The shell is the part of a capsule you cannot change later. It sets the maintenance cycle, the foundation load, the condensation behaviour and the resale value. Aluminium and steel are both legitimate choices, and which one is right depends on where the unit will sit and how long it will stay there. Here are the eight differences that actually matter, including the ones where steel wins.
1. Corrosion in salt air
This is the strongest argument for aluminium and the reason most capsule builders use it. Aluminium forms a stable oxide layer that protects it, and the 6000-series alloys used for extrusion resist marine atmospheres well, particularly when the surface is coated. Steel needs a maintained barrier coating; the moment that coating is breached — a scratch from a crate, a fixing driven without a washer — rust starts and progresses under the paint film. On a coastal site, a steel shell is a maintenance commitment, not a one-off purchase.
2. Weight, and what it does to the foundation
Aluminium is roughly a third the density of steel. On a completed capsule the difference in shell weight is partly offset by the frame sections being deeper, but the unit is still lighter, and that has three consequences: a smaller crane, a lighter foundation, and easier handling on soft or remote ground. On a screw-pile foundation it often means fewer piles. On a site with poor access, a lighter unit is simply easier to deliver.
3. Thermal bridging — both conduct, only one is usually fixed
Both metals conduct heat far better than timber or composite, so both need a thermal break between the inner and outer frame profiles. The difference is cultural: aluminium capsule frames are usually designed with a non-metallic separator because the condensation problem is obvious to anyone who has built one. Steel frames are sometimes assembled without it, on the assumption that the insulation in the panel does the work. It does not. If a supplier cannot show you the thermal break in the wall section drawing, assume there is not one.
4. Coating and the maintenance cycle
Aluminium exteriors are typically finished with a PVDF (fluorocarbon) coating, which holds colour and gloss in high-UV locations far longer than polyester. The realistic schedule is a wash two to four times a year and a sealant inspection twice a year. Steel requires the same sealant inspection plus a recoat cycle — every five to ten years depending on climate — and each recoat is scaffolding, labour and a period where the building looks tired.
5. Fire performance
This is where steel is genuinely better and it should be said plainly. Steel retains strength at temperatures that would already have melted an aluminium frame, so a steel-framed structure has more inherent fire resistance. That advantage can be matched in an aluminium capsule, but only through the specification: a rock wool insulation core rather than polyurethane, fire-rated linings, and correct detailing at penetrations. If fire performance is a regulatory requirement on your site, ask for the core material by name — the frame metal matters less than the core.
6. Repair and modification
Aluminium is easier to cut, drill and modify on site, and a damaged panel can be replaced without touching the structure. Steel is weldable, which matters if a local fabricator has to make a structural alteration — but welding destroys the coating locally and needs recoating. For a unit that will be modified after delivery, that is a real consideration.
7. Resale and recyclability
Both metals are highly recyclable and hold scrap value. Aluminium carries a higher value per kilogram, and an aluminium-bodied unit tends to hold resale value better in the accommodation market because the exterior still looks new after several seasons without repainting. That is a commercial effect rather than a metallurgical one, but it is real.
8. Cost
Steel frame with composite cladding is generally cheaper per square metre of shell, and on a dry inland site with a ten-year horizon that saving is legitimate. The comparison only holds if you include the recoat cycle and the corrosion risk in the steel column — over fifteen years on a coast, the aluminium premium is usually repaid.
When steel is the right answer
- Dry inland climate with low humidity and no salt exposure
- A fixed site with a long but finite horizon, where the unit will not be moved
- Fire performance is the dominant regulatory requirement, and a rock wool aluminium build is not available in time
- The project requires local fabrication and modification after delivery
What to put in the specification, whichever you choose
- Alloy and temper for frame and cladding, stated by designation
- Coating system, with dry film thickness
- Insulation core material and thickness in millimetres
- Thermal break detail, shown on the wall section drawing
- Fastener specification, including isolation washers to prevent galvanic attack
- Corrosion class for the environment the unit will sit in
- Sealant type and the joint detail it is applied to
The fastener point is the one that causes the most avoidable damage. Stainless screws driven directly into an aluminium frame without isolation create a galvanic couple, and the corrosion that follows starts at every fixing — invisible until it blooms through the coating two or three years later.
Related reading
The interior materials guide covers the inside of the same wall build-up, and the maintenance guide sets out the inspection routine for either shell. For the enclosures themselves, specifications are on the M-Series and Y-Series pages. If you tell us the site's distance from salt water and the intended service life, we will tell you which shell the conditions justify.
