Guide · Building
Shipping Container Insulation
Yuri Kan Updated
Closed-cell spray foam is the default container insulation: about R-6 per inch in the US Department of Energy’s guide (R-7.4 on a newer HFO-blown foam’s sheet), a built-in vapour barrier and roughly $2–4 per sq ft installed, by installers’ typical rates. Every interior method costs roughly 75 mm per side, narrowing the 2.352 m (7′8½″) inside to about 2.2 m — and a standard container’s 2.393 m (7′10″) ceiling ends near 2.193 m after a 100 mm floor build-up and 100 mm of ceiling insulation, which is why most insulated conversions start from a high cube, with 2.698 m (8′10″) inside as most published sheets print it.
Uninsulated steel walls track the outside temperature closely 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.
R-values on this page are as the US Department of Energy’s spray foam guide and the makers’ sheets print them. Costs and the 75 mm lining allowance are installers’ typical figures, not figures checked against documents the way the container dimensions are.
The numbers in this chart
| Part | Dimension |
|---|---|
| Insulation build-up, per side | ~75 mm |
What is the best insulation for a shipping container?
| Method | R-value / inch | Vapour barrier | Cost (installed) |
|---|---|---|---|
| Closed-cell spray foam | R-6–7.4 | 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-3.7–4.3 | Separate membrane required | Cheapest materials |
Closed-cell spray foam wins on physics; each of the other two methods carries a catch:
- Closed-cell spray foam: R-6 to R-7.4 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: close on R-value and suited to DIY budgets, but every joint must be foamed and taped; one gap becomes a condensation line.
- Rockwool: cheap and fireproof, but it needs a stud frame and delivers the least R per inch of the three. It fails wet, so 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 standard dry container’s 2.352 m inside width becomes about 2.2 m — acceptable in a 40 ft box, noticeable in a 20 ft one.
The ceiling is where it hurts: a standard container’s 2.393 m inside height ends near 2.193 m after a 100 mm floor build-up and 100 mm of ceiling insulation. That headroom reads as a basement.
A 40 ft high cube starts at 2.698 m and finishes near 2.498 m. Used, the high cube costs $2,100–3,500 against $2,100–3,400 for a standard 40 ft in our price table, for 31 cm more headroom. Full conversion dimensions are in the container home guide.
Insulating outside: the full inside kept, condensation gone
Exterior insulation — rigid panels over the corrugations, then cladding — keeps the full 2.352 × 2.393 m inside. It puts the steel on the warm side of the assembly, eliminating interior condensation entirely.
The cost of exterior insulation 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: the permanent fix for container rain
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: about R-6 per inch in the US Department of Energy’s guide, no air gaps against the corrugated steel, and it acts as its own vapour barrier. PIR panels are the DIY alternative at R-6–6.5 per inch on Johns Manville’s sheet (XPS R-5 on Owens Corning’s); rockwool, R-3.7–4.3 per inch on ROCKWOOL’s sheet, costs least in materials but needs framing and a separate flawless vapour barrier.
How much interior space does insulation take?
Insulation takes about 75 mm (3 in) per side including lining, so the 2.352 m (7′8½″) inside width shrinks to roughly 2.2 m and a standard container’s 2.393 m (7′10″) ceiling ends near 2.193 m once floor and roof are done — the inside per 15 of 18 sheets. That headroom loss is why insulated conversions almost always start from a high cube, with 2.698 m (8′10″) inside.
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 the walls and ceiling of a 20 ft — about 575 sq ft (53 m²) of inside surface — at a typical $2–4 per sq ft, installers’ rates, runs $1,150–2,300 installed. DIY PIR panels cut the materials bill at the cost of a weekend of cutting, foaming and taping joints.