There is barely an American container conversion video without this scene: a person in a protective suit and full-face mask walks a spray lance through the container, and within minutes the corrugated steel wall disappears under a beige, foamy layer. The foam expands, cures, is trimmed flush – insulation done. It looks convincing, which is why the question keeps coming up: why does nobody do this here?
The answer is neither “because it is banned” nor “because the Americans know better”. It lies in a mixture of building regulations, a different building-physics tradition, occupational safety, market structure and an increasingly important end-of-life argument. All legal and regulatory statements below refer specifically to Germany; rules elsewhere differ. The basics are covered under Insulating a container, the wider fit-out context under Living in a container.
What spray foam actually is
Spray polyurethane foam (SPF) is a two-component system. A polyol blend and an isocyanate component are combined only inside the spray gun, react there and foam up on the surface. The insulation is therefore not made in a factory but directly on the wall – both its greatest strength and its greatest weakness: it adapts to any geometry, but its quality is created on site.
Two types exist, and they differ in almost every property.
| Property | Open-cell foam | Closed-cell foam |
|---|---|---|
| Thermal conductivity (rough) | approx. 0.035–0.040 W/(m·K) | approx. 0.022–0.028 W/(m·K) |
| Density | low | considerably higher |
| Feel | soft, sponge-like | hard, compression-resistant |
| Vapour permeability | vapour-open | acts as a vapour retarder |
| Structural stiffening | barely any | noticeable |
| Cost per area | cheaper | more expensive |
| Typical use | interior walls, acoustics | container conversions, refrigeration |
In US container conversions, closed-cell foam is almost always what is meant: it insulates best per centimetre and takes on moisture control at the same time.
Why it is standard in the USA
Three reasons explain its popularity – none of them irrational.
It does three jobs in one pass. Insulation, airtightness and moisture control are created simultaneously. A conventional build-up needs insulation, a vapour control membrane, tapes and careful detailing at corners, penetrations and the floor junction. The foam skips that joint work because it has no joints.
Container geometry suits it. The corrugated steel wall is about the worst possible substrate for board products: boards rest on the ribs and leave cavities in which air can circulate and moisture can condense. Solving that cleanly means stud frames, levelling layers or bonded cut-to-fit boards. Sprayed foam fills every corrugation without gaps or rework.
Centimetres are expensive. With an internal width of roughly 2.35 m, every centimetre of build-up comes straight off the usable floor area – and an insulant at approx. 0.024 W/(m·K) needs far less thickness for the same effect than mineral wool at approx. 0.035 W/(m·K).
On top of that comes the market: North America has many specialised contractors, because the timber-frame construction common there is a major application field anyway. The technology is established, prices are competitive, and a contractor is usually nearby.
Why it is rare in German residential construction
Proof of usability
Insulation materials installed in buildings in Germany generally require a building-authority proof of usability (Verwendbarkeitsnachweis) or CE marking with declared design values. For boards this is routine: the product leaves the factory with defined, tested properties. With foam sprayed in place, the insulant only comes into existence on site – and the evidence base, particularly for habitable rooms, is considerably thinner. Whether a specific product may be used in your case follows exclusively from the manufacturer’s documentation and the relevant proof of usability.
Building-physics tradition
German practice plans in defined layers and assesses moisture protection by calculation – key terms are the sd-value of the vapour control layer, the condensation assessment to DIN 4108-3 and the Glaser method described there. That assumes you know how thick and how vapour-tight each layer is. With sprayed foam, exactly those variables depend on workmanship: layer thickness, mixing ratio, substrate and ambient temperature. A poorly processed batch can perform differently from the data sheet – harder for designers to handle than a board with a printed lambda value.
Fire safety
PU foam is combustible. For habitable rooms, requirements on reaction to fire and smoke development apply – the latter is a particular topic with polyurethane. Depending on use and federal state, a room-side lining may be required. Spray foam is therefore not automatically a finished wall.
Deconstruction and recycling
The foam bonds inseparably to the substrate. Steel with adhering PU foam is harder to recycle as scrap, and the foam itself cannot be reused. Anyone who later wants to repurpose or resell the container has a problem that a screwed stud frame with friction-fitted insulation does not have – an argument gaining weight as circularity moves up the agenda.
Health and occupational safety
Processing involves isocyanates, usually MDI-based. These can act as sensitisers via inhalation or skin contact, and a sensitisation once acquired generally persists. Since 2023, an EU-wide REACH restriction on diisocyanates has required mandatory training for professional and industrial users before these substances may be handled – one of the main reasons why spray foam is not a typical DIY application in Europe.
Market size
Because the focus here is not on housing, there are few specialised contractors. That means longer travel distances, higher prices, and a single container often being simply too small a job.
Does it exist in Germany at all?
Yes – just not where the US videos are set. The technology is established, but its centre of gravity lies well outside housing.
| Sector | Typical application |
|---|---|
| Agriculture | storage barns, potato and vegetable stores, livestock buildings |
| Commerce and industry | hall roofs, refrigeration and cold storage |
| Plant engineering | tanks, vessels, pipework insulation |
| Roof refurbishment | flat roof refurbishment with sprayed foam |
Here the foam plays to its strengths: large continuous surfaces, complex geometries, no habitable-room requirements. A contractor who insulates potato stores can in principle also foam a container – the question is whether they offer it for residential use, and on what evidence.
What does it cost?
All figures below are very rough orientation values for the German market, including material and installation. They vary considerably by region, thickness, accessibility and job size. Only a written quotation is binding.
| Option | Rough range per m² | Note |
|---|---|---|
| Closed-cell spray foam, contractor | approx. €40–90 | depends strongly on thickness |
| Open-cell spray foam, contractor | approx. €25–50 | rarely sensible for containers |
| Mineral wool in a stud frame | approx. €15–35 | incl. vapour control layer |
| PU/PIR boards, bonded | approx. €25–50 | incl. vapour control layer |
Ancillary costs matter: travel, a minimum order value and masking off every surface that must stay foam-free. Given the small area of a 20-foot container, these fixed costs quickly dominate the price per square metre – spray foam gets more expensive per square metre the smaller the project. That is the economic core of the problem.
Can you do it yourself?
Two-component kits for self-application do exist in Europe: two pressurised cylinders with hose and gun, covering a modest area. Honest assessment: conceivable for small repairs, generally a bad idea for a whole container.
- Cost per square metre. Small kits are expensive relative to their yield; across a container you often end up above what a contractor with professional equipment charges.
- Occupational safety. Depending on product and situation you need suitable respiratory protection – up to supplied-air systems – plus protective suit, gloves, eye protection, effective ventilation and a re-entry waiting period. A container is a very small enclosed space, so this is more demanding than an open roof void.
- Processing window. Substrate and ambient temperature must be right, the components dispensed in the correct ratio, the layer thickness even. Too thick in one pass, the foam can overheat; too thin or badly mixed, it misses its declared properties.
- Evidence. For a residential use requiring approval, it stays unclear how the build-up would be substantiated.
In short: if spray foam, then through an experienced specialist contractor – not as DIY.
Better alternatives for a container
| Build-up | Rough lambda | Advantage | Disadvantage |
|---|---|---|---|
| Mineral wool in stud frame + vapour control layer | approx. 0.032–0.040 | cheap, non-combustible, easy to substantiate | thick, membrane must be sealed |
| Bonded PU/PIR boards + vapour control layer | approx. 0.022–0.028 | slim, high performance | joints and corrugations need levelling |
| External insulation / ETICS on the shell | system-dependent | best against thermal bridges, keeps internal size | changes appearance, weather protection needed |
| Vacuum insulation panels | very low | minimal build-up | costly, cannot be cut, pressure-sensitive |
| Wood fibre | approx. 0.038–0.045 | vapour-open, good summer performance | thick, heavier |
For most container projects in Germany, external insulation is the cleanest solution: the steel shell then sits on the warm side and condensation on the steel is ruled out by design. Where that is not possible, a careful internal build-up with a fully sealed vapour control layer is the standard route – see also Tiny house from a container.
When spray foam can still make sense
There are cases where the calculation comes out differently: storage and workshop use without habitable-room requirements, several containers at once (the fixed-cost share drops), convoluted geometries with many penetrations, and refrigeration. The technique is also used commercially purely as a condensation barrier under a container roof.
Important note
This article is general information and replaces neither construction nor legal advice. Binding are exclusively the manufacturer’s specifications, the proof of usability or declaration of performance for the specific product, and the assessment by qualified design professionals and an executing specialist contractor. Requirements for fire safety, moisture protection and planning approval differ by use, federal state and individual case; all regulatory statements here relate to Germany. Prices are non-binding orders of magnitude, not an offer.
Conclusion
Spray foam is not a bad insulant – closed-cell sprayed polyurethane solves several container-typical problems in one pass and is hard to beat per centimetre. That it barely appears in German housing has nothing to do with thermal performance and everything to do with the surrounding conditions: the harder evidence situation for site-made build-ups, the design practice of calculated moisture protection, fire safety, occupational safety rules for diisocyanates, and the impossibility of clean material separation at end of life – plus a small market that makes single containers expensive jobs.
For a residential project in Germany, a properly planned layered build-up – ideally from the outside – is usually the better route: verifiable, reversible, predictable. Anyone still drawn to sprayed foam should talk to a specialist contractor early and make proof of usability the very first question they ask.