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