Why steel is the real problem
A shipping container is a self-supporting structure made of Corten steel. Exactly what makes it stackable, transportable and durable is its biggest drawback in building physics terms: steel conducts heat extremely well. The thermal conductivity of structural steel is in the order of roughly 50 W/(m·K), while common insulation materials such as mineral wool, EPS or PUR sit around 0.035 W/(m·K). That is a difference of roughly a factor of 1,000 to 1,500.
Put visually: every continuous steel component is a motorway for heat, while the insulation next to it is a dirt track. If you fit a container with 80 millimetres of interior insulation but heat can still travel unhindered through a few centimetres of steel in the corner post, even the best insulation layer helps only so much. This deliberate bypass route for heat is called a thermal bridge.
It helps to distinguish two types. Geometric thermal bridges occur where a small internal surface meets a large external surface – the classic building corner. Structural or material-related thermal bridges occur when a highly conductive component penetrates the insulation layer. In a container, both types coincide at the same locations: the corners and the perimeter frame. That is why the effects are far more pronounced here than in a masonry building.
Where thermal bridges sit on a container
The critical points are practically identical on every container because the form is standardised. The overview below links component, cause and typical consequence.
| Component | Why it is critical | Typical consequence |
|---|---|---|
| Corner posts and corner castings | Solid steel volume, continuous inside to outside, no separation possible | Coldest points in the room, mould in room corners |
| Upper and lower rails | Perimeter steel beam running the full container length | Cold strips at wall-to-ceiling and wall-to-floor junctions |
| Door frame and door sill | Heavy frame with sealing plane, hard to over-insulate | Condensation at the sill, corrosion, draughts |
| Floor cross members under the timber floor | Steel profiles spaced a few decimetres apart, directly beneath the floor build-up | Cold floors, streak-shaped damp marks |
| Corrugated side wall profile | Corrugations enlarge the surface, cavities hinder gap-free insulation | Uneven surface temperature, ventilation gap errors |
| Window and door reveals after cutting | Welded-in frames create new continuous steel profiles | Condensation on the reveal, mould at the window corner |
| Bearing points on the foundation | Direct steel-to-concrete contact at pads or strip footings | Heat drain into the ground, moisture from below |
| Penetrations for services and ventilation | Every opening interrupts insulation and airtightness | Localised condensation, draughts |
Walk this list around a real container and the picture is clear: thermal bridges are not the exception, they are the rule, running around the entire structure.
What happens: from heat loss to mould
Energy loss is the less dramatic consequence. A few per cent more heating demand is annoying but manageable. The real risk lies in the surface temperature on the inside.
At a thermal bridge, the internal surface cools more than the surrounding area. Warm room air touching that cold surface can hold less water vapour. If the surface temperature drops below the dew point of the room air, moisture condenses out. And well before the dew point, relative humidity directly at the surface rises far enough for mould to find growth conditions. As a rough orientation: if relative humidity at a surface stays above roughly 70 to 80 per cent for extended periods, mould growth becomes likely.
Building physics uses the temperature factor fRsi to assess this. It describes how close the internal surface temperature is to the room air temperature. DIN 4108-2 names a guide value of 0.7 for residential use as orientation. That is a planning approach and explicitly not a promise for any individual case: whether a specific container meets the requirement can only be judged by calculation or measurement on site. Treat it as a mental model, not a guarantee.
The decisive question is not “how much insulation is in the wall” but “how warm does the coldest spot in the room stay”.
Why interior insulation only partly solves it
Interior insulation is the usual container solution because it is cheap, preserves the external dimensions and keeps the container look. In building physics terms, however, it is the most difficult option.
The reason is simple: with interior insulation the entire steel frame sits outside the insulation layer. It takes on the outside temperature and stays cold in winter. Wherever that cold steel meets the interior – at every corner, every frame junction, every reveal – the cold travels past the insulation into the room. The result is cold strips along posts and rails that are often noticeable by hand in winter and show up as dark lines in wallpaper or paint.
On top of that, interior insulation reduces the already tight internal dimensions and shifts the dew point into the construction. Both are manageable but demand care. You will find the build-up basics in Insulating a container, and the seamless variant in Spray foam insulation.
Solutions in order of effectiveness
Not every measure works equally well. The order below reflects thermal effectiveness, not price.
| Measure | Effect | Effort and cost (rough orientation) |
|---|---|---|
| External insulation of the whole envelope | Best solution: steel frame sits in the warm zone, thermal bridges largely neutralised | High; additional cladding and substructure needed, external dimensions grow |
| Combination of external and internal | Very good; even a thin external layer significantly de-risks the frame | Medium to high, two trades |
| Interior insulation with thermally separated studwork | Clearly better than metal studs; carry insulation over the frames with an overlap | Medium; timber battens instead of metal profiles, small extra cost |
| Reveal insulation at windows and doors | Reduces the most common mould pattern at the window corner | Low; thin insulation boards, high effect per euro |
| Insulation beneath the floor | Neutralises cross members and cold floors | Medium; requires a raised container or build-up from inside |
| Thermal separation at bearing points | Reduces heat drain into the foundation, protects against moisture from below | Low to medium; load-bearing insulation elements or elastomer pads |
The message of this table is uncomfortable but clear: anyone intending permanent living or working is safest with a closed external insulation layer. Everything else is damage limitation. For residential projects it is worth reading Living in a container before the first batten goes up.
Details that decide the outcome in practice
- No continuous metal profiles in the interior fit-out. Metal studwork connecting the internal lining to the steel wall creates additional small thermal bridges every few decimetres. Timber battens conduct far less and are the better choice here.
- Run insulation without interruption. Otherwise gaps form at joints, corners and behind service voids. Insulation should run past the frame profiles with an overlap rather than stopping at them.
- Make junctions airtight. Convection through gaps carries far more moisture into the construction than diffusion does. One leaky corner can devalue an otherwise correct insulation layer.
- Seal the vapour control layer properly. Overlaps, junctions to floor and ceiling and all penetrations for sockets, cables and ventilation need suitable tapes and grommets.
- Bundle penetrations. Fewer holes mean fewer weak points. One central service entry beats ten scattered ones.
Detecting and checking thermal bridges
Verification is part of the fit-out and is easier than many assume.
| Method | What it shows | Effort |
|---|---|---|
| Thermal imaging in winter | Area-wide view of cold zones, ideal for handover | Medium; specialist or rented camera, temperature difference required |
| Infrared surface thermometer | Spot temperatures at corners, frames, reveals | Low; devices from around 30 euros |
| Temperature and humidity data logger | Shows trends over days and weeks instead of snapshots | Low to medium |
| Placing a hand on surfaces during frost | Quick first diagnosis at corners and door frames | Very low |
| Visual check after the first winter | Dark strips, damp stains, rust at the sill | None |
All cost and numerical figures in this article are rough orientation values and do not replace project-specific planning. Standards mentioned serve as context, not as a binding commitment.
If you insulate from the inside, treat the frame zones with particular care – or better, insulate from the outside in the first place.
Conclusion
Thermal bridges are not a side issue in containers, they are the central building physics challenge. The continuous steel frame is structurally necessary and thermally fatal. Ignoring it with pure interior insulation produces cold corners, condensation and, in the medium term, mould – regardless of how thick the insulation layer is across the surface.
The clear recommendation: for permanent occupied use, insulate externally or at least add a thin external layer. Where that is not possible, apply consistent care at frames, reveals, floor and bearing points, combined with thermally separated studwork and a genuinely sealed vapour control layer. Air movement matters too: without controlled moisture removal, even the best insulation helps only so far. Pure storage use without heating follows different rules, where ventilation takes priority – see Using storage containers properly.