🚌 E350 Travel Hub

Insulation & Frame

Aluminum extrusion (80/20) interior frame design and insulation plan for the shuttle bus build-out.

Goal

Use 80/20 T-slotted aluminum extrusion as an interior framing system around the living space of a shell bus (2014 E350 shuttle cutaway, thin walls, very large windows) that holds insulation in place and provides mounting points for walls/ceilings/windows — while making off-grid AC viable. See Air Conditioning — insulation and AC are one system: the envelope halves the AC load before the AC runs.

Research Update (2026-09) — What Changed in This Approach

The original plan (batt insulation pressed into 80/20 bays + Reflectix + skin) is workable but leaves the three bus-specific failure modes open. Research from skoolie/bus builders and insulation manufacturers refines it:

  1. Seal first, insulate second. The #1 heat (and moisture) path in a bus is air leakage — gaps around ribs, window frames, floor seams. Batt insulation in a frame does NOT seal; it reduces convection inside the frame but the shell still breathes around it. Bus builders’ consensus: closed-cell spray foam (ccSPF) or polyiso board with taped/sealed seams is what kills the heat, because it’s also the air seal. Plan: seal + insulate with foam/polyiso against the shell, then use 80/20 as the finish substrate + window frames + wire raceway mounted over it — not as the primary insulation holder.
  2. Reflectix / foil bubble wrap is a radiant barrier, not insulation. Its measured material R-value is ~R-1.0 (ASTM C518, cited in building forums). Its real value comes ONLY from an enclosed air space ≥ 0.75–1” on the reflective side (manufacturer requirement), and it degrades with dust/contact. Laid flat against bus metal or under plywood = mostly useless. Use foil-faced polyiso instead — it delivers conduction R-value AND a reflective face in one sheet, no air-gap trap.
  3. Windows are the biggest thermal hole (confirmed by skoolie builders — “the original windows became our pain point”). Treat them with three layers: permanent ceramic IR film, removable insulated inserts for boondocking, curtains. The 80/20 window frames are exactly the right mount for the inserts — and the AC seat (see air-conditioning.md).
  4. Thermal breaks on 80/20 are mandatory (original doc had this right, keep it): every aluminum member is a ~300× conduction path vs foam. Anywhere extrusion touches rib/metal: 1/2” XPS strip or adhesive foam tape between them; anywhere it touches interior skin: closed-cell foam. Cheaper busy-builder trick validated on skoolie.net: XPS strips are 90% of the value of aerogel at ~5% of the cost.
  5. Moisture strategy depends on the insulator:
    • ccSPF (2-lb): R-6.5/in, at ≥1.5” it IS the vapor barrier and air seal — no separate barrier needed. This is why it’s the skoolie gold standard.
    • Foil-faced polyiso: R-5.6–8.0/in (typical ~6.5; ~R-13 at 2”); vapor-closed with taped foil seams → acts as vapor barrier on the interior/warm side. Best $/R for DIY in an 80/20-frame build.
    • Rockwool/mineral wool (R-3.1–3.4/in): vapor-open — good at handling moisture that gets in, but needs a separate vapor barrier on the interior side and ventilation discipline. Fine for a split duty (e.g., behind beds) but gives up ~half the R per inch vs foam.
    • Bus reality: the steel shell sweats with inside/outside temp swing (windows fog, ribs drip in winter). Foam-family (ccSPF or polyiso w/ sealed seams) on the interior of the shell is the direct fix.

Why Aluminum Extrusion (unchanged — still the right call)

  • Lighter than wood framing (~1/3 density; ~0.76–1.7 lb/ft depending on profile — see weights below)
  • T-slots accept drop-in nuts, brackets, accessories without drilling into bus structure
  • Modifiable forever: shelves, racks, curtain rods, AC panel, cabinet mounts
  • Wire raceway bonus: T-slots run wiring through the framing before the insulation is closed in — solves the “spray foam before wiring = carved foam” problem
  • Bolts to existing rib structure or U-channels over ribs

System Design (revised)

Layer Strategy (outside → in)

  1. Shell metal — fix ALL leaks, treat rust BEFORE insulating (foam/board over rusty steel traps moisture and hides corrosion — documented skoolie regret). Paint/seal penetrations.
  2. Service raceways — run wiring/conduit in 80/20 T-slots or chase tubes along the shell FIRST. Foam/board goes in after; no carving.
  3. Air seal + mass insulation — ccSPF spray OR polyiso board with seams taped (foil tape + minimal-expansion foam can at edges). This layer carries the R-value AND stops the shell breathing.
  4. 80/20 frame — bolted through/seated over the insulation to ribs (with thermal-break strip between extrusion and rib). Holds panels, windows, and the finish skin.
  5. Interior skin / finish — 1/4” plywood or MG, screwed to 80/20 T-nuts.

R-Value Targets (desert-heavy use: AZ/CO per destinations)

| Surface | Target R | How | Notes | |———|———-|—–|——-| | Roof | R-10–15 | 2” ccSPF or 2” polyiso + radiant face | #1 sun load; also where solar panels mount — keep walkable space, don’t overshoot depth | | Walls | R-8–13 | 1.5–2” ccSPF or polyiso in 80/20 bays | Thin walls — every inch counts; polyiso wins $/R | | Floor | R-5–8 | 1–1.5” XPS/polyiso under subfloor | Less critical in desert heat; critical winter | | Windows | see strategy below | film + inserts + curtains | The biggest single hole on this bus |

Reference points (insulation literature + skoolie builds, 2026): 2-lb ccSPF R-6.5/in; polyiso R-5.6–8/in; XPS R-5/in; rockwool R-3.1–3.4/in; fiberglass R-3.1–3.4/in. Typical skoolie results: spray foam walls/ceiling land R-9–14.

Extrusion Profile

  • 80/20 Series 15 (1515) or 20 (2020) T-slotted (6063-T5) — imperial 15/25-series or metric 20-series are the right class for interior framing; 40-series (40×40mm) only for structural spans (AC panel, bed frames). Body is a shuttle bus, not a CNC machine — don’t over-build.
  • Weights: 1515 ≈ 0.76 lb/ft; 2020 ≈ 1.08 lb/ft; 40-series 4040 ≈ 1.7 lb/ft. Budget ~80–120 ft total framing → ~65–200 lb depending on profile mix. [VERIFY] exact per 80/20 spec sheet when ordering.
  • Costs (retail, 2026): 1515/2020 class ~$5–9/ft; 40-series ~$10–14/ft. Bus-scale frame (walls + roof sections + window frames) roughly $600–1,500 (vs $200–300 wood) — pays for itself in modularity + weight.
  • Connectors: corner brackets, T-nuts, end plates, joining plates.

Thermal Break (critical — keep from original plan)

  • Aluminum conducts heat/cold ~300× better than foam. Without breaks you get condensation on interior aluminum in winter + heat loss year-round.
  • Solution 1 (preferred): 1/2” XPS strip or adhesive foam tape between extrusion and EVERY rib/metal contact point. (XPS ≈ 90% of aerogel’s value at ~5% cost — skoolie.net validated.)
  • Solution 2: closed-cell foam / Reflectix-style (with air gap on the reflective side only) between aluminum and interior skin.
  • Solution 3: fabric/panel-only interior with air gap — cheap, covers most surfaces, but leaves the aluminum-to-metal contacts bare; use Solution 1 for those.

Mounting to Existing Bus Structure

  • Option A (preferred): bolt extrusion directly to wall ribs (E350 shuttle: vertical ribs ~16” OC + horizontal rails at floor/ceiling — [VERIFY] ribs on this bus: measure twice). Thermal-break strip under every contact.
  • Option B: U-channels snap over ribs, extrusion to U-channel.
  • Option C: free-standing 80/20 rectangles resting on floor/ceiling, pressing against walls — removable, but poor seal; only for panels you want to move.

Large Window Strategy (the biggest lever on this bus)

Large windows are the bus’s best feature and worst thermal hole. Three layers:

  1. Permanent: ceramic infrared (IR) window film on the glass — rejects the majority of solar IR while staying visibly clear. Cheap, invisible, permanent. (Tint laws: verify for the driver area; passenger windows fine.)
  2. Boondocked/parked: removable insulated inserts — 1.5–2” foil-faced polyiso sandwich (foil to the glass/outside with ~5/8” gap or foil to inside; tape edges), sized to the window reveal, held by the 80/20 window frames with cam locks or spring clips. These are the same 80/20 frames that seat the AC panel (air-conditioning.md) and can be removed for view/driving.
  3. Driving/living: dual-layer curtains on 80/20 curtain rods (T-slot mounts make this trivial).

AC seat: one window goes permanently to the Midea U-shaped 12K AC panel during camp season (air-conditioning.md) — that panel is also an insulated plug when the AC is removed for travel.

Vapor Barrier & Moisture

  • Bus interiors condense from breath/heat against the steel shell — the single most common skoolie damage mode.
  • If ccSPF or sealed polyiso: vapor barrier built in; keep seams taped; still ventilate (roof vent / cracked window when parked).
  • If rockwool (vapor-open): install a vapor barrier on the interior/warm side + ventilation. Rockwool handles incidental moisture better than fiberglass but needs the barrier to work as a system.
  • Foil orientation: foil toward the warm side (interior in desert heat, exterior in cold climate) — this bus is desert-first: foil/reflective faces into the interior air gap.

Estimated Material List (one bus side, ~8 ft wall — revised)

| Item | Qty | Notes | |——|—–|——-| | 80/20 1515/2020 extrusion | 8–10 ft | Cut to bay lengths; thermal-break strip under all rib contacts | | T-nuts (1/4-20) | 20–30 | Drop-in for T-slots | | Corner brackets (internal) | 8–12 | Aluminum or 3D-printed | | 1/2” XPS strip or foam tape | roll | Thermal break at ribs — mandatory | | 2” foil-faced polyiso board | 1–2 sheets | Primary insulation + radiant face + R-13 | | Foil tape + expanding foam can | 1 roll + 1 can | Seams, edges, penetrations = the actual air seal | | OR ccSPF DIY kit (600 bf) | 1 | ~$900 (2026 skoolie pricing); R-13 @ 2” + vapor barrier — the premium route | | 1/4” plywood or MG skin | 1 sheet | Interior finish, screw to T-nuts | | Window film (ceramic IR) + inserts | per window | Film once; inserts permanent removable |

Cost & Weight (whole-living-space ballpark, est.)

  • Framing: $600–1,500 / ~65–200 lb (80/20)
  • Insulation: polyiso ~$35–55/sheet (4×8, 2”) ≈ $400–800 total; ccSPF DIY ~$900–1,800; rockwool ~$50–70/bag (cheaper, half the R/in)
  • Complete build-out (frame + insulation + skin + window inserts + AC panel): rough $1,500–3,000 material, est. +350–550 lb to the bus — see weight-capacity.md for the tally against the still-unweighed payload.

Installation Sequence (revised)

  1. Measure each wall bay between ribs/windows/doors; measure rib spacing; count windows + openings (feed to ford-e350.md).
  2. Fix leaks + treat rust first. Seal all shell penetrations. Do NOT skip.
  3. Run wiring/conduit in 80/20 T-slots or chase tubes along the shell.
  4. Seal + insulate: polyiso cut to bay size, edges foamed, seams taped (or ccSPF sprayed). Seal any shell gaps around ribs/window frames with foam can first.
  5. Cut 80/20 extrusion, deburr; install T-nuts before assembly.
  6. Assemble frame with thermal-break strips at every rib/metal contact, corner brackets, end plates.
  7. Windows: film the glass; build 80/20 window frames + insulated inserts; one window panel doubles as the AC seat (air-conditioning.md).
  8. Skin with 1/4” plywood/MG into T-nuts; interior finish over that if desired.
  9. Install interior vapor strategy per insulator choice; add curtains on T-slot rods.

Bus-Specific Tips (verified from original plan + shuttle reality)

  • E350 shuttle: vertical ribs ~16” OC, horizontal rails floor/ceiling — [VERIFY] on this bus; the 80/20 grid is designed around actual rib spacing.
  • Large windows already have aluminum surrounds — integrate 80/20 frames inside the existing reveals, or adapt interior trim; the window inserts + AC panel live here.
  • Floor/ceiling: 80/20 can butt against existing rails without permanent fastening for removable panels.
  • Spray foam + ribs: ccSPF bonds to the shell and adds shear rigidity — a genuine structural bonus on a thin-wall shuttle body. It also means future wiring requires carving; wires/conduit go in BEFORE (step 3).

Pending / Verify

  • [VERIFY] rib spacing, window count + opening sizes, roof hatch/vent state, shell leak status (fix before insulating)
  • Decide: polyiso (DIY, cheap, removable) vs ccSPF (premium, sealed, structural) vs hybrid (polyiso walls + foam can sealing)
  • Source 80/20 (distributor or McMaster), pick skin (1/4” plywood vs 1/8” MG vs aluminum sheet)
  • Fold window-insert and AC-panel designs into the 80/20 window frames

Alternative: Aluminum Framing + Fabric Only

Weight-absolute priority path: skip rigid skin, stretch fabric (cuben/ripstop) over the 80/20, bungee/spline fasten. Saves 15–20 lb/wall but gives no rigid mount surface and no insulation retention against the skin — only viable with ccSPF on the shell so the frame is pure finish, not insulation holder. With polyiso/batt it doesn’t work: nothing holds the insulation against the shell.

Cross-References

  • Air Conditioning — the AC side of this envelope; Midea 12K U-shape installs into the window-panel system
  • Power & Solar — power budget; AC as a load, solar implications
  • Weight & Capacity — build-out weight tally vs unweighed payload
  • Ford E350 Specs — bus dimensions/state to verify before this plan’s numbers harden