PIR or mineral wool: choosing container insulation that survives winter

Insulation is the least visible part of a container build and the one occupants feel every single day. Two materials cover almost every container project in Europe: PIR sandwich panels and mineral wool. Both work. They are not interchangeable, and the right choice depends on what the unit is for, where it will stand, and which regulations apply to it.

This guide explains the practical differences the way we would across a table, without laboratory language.

What is a PIR sandwich panel?

PIR (polyisocyanurate) sandwich panels are two steel skins bonded to a rigid foam core, delivered as a finished building element: structure, insulation and interior surface in one panel. In container construction they form the wall and roof envelope of office, living and sanitary units.

PIR's defining property is thermal efficiency per centimetre. A PIR panel reaches the same insulation value as mineral wool at roughly two thirds of the thickness, sometimes less. In a container, where every centimetre of wall build-up is taken from a fixed external envelope (the standard office module measures 6058 x 2438 x 2800 mm outside), that efficiency converts directly into usable interior space and headroom.

Panels also assemble quickly and cleanly, with tight, repeatable joints. For serial production of office and accommodation units, PIR is the default for good reasons.

What does mineral wool do better?

Mineral wool is fibre insulation fitted between structural members and finished with separate inner lining. It needs more thickness for the same thermal result, but it brings two properties PIR cannot match.

First, fire behaviour. Mineral wool is non-combustible, which makes it the natural core for builds where fire-resistance classes such as EI 30 or REI 30 are part of the specification. Where a fire-rated wall or roof assembly is required, the build-up is engineered around it, and wool is usually in it.

Second, acoustics. Fibre absorbs sound; rigid foam mostly does not. For sleeping accommodation near machinery, generators or a road, a wool build-up audibly outperforms a thin foam wall.

The cost is space and weight: thicker walls, more structure, more kilograms per unit, which matters when units are craned and trucked repeatedly.

How much insulation does a container actually need?

A reference point from real deliveries: seasonal leisure-sector accommodation we built for a mountain location carried 100 mm of mineral wool in the walls, 140 mm in the roof and 100 mm in the floor, and stayed comfortable through alpine winters. A PIR build reaches comparable comfort with visibly thinner panels.

Three rules of thumb transfer to almost any project:

• The roof always gets more insulation than the walls. Heat leaves upward; 40 percent more thickness in the roof than the walls is a sound starting ratio.
• The floor is not optional. An uninsulated steel floor over open air will undo whatever the walls achieve, and occupants feel it within an hour.
• Continuity beats thickness. A thinner envelope without gaps outperforms a thicker one interrupted at every joint, opening and penetration. This is a production-quality question, which is why panel fit and joint detail are checked at QA rather than assumed.

Which one should you specify?

For a heated office or living unit in year-round use: PIR, almost every time. Maximum interior space, clean serial quality, fast build.

For sleeping quarters where acoustics matter, or any unit whose specification names a fire-resistance class: engineer the assembly, and expect mineral wool in it. Fire-rated container builds (EI 30 / REI 30 on request in our special-purpose family) are designed as complete assemblies, not as a panel swap.

For unheated storage: often neither. A plain ISO standard container with ventilation handles most storage duties; insulation earns its cost only when condensation-sensitive goods enter the picture.

If you are weighing families rather than materials, the comparison guide on choosing between ISO, office and special-purpose units covers that decision.

FAQ

Is PIR insulation safe in terms of fire?

PIR is a thermoset foam that chars rather than melts and is produced in classified grades for construction use. For ordinary office and accommodation units it is the European standard solution. Where the specification requires a formal fire-resistance class for the assembly, non-combustible cores such as mineral wool come into play.

Can an insulated container be used in winter at minus 20 degrees C?

Yes. With a correctly sized envelope (walls, roof AND floor) and adequate heating, office and living containers operate through Central and Northern European winters as a matter of routine. The failure mode is almost never the panel; it is a missing floor build-up or an interrupted joint.

Does insulation change the outside dimensions of the container?

No. The envelope is built inside the standard external frame, which is why thermal efficiency per centimetre matters: every centimetre of wall build-up comes out of the interior, not added to the exterior.

What about condensation?

Condensation appears where warm moist air meets a cold surface, so the answer is continuity of insulation plus ventilation, not thickness alone. Sanitary and kitchen units need engineered ventilation regardless of insulation choice.

The takeaway

Specify the use, not the material: "year-round office, comfortable at minus 15" or "sleeping quarters, 30 dB attenuation, EI 30 wall" gives a production engineer everything needed to choose the assembly. If you name the material yourself, name the thickness per surface too, and remember the roof-floor rule.

The Office & Living family page shows the PIR reference build. For a project-specific recommendation, send the use case through the quote form; the engineering review comes back with a fixed specification and price, typically within 3 to 5 working days.