The right shipping container insulation depends on what the box is for. If people will spend time inside, use closed-cell spray foam and accept the cost. If it's storage that needs to stop sweating, rigid foam panels do the job for about half the money, and you can fit them yourself over a weekend. Batts are cheap but the most likely to leave you with wet walls, so they only make sense in dry climates or on top of a layer of foam. An insulation blanket costs least of all and mainly cuts summer heat in a storage box.
Below are the numbers for each: R-value per inch, typical cost for a 20ft and a 40ft, how much of your 7 ft 8 in interior width it takes, and how it handles condensation, the bigger problem in a steel box.
Do you need insulation at all?
A steel box follows the outside temperature. On a summer afternoon the roof can be too hot to touch, and on a clear night the walls can cool below the dew point of the air inside, so water forms on them. In a humid climate that "container rain" drips onto whatever you've stored.
You don't need insulation to store things that don't mind damp, such as lumber, fencing, equipment or anything in sealed bins. Two or more vents and a gravel pad usually solve the dripping. The ventilation guide covers how many vents and where, and stopping condensation covers the cheap fixes.
You do need it for anything people sit or work in, such as a container office or workshop. You also need it for paper, fabric, electronics or furniture you care about, for anything you heat or cool, and for a grow room, where you're controlling temperature and humidity. See grow room insulation for that case.
Start with the ceiling, where most of the heat comes in and most drips start.
The four methods compared
Typical values, marked ~ because products and installs vary.
| Method | R-value per inch (typical) | Interior width lost (both sides) | Condensation control |
|---|---|---|---|
| Closed-cell spray foam, 2 in, plus furring and plywood | ~R-6 to R-7 | ~5 to 6 in | Excellent: bonds to the steel, leaves no air gap and acts as its own vapor retarder |
| Rigid foam panels (polyiso, XPS or EPS) between furring strips | ~R-4 (EPS) to ~R-6.5 (polyiso) | ~4 in | Good if seams are taped and valleys foamed, poor with gaps behind |
| Mineral wool or fiberglass batts in 2x4 framing | ~R-3.1 to R-4.3 | ~8 in | Poor alone: moist air reaches cold steel through the batts |
| Insulation blanket (reflective bubble or foil-faced) or ceramic paint | ~R-1 for the material itself, more only with a sealed air gap. Paint has no rated R-value | About 2 in on furring, none for paint | Cuts radiant heat and some sweating, but won't keep a heated room warm |
The last two columns matter most. Spray foam costs the most and takes more width than panels, but it's the only method that leaves no cold steel for indoor air to reach. Every other method leaves steel somewhere behind the insulation, and any moist air that gets back there can condense on it.
Closed-cell spray foam
Two inches of closed-cell foam sprayed onto the corrugations gives roughly R-12 to R-14, seals every seam and bolt head, and at that thickness acts as its own vapor retarder. There's no gap between the insulation and the steel, so there's nowhere for moisture to condense. That's why it's the standard for a spray foam shipping container office, workshop or studio anywhere with real humidity.
The downsides are that it's a professional job, it's messy and it's permanent. In a space people use, building codes generally require foam to be covered by a thermal barrier such as half-inch drywall or, under some code editions, 23/32 in plywood. Half-inch plywood doesn't qualify. Open-cell foam is cheaper, at about R-3.5 to R-3.8 per inch, but it lets vapor through and can hold water, so don't put it against container steel. For closed-cell, 1 in stops condensation in mild regions, 2 in suits anything heated or cooled, and 3 in (about R-18 to R-21) suits the northern tier.
Rigid foam panels
Polyiso (foil-faced), XPS (pink or blue) and EPS (white bead board) come in 4 x 8 sheets. They go between furring strips screwed to the corrugation ridges and get covered with plywood. Polyiso gives the most per inch, about R-6 to R-6.5 (less in hard cold), and its foil face works as a vapor barrier if you tape the seams. XPS is about R-5 and handles moisture well. EPS costs the least, at about R-4. With 1.5 in panels between 2x2 furring and half-inch plywood, you lose about 2 in per side and get roughly R-6 with EPS, R-7.5 with XPS or R-9 to R-10 with polyiso.
The weak point is the air space. A flat panel touches the corrugation ridges and leaves a gap over every valley, and indoor air that reaches those gaps condenses on the steel behind your dry panel. Fill the valleys with canned spray foam before the panels go on, and tape every seam with foil tape.
For a DIY insulated shipping container in a moderate climate it's the best method, and it can come out later.
Mineral wool or fiberglass with framing
Build 2x4 stud walls inside, fill them with batts, add a vapor barrier on the warm side and finish with plywood or drywall. Mineral wool gives about R-4 to R-4.3 per inch and handles fire and water better than fiberglass. Fiberglass costs the least, at about R-3.1 to R-3.7 per inch in standard R-11 and R-13 batts.
The problem is a 3.5 in cavity against cold steel with nothing bonded to it. Vapor passes through the batts, reaches the steel, condenses and soaks the insulation, and within a few years the wall can hide mold and rusted corrugations.
Use batts only in a dry climate, or over 1 in of closed-cell foam on the steel with a continuous vapor barrier on the room side. Done that way, a 2x4 wall gives around R-20 and the warmest container you can build, at a cost of about 10 in of width. In a standard 40 that leaves about 6 ft 10 in of width and about 7 ft 5 in of height under a lined ceiling. In a 40ft high cube the ceiling stays near 8 ft 5 in, the best argument for a high cube on any insulated build.
Insulation blankets and ceramic paint
A reflective blanket on furring strips, with an air gap toward the steel, noticeably cuts the radiant heat from a summer roof. The material itself is rated at around R-1, and manufacturers quote higher figures only for assemblies with a sealed air space. It makes the box less of an oven and cuts dripping, but it won't keep a heated office warm in January.
Ceramic paint on the inside of the steel is sold to cut condensation in marginal conditions. Treat it as a condensation treatment, because it won't insulate.
Cost by method and interior space lost: a 20ft worked out
A standard 20ft is about 19 ft 4 in long, 7 ft 8 in wide and 7 ft 10 in high inside. Leaving the doors alone, the surface to insulate is two long walls at about 151 sq ft each, a ceiling at about 148 sq ft and an end wall at about 60 sq ft. That's around 510 sq ft, or 18 to 20 sheets of 4 x 8 material in practice.
| Method on a 20ft | Interior after (W x H) | Floor area after |
|---|---|---|
| 2 in closed-cell spray foam | About 7 ft 2 in x 7 ft 7 in | About 137 sq ft |
| 1.5 in polyiso between furring | About 7 ft 4 in x 7 ft 8 in | About 141 sq ft |
| 2x4 framing with mineral wool | About 7 ft 0 in x 7 ft 6 in | About 133 sq ft |
| Reflective blanket on 1x furring | About 7 ft 6 in x 7 ft 9 in | About 144 sq ft |
Plywood prices move a lot, so check them the week you buy (the low end is sheathing grade, the high end sanded panels). Heights assume the ceiling gets the same build-up as the walls and the floor stays bare. A 40ft takes roughly double the material and labor for the same finished width and height. In an office or studio, add 1 in of XPS under new 3/4 in plywood on the floor, which takes about another 1.75 in off the height.
Step-by-step: panels on a 20ft
This is the usual DIY method. Budget two long days with two people.
- Settle the openings first. Cut any personnel door, window or vent before you insulate, ideally before the container is delivered.
- Fix leaks and rust. Look for daylight with the doors shut, then wire-brush any rust, treat it with a converter and prime it.
- Rough in the electrical. Surface conduit and boxes go on the steel now, so the panels and plywood can be cut around them.
- Fill the corrugation valleys with canned spray foam and shave it flush once it cures. This is the step that stops condensation behind the panels, and the one people skip.
- Fit the furring strips. Use 2x2s (they match 1.5 in panels) vertically on the walls and across the ceiling at 16 in on center, fixed to the ridges with self-drilling screws. Shim them plumb.
- Fit the panels. Cut 1.5 in polyiso to fit snugly between the strips, ceiling first, then walls. Foam any gap wider than a pencil and tape every seam with foil tape.
- Check the vapor control. Foil-faced polyiso with taped seams is your vapor barrier. With unfaced EPS or XPS, add 6 mil poly over the panels before the plywood.
- Line it with half-inch plywood or OSB screwed to the furring, and you can hang things anywhere. For a space people use, see the thermal barrier note in the spray foam section.
- Keep two louver vents high at opposite ends, or add a small fan if people use the space.
Expect about R-9, and dry walls if step 4 was done properly.
Climate-based pick
- Hot and humid (Gulf Coast, Southeast, the Midwest in summer): 2 in of closed-cell spray foam, the most reliable way to keep the steel dry.
- Hot and dry (Southwest): rigid polyiso with the reflective face toward a small air gap, plus a light-colored roof.
- Cold (northern tier, mountains): 2 to 3 in of closed-cell, or 1 in of closed-cell plus batts in framing if you heat it all winter. Buy a high cube.
- Mild and damp (Pacific Northwest, coasts): rigid panels with sealed valleys, and vents.
- Storage only, anywhere: vents first, a reflective blanket on the ceiling if the roof bakes, panels if you still get drips.
Frequently asked questions
What is the best insulation for a shipping container?
For any space people use or heat, it's closed-cell spray foam about 2 in thick. It bonds to the steel, has one of the highest R-values per inch and keeps condensation off the steel. For storage and DIY, rigid foam panels are the best value.
How much does it cost to insulate a shipping container?
Professional spray foam costs the most, DIY rigid panels come next, and batts in framing cost the least in materials, with the plywood lining on top of any of them. A 40ft costs roughly double a 20ft, and spray foam is priced by the installer, so get local quotes.
Does insulation stop condensation in a shipping container?
Spray foam does, because it leaves no cold surface exposed. Panels and batts reduce it, but they can move the problem behind the insulation unless the gaps to the steel are foamed and the seams sealed.
Should I insulate a used container or buy new?
A new one-trip box is the better base if the budget allows. Foam and framing go onto straight, unrusted steel more easily, and a container you're putting a full fit-out inside should have the most years left in it. A clean cargo-worthy box is fine for a panel job.
Planning to insulate? Start with a one-trip box, and make it a high cube if ceiling height matters. Send your ZIP and what you're building for one delivered price. See my price.
