A shipping or storage container is made of just a few millimetres of Corten or structural steel. This steel is extremely load-bearing, but thermally it is about the worst building material imaginable: it conducts heat almost unhindered from inside to outside and vice versa. Anyone who wants to use a container for permanent occupancy, as an office, workshop or living space, cannot avoid proper insulation. This guide deliberately focuses on the most critical single aspect: the insulation itself and the avoidance of condensation. The complete interior fit-out is covered separately under Living in a container: fit-out & insulation.
Why insulation is especially critical for a container
The steel shell as a continuous thermal bridge
Unlike a masonry house, a container has no natural insulating layer. The entire outer skin of profiled steel sheet (the so-called corrugated or trapezoidal profile) forms a continuous, highly conductive surface. Heat generated indoors migrates directly to the cold exterior. In winter an uninsulated interior cools down rapidly; in summer the steel heats up to over 60 °C and turns the container into a sauna.
Condensation and the dew point
The real problem, however, is less the temperature than the moisture. Warm indoor air can hold considerably more water vapour than cold air. When warm, humid indoor air meets the cold steel wall, it falls below the so-called dew point there: the water vapour condenses and settles as liquid water. In a container this often happens invisibly behind a cladding, directly on the steel.
Rule of thumb: condensation forms wherever warm air meets a surface below the dew-point temperature. On an uninsulated steel container that is effectively the entire wall area.
Risk of mould and corrosion
Persistent condensation has two consequences. First, the steel corrodes from the inside; even with Corten steel the rust protection is limited. Second, in combination with organic materials (timber studs, insulation, dust) mould develops quickly. Incorrectly executed insulation can even make the problem worse, because it shifts the dew point into the construction and traps the moisture. That is why not only the insulation thickness matters, but above all the correct build-up with a vapour barrier and rear ventilation.
Interior insulation: the most common approach
Interior insulation is the most widespread method because it leaves the container’s external appearance unchanged and can be realised with manageable effort. The drawback: it reduces the usable interior space and requires special care in terms of building physics.
Stud frame with mineral or rock wool
The classic build-up uses a timber or metal stud frame fitted into the trapezoidal profiles. Mineral or rock wool goes into the bays. Rock wool is non-combustible (building material class A1) and additionally offers good sound insulation. Important: ideally a ventilated air gap should remain between the steel sheet and the insulation so that any moisture can dry out.
PU/PIR rigid foam boards
Polyurethane (PU) and polyisocyanurate (PIR) boards offer very good insulation performance at low thickness (low lambda value). With a container’s tight ceiling height and width, this is a real advantage. The boards are bonded directly to the inside face or set into the stud frame. Joints must be carefully taped to avoid thermal bridges and moisture ingress.
The vapour barrier – decisive
The most important and most frequently mis-executed point: on the warm interior side of the insulation an airtight vapour barrier or vapour retarder must be installed. It prevents warm, humid indoor air from penetrating into the insulation layer and condensing there on the cold steel. The membrane is taped airtight at all joints, penetrations (sockets, pipes) and connections. A single leak can transport considerable amounts of water into the construction over the years.
Exterior insulation / ETICS
In terms of building physics, exterior insulation is the cleanest solution. Here an external thermal insulation composite system (ETICS) is applied to the outside of the container. The big advantage: the steel shell lies entirely within the warm zone and therefore stays above the dew point. Condensation on the steel is thus practically eliminated, and thermal bridges are largely removed. The interior space is retained at full size.
The disadvantages are the greater effort, the cost, and the fact that the characteristic container look is lost. In addition, the substructure has to carry the load of the ETICS and receive a suitable render or facade layer. For permanently occupied containers, exterior insulation is nonetheless often the technically superior choice.
Spray PU as a special route
An intermediate solution is closed-cell polyurethane spray foam applied directly to the inside face of the steel. It bonds seamlessly to the substrate and acts as a vapour barrier itself, provided it is applied closed-cell in sufficient thickness. This considerably reduces the risk of condensation behind the insulation, because no air layer remains between foam and steel.
Note: only closed-cell foam is vapour-tight; open-cell foam soaks up moisture and is unsuitable for this application. The work should be carried out professionally, since layer thickness and material quality determine whether it functions.
Don’t forget the roof and floor
Insulation does not end at the walls. The container roof is the surface that heats up most in summer and cools down most in winter – generous insulation is particularly effective here. Frequently a suspended, insulated inner ceiling is fitted; alternatively an additional, rear-ventilated roof can span the entire container and keep off heat and rain.
The floor must be insulated too, otherwise cold draws up from below and condensation can form on the underside of the floor. A common build-up consists of bearer battens, insulation between the battens and a load-bearing floor board on top. Since every centimetre of floor build-up reduces the interior height, low-lambda rigid foam boards are often used here as well.
Insulation materials compared
The following table gives rough guidance. All values are approximate and can deviate significantly depending on product, density and manufacturer.
| Insulation material | approx. lambda value (W/mK) | Price level | Suitability for containers |
|---|---|---|---|
| Mineral/rock wool | approx. 0.035–0.040 | low | Good; non-combustible, good sound insulation, vapour barrier essential |
| Glass wool | approx. 0.032–0.040 | low | Good; lightweight, vapour barrier essential |
| PU/PIR rigid foam | approx. 0.022–0.028 | medium–high | Very good at low thickness; ideal where space is tight |
| EPS (polystyrene) | approx. 0.032–0.040 | low | More suited to exterior insulation/ETICS |
| XPS (extruded) | approx. 0.030–0.035 | medium | Very good for floors (load-bearing, moisture-resistant) |
| Closed-cell PU spray foam | approx. 0.022–0.028 | high | Very good; acts as its own vapour barrier, seamless |
| Wood fibre | approx. 0.038–0.045 | medium–high | Ecological, good summer behaviour, thicker build-up needed |
Disclaimer: The figures are non-binding approximate values for initial orientation. The technical data sheets of the respective products and a project-specific building-physics design are authoritative.
Typical mistakes when insulating a container
Most moisture damage on an insulated container comes down to a few recurring mistakes:
- Missing or leaky vapour barrier: without an airtight vapour barrier on the warm side, moisture enters the construction and condenses on the steel. The most common and most damaging mistake.
- Thermal bridges through metal studs and fixings: metal profiles connecting the steel wall directly to the interior cladding conduct cold inward at points and create local condensation spots. Decoupling or thermally separated constructions help.
- Insulation too thin: to save space, insulation is often applied too thinly. The result is high heating costs and a dew point that sits unfavourably within the construction.
- Trapped moisture with no drying reserve: if the steel is packed vapour-tight on both sides, moisture that has once entered can no longer dry out. Well-considered rear ventilation or a build-up that is diffusion-open to the outside is important.
- Roof and floor neglected: if only the walls are insulated, the roof and floor remain as large thermal bridges and condensation surfaces.
Conclusion
For a container, insulation is not an optional extra but the decisive measure that turns a steel box into a usable, moisture-free space. The technically cleanest option is exterior insulation, because it keeps the steel above the dew point. Interior insulation is cheaper and visually neutral, but it absolutely requires a correctly executed, airtight vapour barrier and well-considered rear ventilation. Closed-cell PU spray foam is an interesting special route that combines insulation and vapour barrier in a single step. Whatever the method: consider the roof and floor, avoid thermal bridges, and when in doubt leave the building-physics design to a specialist. Anyone who observes these points will permanently avoid condensation, mould and corrosion. For further information on the complete interior fit-out, see Living in a container: fit-out & insulation.