Spray foam · PIR panels · Rockwool
Shipping Container Insulation
Uninsulated steel walls track outside temperature within a degree or two and condense water on every cold night. Insulation fixes both — at a price paid in dollars and, less obviously, in interior centimetres.
This guide compares the three methods that work in containers, what each costs in money and space, and when insulating outside the box beats insulating inside it.
What is the best insulation for a shipping container?
| Method | R-value / inch | Vapour barrier | Cost (installed) |
|---|---|---|---|
| Closed-cell spray foam | R-6 | Built-in | ~$2–4 / sq ft |
| PIR / XPS rigid panels | R-5–6.5 | Foil facing + taped joints | Material cheap, labour-heavy |
| Rockwool + stud frame | R-3.3 | Separate membrane required | Cheapest materials |
Closed-cell spray foam wins on physics: highest R per inch, it adheres to the corrugations with no air gaps, and it is its own vapour barrier — moist interior air can never reach the cold steel. PIR/XPS panels get close on R-value and suit DIY budgets, but every joint must be foamed and taped; one gap becomes a condensation line. Rockwool is cheap and fireproof but needs a stud frame, delivers the least R per inch, and fails wet — a flawless separate vapour barrier is mandatory, and flawless is a high bar inside a corrugated box.
How much interior space does insulation take?
Budget roughly 75 mm per wall for insulation plus lining. A dry container’s 2.35 m internal width becomes about 2.2 m — acceptable in a 12 m long room, noticeable in a 6 m one. The ceiling is where it hurts: a standard container’s 2.39 m internal height drops below 2.2 m after insulating floor and roof, which reads as a basement. A 40 ft high cube starts at 2.69 m and finishes near a normal domestic 2.4 m — the extra ~$300–500 a high cube costs is the cheapest headroom you will ever buy. Full conversion dimensions are in the container home guide.
Insulating outside instead
Exterior insulation — rigid panels over the corrugations, then cladding — keeps the full 2.35 × 2.39 m interior and puts the steel on the warm side of the assembly, eliminating interior condensation entirely. The cost is aesthetic and financial: the container no longer looks like a container, and cladding plus weatherproofing typically exceeds interior spray foam. It is the right call when interior space is non-negotiable, or when several containers join into one building and share exterior walls.
Insulation and condensation
Condensation is the reason most storage containers get insulated at all: warm humid air on cold steel produces “container rain” on the ceiling. Insulation with a proper vapour barrier is the permanent fix; ventilation and desiccants are the cheap partial ones. The physics and the cheaper options are covered in the ventilation and condensation guide.
Frequently asked questions
What is the best insulation for a shipping container?
Closed-cell spray foam for most builds: R-6 per inch, no air gaps against the corrugated steel, and it acts as its own vapour barrier. PIR panels are the DIY alternative at R-5–6.5 per inch; rockwool is cheapest but needs framing and a separate flawless vapour barrier.
How much interior space does insulation take?
About 75 mm per side including lining. Width shrinks from 2.35 m to roughly 2.2 m; a standard container’s 2.39 m ceiling drops below 2.2 m once floor and roof are done. That headroom loss is why insulated conversions almost always start from a high cube (2.69 m internal).
Can you insulate a shipping container from the outside?
Yes — rigid panels over the exterior, then cladding. You keep the full interior width and height and move condensation risk outside the steel entirely. It costs more than interior spray foam and hides the container aesthetic, so it is chosen when interior space matters most.
Do you need a vapour barrier in a container?
Yes, without exception. Steel is completely vapour-tight, so any moist interior air that reaches it condenses inside your wall assembly. Closed-cell spray foam is its own barrier; panel and rockwool builds need taped foil facings or a dedicated membrane with sealed joints.
How much does it cost to insulate a 20 ft container?
Spray-foaming walls and ceiling of a 20 ft (roughly 600 sq ft of surface) at $2–4 per sq ft runs $1,200–2,400 installed. DIY PIR panels can halve the materials bill at the cost of a weekend of cutting, foaming and taping joints.