LOCKED strategy finish tier needs designer

LDD-11 · Exterior Envelope (IMP)

One-line intent

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

Locked decisions

Primary wall (outside → inside)

Factory-finished exterior metal skin (matte dark)
Continuous rigid foam insulation core
Factory-finished interior metal liner

Secondary interior wall (in habitable zones)

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

Material strategy

Exterior: matte dark brown — Kynar 500 PVDF finish (LOCKED, per builder)
Color logic: dark brown integrates with Delaware mid-Atlantic landscape (deciduous canopy + native grasses + stream-adjacent context) more naturally than gray-black would
Interior: hybrid by zone — exposed IMP in infrastructural zones, layered drywall + mineral wool in living-oriented

Open items / engineer review

IMP panel thickness — 4", 5", or 6" (R-26 to R-42)
~~Panel finish tier~~ — Resolved: Kynar 500 PVDF, dark brown (per builder, 2026-05-15)
Vapor strategy — IMP is vapor-tight both sides; need air control layer detail at joints/penetrations
Air sealing detail at IMP-to-IMP, IMP-to-foundation, IMP-to-roof, IMP-to-glazing
Slab-edge insulation continuity at IMP-to-slab
Lightning protection for large metal building
Airtightness target (ACH50) — currently unspecified

Cost drivers (corrected 2026-05-15 PM — arithmetic error)

IMP wall area: ~7,000–8,000 sqft (perimeter 360' × ~22' avg height = 7,920 sqft, net of openings ≈ 7,070 sqft). NOT 13,500 sqft as previously stated — the earlier 13,500 figure was an arithmetic error (`360 × 22` ≠ 14,400). IMP wall installed at $17/sqft avg = ~$120–135K (was $230K). IMP roof $85–145K (LDD-01). Interior framing + mineral wool + drywall ~3,000 sqft = $66–105K.

4' perimeter overhang (added per builder)

The design includes a 4-foot perimeter overhang wrapping the building. Functions: rain-shedding well away from wall-roof joints; summer solar shading on south face (critical for Delaware sun angles); year-round usable shade band; coordinates with monopitch to keep eaves consistent west-to-east. Significant feature missed in earlier drafts.

Air-gap concerns

Condensation risk on interior IMP liner — both sides vapor-tight. Hybrid mitigates in habitable zones; verify utility zone exposure.
Thermal bridging at fasteners + clips — verify effective R, not nominal.
Roof IMP at ½"/ft is below most manufacturers' preferred minimum for cold climate.
Acoustic underperformance for gym west IMP wall (low-frequency basketball impact).
Lightning + grounding — large metal shell needs bonding strategy.
Finish tier matters long-term. Kynar 500 holds color 25–35 years vs SMP 8–15 years. $1–3/sqft × 13,500 = $13–40K premium worth considering.
No airtightness target — without it, you don't know what you got. ≤1.5 ACH50 reasonable; ≤0.6 ACH50 is Passive House level.

Builder's question (Peter, 2026-05-15 PM, via David) — IMP vs alternatives?

"Can you ask Claude if he agrees with the use of IMPs as a clean and cost-efficient solution for the building envelope vs other solutions?"

Yes — IMP is the right envelope for this PEMB residential build.

Alternative	vs. IMP	Verdict for this build
Site-built layered wall (cladding + WRB + sheathing + insulation + framing + drywall)	Cheaper materially; much more labor (5+ trades vs 1); thermal bridging at every stud; dozens of joint failure points	IMP wins on labor + air/vapor control
SIPs (Structural Insulated Panels)	OSB facings; doesn't pair structurally with PEMB	Wrong tool for PEMB
ICF (Insulated Concrete Forms)	Great thermal mass + soundproofing; doesn't integrate with PEMB cleanly	Wrong tool for PEMB
Stud wall + closed-cell spray foam	Comparable R-value; more VOCs; fire considerations at thickness; harder to demo/modify	IMP wins on factory QC + recyclability
Agricultural barn enclosure (R-Tac, vinyl-faced fiberglass)	Cheapest PEMB enclosure	Poor envelope performance; not residential-appropriate
IMP (Kynar 500 dark brown — spec'd)	Continuous insulation, factory air/vapor control, single-trade install, integrated finish, 25–35 year color stability	Right call

Where IMP loses: future modifications. Cutting in a new window after the fact is harder than stick-built. For a build-once-and-live-with-it project, IMP wins.

The 4' perimeter overhang makes IMP even better — sheds rain away from joints (the IMP failure mode if poorly detailed), shades the south face in summer, gives the building a "hat" that conventional wall systems can't easily replicate.

Cost positioning: IMP at corrected $130K wall is mid-range — more than agricultural-barn (~$70–90K, performs poorly), less than stick-built layered wall (~$160–200K all-in), comparable to SIPs/ICF. Premium over agricultural is justified by 50-year performance + maintenance reduction.

Cross-references

← Inputs from

LDD-01 PEMB girts · LDD-02 slab edge · LDD-04 clerestory rough opening

→ Outputs to

LDD-18 habitable zone wall finish

Diagram

IMP envelope wraps the perimeter rectangle.