# LDD-11 · Exterior Envelope (IMP)

> **Status:** 🟡 LOCKED strategy — finish tier + thermal continuity detail need designer-level resolution.

## One-line intent

A robust, forgiving, high-performance environmental shell using insulated metal panels (IMP), combined with selective interior framed assemblies in habitable zones for acoustic and human warmth.

## Why this matters

The envelope decision is the highest-leverage single decision for long-term comfort, energy use, durability, and maintenance burden. IMP is the right call for this PEMB shell — it gives airtightness, thermal continuity, and construction efficiency that a conventional layered wall can't match without much more labor and more failure modes.

## Locked decisions

**Primary enclosure strategy**

- High-performance PEMB-compatible enclosure
- Insulated metal panel (IMP) wall system as primary exterior wall assembly
- Emphasis on airtightness, thermal continuity, durability, construction efficiency

**Primary wall assembly (outside → inside)**

1. Factory-finished exterior metal skin (matte dark finish)
2. Continuous rigid foam insulation core (integrated IMP)
3. Factory-finished interior metal liner face

**Secondary interior habitable wall systems**

- Independent framed wall systems used selectively in primary habitable zones
- Provide acoustic control, service routing flexibility, lighting integration, psychological warmth, reduced envelope penetrations, future adaptability

**Typical interior habitable wall (IMP interior face → room side)**

- Optional service cavity as required
- Independent wood stud framing
- Mineral wool acoustic insulation (LOCKED)
- Drywall and/or selective plywood or wood finish

**Material strategy**

- Exterior: **matte dark brown — Kynar 500 PVDF finish** (LOCKED, per builder)
- Color logic: dark brown integrates with the Delaware mid-Atlantic landscape (deciduous canopy, native grasses, stream-adjacent context) more naturally than a gray-black metal palette would
- Interior: hybrid based on zone hierarchy
  - **Exposed IMP** permitted in infrastructural zones (gym, workshop, mech, utility)
  - **Layered drywall + mineral wool** prioritized in living-oriented zones (downstairs living, upstairs living, ILS, bedrooms, LOW, UCR, office, reading/lounge)

**Insulation strategy**

- Primary: continuous integrated IMP core
- Secondary: mineral wool for acoustic in interior assemblies

**Performance goals**

- Real-world airtightness
- Thermal continuity, reduced thermal bridging
- Drying resilience, durability, acoustic control
- Condensation resistance
- Reduced field-installed assembly complexity
- Long-term serviceability and adaptability

## Open items / requires engineer review

- **IMP panel thickness** — typical options 4", 5", 6". Affects R-value (R-26 to R-42) and cost.
- ~~Panel finish tier~~ — **Resolved: Kynar 500 PVDF, dark brown** (per builder, 2026-05-15).
- **Vapor strategy** — IMP factory liner is typically vapor-tight on both sides. Need explicit vapor and air control layer detail at joints, fasteners, openings.
- **Air sealing detail at joints** — IMP-to-IMP joint, IMP-to-foundation, IMP-to-roof, IMP-to-glazing. Each is a leak source if mis-detailed.
- **Slab-edge insulation continuity** at IMP-to-slab joint — coordinate with radiant LDD-02.
- **Window/door rough opening detail** — water management at penetrations.
- **Lightning protection** for a large metal building.
- **Acoustic performance of IMP itself** — IMP is a thin metal sandwich, not great at low-frequency sound. Interior layered assemblies must compensate in habitable zones.

## Cross-references

- ← [LDD-01 structural](01-structural-pemb.md) — IMP attaches to PEMB girts; coordinate panel module with girt spacing.
- ← [LDD-02 radiant](02-radiant-slab.md) — slab-edge insulation continuity at IMP-to-slab interface.
- ← [LDD-03 spine wall](03-spine-wall.md) — spine wall is interior, not envelope; IMP system terminates at exterior PEMB perimeter.
- ← [LDD-04 west gym hero wall](04-west-gym-hero-wall.md) — clerestory glazing rough-opening detail in IMP wall.
- → [LDD-18 interior materials](18-interior-materials.md) — habitable-zone wall finish.

## Cost drivers

- **IMP wall panels** installed: $14–24/sqft depending on panel thickness and finish tier. Wall area ≈ 14,400 sqft gross (perimeter 360' × 22' avg height incl. clerestory area) – glazing 600 sqft – garage openings 250 sqft = ~13,500 sqft. At $17/sqft average ≈ **$230K**.
- **IMP roof panels** (separate or integrated): $12–20/sqft × 7,200 sqft ≈ **$85–145K**. Budgeted in LDD-01.
- **Interior habitable framing + mineral wool + drywall**: ~$22–35/sqft of finished interior wall area. ~3,000 sqft = **$66–105K**. Budgeted as part of interior framing line.
- **Exterior windows + doors** (separate from spine wall glazing): rough openings, frames, hardware = $35–70K depending on glazing spec.

**Likely-case rollup: $230K for IMP envelope** (in budget waterfall). Plus interior assemblies budgeted separately.

## Air-gap concerns

1. **Condensation risk on the interior liner side in cold weather.** IMP panels are vapor-tight on both sides. Any indoor humidity migrating to a cold spot can condense on the inside face of an exposed IMP wall. The hybrid strategy mitigates this in habitable zones (added stud wall traps cold liner face), but exposed-IMP utility zones can show condensation. Verify with the IMP manufacturer's climate-zone guidance.
2. **Thermal bridging at fasteners + clips.** IMP joints and structural clips are thermal shorts. Manufacturers publish R-value with bridging accounted for; verify the effective R, not the nominal R, in spec.
3. **Roof IMP at the shallow ½"/ft slope.** Drainage detail at panel laps and roof penetrations needs attention; ½"/ft is below most IMP manufacturers' preferred minimum (typically ¼"–½"/ft minimum).
4. **Acoustic underperformance for the gym west wall.** IMP transmits low-frequency basketball impact and music. The mini-splits and the slab radiant systems are not enough; consider exterior acoustic mass at the gym west wall if there's any neighbor concern.
5. **Lightning + grounding.** Large metal shells need bonding strategy. Coordinate with electrical.
6. **Finish tier resolved — Kynar 500 PVDF dark brown is the right call** for the Delaware setting and a 25-35 year color-stability target. Premium baked into the rollup ($13–40K above SMP polyester baseline).
7. **No specified airtightness target.** A high-performance envelope should have a blower-door target (e.g., ≤ 1.5 ACH50 for tight, ≤ 0.6 ACH50 for Passive House-level). Without a target, you don't know what you got.

## Diagram

(no dedicated SVG — envelope is the perimeter rectangle in [building footprint](../../diagrams/01-building-footprint.svg))

## Status

🟡 **Yellow — strategy is right; finish tier, vapor/condensation detailing, and airtightness target need to lock before procurement.**