By the ZN House Editorial Team | Updated October 21, 2026 | Reading time: about 8 minutes
In this guide: the short answer · how the three cores compare · what the numbers mean for a 75 mm panel · what independent fire research shows · matching panels to project types · what to ask a supplier · FAQs · sources
No single wall panel is best for every flat pack container house. Expanded polystyrene (EPS) is often the least expensive core with fair insulation but is the weakest material on fire. Rock wool, on the other hand, is the most fire-resistant material and is often the choice for sleeping areas; however, it is denser and may also be more expensive than the other substances mentioned here. PU gives the highest insulating properties for thickness, but is still a flammable foam, resulting in having to pay more. The choice is made based on the users of the building, climate, and fire regulations, and not based on the brochure rating.
A flat pack container house is a good example of why these matter. The steel frame will remain constant regardless of the choice of core, thus leaving the wall panel as one of the few final choices for the buyer, which can influence a multitude of aspects including insulation properties, behavior in fire, weight and cost.
The steel structure sustains the loading in a flat pack container home. Fillers of sandwich panels between the columns also act as insulating material and sound proofers and provide protection for the living space against environmental factors such as heat, weather, and fire during emergencies. Being the outer covering of a container house, the type of core material of sandwich panels influences performance and comfort levels significantly.
Three properties pull against each other. The best insulator per inch is not the best performer in a fire. The best performer in a fire is heavier and often costs more. The cheapest option is acceptable in some uses and not in others. Choosing well means deciding which of those trade-offs your project can live with.
Table 1 combines independent insulation data with the relative ratings printed on the ZN House flat pack product page. The page ratings are the manufacturer's own and are useful for comparison between its options only; they are not laboratory results.
Table 1. Core comparison for flat pack container house wall panels
| Core | Typical insulation (R-value per inch) | Fire behavior | Manufacturer's relative rating (product page) | Main trade-off |
| EPS | About 3.8 to 4.4 | Standard EPS can sustain combustion; fire-retardant grades self-extinguish once the ignition source is removed | Fireproof 3 of 5; heat insulation 4 of 5; lowest cost | Weakest fire performance |
| Rock wool | About 3 to 4 | Non-combustible mineral fiber; stays largely intact in a fire | Fireproof 5 of 5; heat insulation 4 of 5 | Heavier; price depends on density and supplier |
| PU (sealed rock wool or full PU) | Roughly 6 for polyurethane and polyiso foams | Combustible foam; behavior depends on formulation and detailing | Fireproof 4 to 5 of 5 depending on type; heat and sound insulation 4 to 5 of 5 | Higher cost; foam core still needs fire testing |
The R-value ranges come from the Pacific Northwest National Laboratory's building science reference table. Treat them as typical values, since density, facing and age all shift the real figure.
The ZN House flat pack wall that is considered standard is a rock wool sandwich panel with a thickness of 75 mm. The outer layer of the panel is made of steel with a thickness of 0.5 mm and a density of 140 kg/m3. Thus, converting a thickness of 75 mm to inches gives approximately 2.95 inches. Therefore, it can be compared only based on the density of its core.
These values pertain only to the core. Actual walls exhibit losses at panel joints, door and window frames, and at points of contact between the steel structure and the skins. Therefore, in temperate regions, both the 75 mm rock wool board and the 75 mm EPS board will feel the same. The gap that matters is fire, not heat. In very cold or very hot climates, PU's extra resistance per millimeter may justify its price, or you may simply specify a thicker panel of a cheaper core.
Fire is the part of this decision where marketing language is least reliable, so it helps to look at outside evidence.
A 2026 technical report by the fire engineering consultancy ASP Fire, summarized by Engineering News, compared EPS, PU, PIR, phenolic foam, mineral wool and vacuum insulated cores. It effectively distinguishes between ordinary EPS, which is able to burn and assist a fire in spreading, and fire-retardant EPS with high limit oxygen index above 24%, which typically will extinguish itself when the source of ignition has gone. Furthermore, it indicates that PU and PIR panels may cost nearly twice as much as fire-retardant EPS and mineral wool and vacuum panels may cost even more.
The report ends with a relevant finding: reducing fire hazards (wiring, heating, cooking, smoking rules) is often far more effective for safety than substituting every item. This indicates that a rock wool barrier in a poorly wired building may not guarantee safety, and at the same time, a fireproof wall made of EPS in an office that is well taken care of does not instantly mean any fire safety problem. The evaluation occurs as well in the industrial and agricultural sectors.
This specific instance is not a ZN House project; it instead refers to a laboratory investigation that didn’t involve using EPS panels. In Fire Technology (2018), Crewe, Hull, and co-authors created four small test rooms out of steel-faced sandwich panels and followed ISO 13784-1 small room tests to make a comparison between PIR panels and stone wool panels, both with, and without additional wooden combustibles. The published summary reports significant differences in fire growth and burning behavior between the two.
Gases emanating from the spaces between the panels of PIR rooms ignited after a few minutes, and flames warped out the panels, allowing more foam to be exposed, and large fires occurred both inside and outside the chamber. Thus, the authors state that when subjected to large fire loads, the steel-faced PIR panels cannot be considered reliable fire barriers, and could even facilitate flame spread. The PIR rooms also produced dense smoke and poisonous effluents while the stone wool rooms produced very little light smoke.
For customers, this means that the foam core cannot be considered a non-flammable material just because it is encased in a steel shell, and that it is at the areas of the connections of the core that concerns arise. What is not said to the customers is how the particular EPS or PU container panel behaves in terms of fire safety as this depends on the testing results for the specific product being sold. It is worth bearing this gap in mind before taking a glance at the fire claims put forward by different suppliers.
Starting framework can be found in Table 2. Local codes should always prevail and any project that involves sleeping people or large public audiences needs to be assessed by a fire engineer or relevant local authorities.
Table 2. Panel selection by project type
| Project type | Often suitable core | Why | What to watch |
| Temporary site office or storage | Fire-retardant EPS | Short occupancy, low occupant count, tight budget | Confirm the EPS grade and test report; avoid standard EPS near heat sources |
| Worker dormitory or container camp | Rock wool | People sleep, escape time matters, and many units sit close together | Weight per shipment; spacing between blocks |
| Container office used for years | Rock wool or sealed PU rock wool | Longer service life and heavier daily use | Joint sealing and moisture at the base |
| Extreme hot or cold climate | PU, or thicker rock wool | Higher insulation per millimeter, or more thickness for the same cost | Foam fire behavior; thermal bridging at the frame |
| Multi-story or public-facing use | Rock wool, verified against local code | Stricter fire requirements usually apply | Structural review for stacked units |
The standard wall specification uses 0.5 mm skins. Thinner skins dent more easily during handling and re-assembly, which can open joints.
Heat and fire issues often originate at the panel joints instead of the center of panels. Make sure to get details on joint sealing and the sealant being used.
The standard ceiling uses a 50 mm rock wool panel with a 0.3 mm steel sheet, and the roof structure uses glass wool under corrugated sheet. A strong wall core does not fix a weak roof.
Flat pack units are shipped in kit form to cut freight volume. Denser cores such as 140 kg/m³ rock wool add weight, so check whether a container will reach its weight limit before its space limit, especially on large modular container house orders.
EPS and sealed PU have good resistance to water. Rock wool does not perform as well when it gets wet, so sealing the edges and skin of the panel is more important in wet areas.
Table 3. Verification checklist for flat pack container house wall panels
| Ask for | What a good answer looks like | Why it matters |
| Fire test report for the exact panel | A named test method and classification (for example EN 13501-1 or ASTM E84) issued by an independent lab | A brochure word such as fireproof is not a rating |
| Core density and, for EPS, the fire-retardant grade | A stated density in kg/m³ and a limiting oxygen index figure | Density and grade change both fire and insulation performance |
| Skin thickness and coating | Stated in mm, with coating type | Affects durability and joint quality |
| Joint and edge detail | A drawing or sample showing the connection | Joints are where fire and air leakage usually start |
| Panel weight per unit and units per container | Figures for your chosen core | Prevents surprises on freight and handling |
| Written confirmation of the core in the quote | The core named on the quotation, not just panel thickness | Prevents a cheaper core replacing the one discussed |
The ZN House flat pack product specifications, the sources detailed below, and the general range of building science are referenced in this article. However, it is not a substitute for a site-specific fire strategy and does not compare one supplier with another. The R-values provided are typical and not necessarily guaranteed.
Not at all times. A location’s fire code and whoever is reviewing the project have definitely imposed this requirement. Rock wool, as a non-combustible material, is usually widely adopted for sleeping arrangements, but jurisdictions might accept some additional materials if correctly tested and reported. It’s recommended to verify this with local authorities before making up your mind based on price.
The thicker any panels are, the stronger the core will be. However, this is not true after the factor of joints and frame contact and door and window openings becomes vital for heat loss. Once that occurs, sealing and glazing aid comfort more than thickness.
According to the product page of ZN House, there are different types of panels’ core, such as EPS, rock wool or PU. Changing the type of panels’ core later means that a manufacturer has to address various technical issues, like changes in terms of weight, fire protection aspects and connections; hence, it should be treated as an engineering issue raised in the course of construction.
This article provides general information as published by ZN House and does not constitute engineering, fire protection, or legal advice. Different projects may have various specifications that should be included in the final estimate and be approved. It's important to remember that regulations for construction and fire protection vary from country to country, hence consulting with experts is necessary.
If you are comparing options for a project, share your occupancy type, climate, destination port and local fire requirements, and ask for the panel test report with the quotation. You can browse the full ZN House product range to compare configurations first. Watch the ZN House video below to see how container accommodation is laid out on site:
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