intake/exhaust) |

Window & Door Openings in Steel Frame

Window in Exterior Wall at Steel Column

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┌─────────────────────────┐ │ Steel beam above │ ├─────────┬───────┬────────┤ │ │ │ │ │ Wall │WINDOW │ Wall │ │ │ │ │ │ │ │ │ ├─────────┴───────┴────────┤ │ Steel beam below │ └─────────────────────────┘

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• Steel angle lintel NOT needed (beams carry load above)

• Window rough opening framed with wood header + jack studs

• Flashing: sill pan → jamb → head (shingle-lapped up)

• Sealant: backer rod + silicone at exterior, spray foam at interior gap

Performance Targets (IECC 2021 CZ5A)

| Parameter | Requirement | |-----------|-------------| | U-factor | ≤ 0.30 | | SHGC | ≤ 0.40 (or no limit if N-facing) | | Air leakage | ≤ 0.30 CFM/sf at 6.24 PSF | | Window-to-wall ratio | 15–25% optimal (energy + daylight) |

Exterior Cladding Options

Fiber Cement (Transitional/Colonial)

• Material: Portland cement + cellulose fiber

• Thickness: 5/16" lap or panel

• Fire resistance: Non-combustible (Class A)

• Warranty: 30-year (painted factory finish)

• DfD: Bolted aluminum clips to steel frame — no adhesive

• R-value: Negligible

Metal Panel / IMP (Modern/Industrial)

• Material: Insulated metal panel (steel skins + polyiso core)

• Thickness: 2"–4" (R-15 to R-30 integral)

• Fire resistance: FM/UL listed assemblies

• DfD: Bolted standing seam or clip system — fully reversible

• Best for: Maximum DfD alignment — panel = structure

Cedar Shingle (Cape Cod/Craftsman)

• Material: Western red cedar, #1 grade

• Thickness: 3/4" butt

• Fire resistance: Class C (treat for Class A in fire zones)

• Warranty: 25+ years (natural weathering)

• DfD: Stainless ring-shank nails to furring — removable but labor-intensive

• Note: Highest maintenance of the three options