How to Choose Thermal Insulation Core Materials?

In steel-structure buildings, the sandwich panels used for walls and roofs serve the critical function of thermal insulation. The very "soul" of these sandwich panels lies in the insulation core material situated within them.

When making a purchase, many clients focus on factors such as panel thickness and steel brand, yet often overlook the selection of the core material itself. In reality, choosing the wrong core material can lead to consequences ranging from failing fire safety inspections (a minor issue) to creating serious safety hazards in the event of a fire or prolonged exposure to humid environments (a major issue).

Today, rather than simply listing the names of various core materials, I aim to help you establish a professional decision-making framework based on the principle that "Choosing a Core Material = Choosing Safety, Longevity, and Operational Efficiency."

 

I. Ask Yourself Three Questions Before Choosing-Clarifying the Application Scenario

 

Not every setting calls for the same type of core material. Before comparing specific materials, please answer the following questions:

 

1. Are there mandatory fire safety requirements?

Areas under strict fire safety regulation (factories with high occupancy, logistics warehouses, supermarkets, hospitals) → *Must* be Class A (Non-combustible).
Temporary structures or small, unmanned storage units → Class B1/B2 materials may be considered.

 

2. Is the environmental humidity high?

Cold storage facilities, livestock farms, or regions prone to seasonal dampness (e.g., southern China) → *Must* feature low water absorption and resistance to condensation.

 

3. Is it for roofing or wall cladding?

Core materials used for roofing must also be evaluated for compressive strength, flexural strength, and the ability to withstand long-term self-weight without sagging.

The answers to these three questions will determine the range of core materials suitable for your project.

 

 

II. A Comparative Overview of Mainstream Insulation Core Materials

 

Core Material Type	Fire Rating	Thermal Conductivity	Water Absorption Rate	Main advantages:	Main Disadvantages
Rock Wool	Class A (Non-combustible)	0.036–0.043	High (Highly hygroscopic)	Best fire resistance, good sound insulation	Loss of thermal insulation upon water absorption; causes skin irritation during installation
Glass Wool	Class A (Non-combustible)	0.032–0.040	Medium-High	Low price, quick construction	Low compressive strength; prone to sagging after absorbing water
Polyurethane (PU/PIR)	B1/B2 (Flame-retardant / Combustible)	0.022–0.028 (Optimal)	Extremely Low (Closed-cell content > 90%)	Best thermal insulation, good compressive strength, moisture-proof	Fire resistance falls short of Class A standards; poses risks of toxic smoke
Extruded Polystyrene (XPS)	B1/B2	0.028–0.033	Extremely Low	High compressive strength, good moisture-proof	Poor breathability; prone to shrinkage with aging
Molded Polystyrene (EPS)	B2 (Combustible)	0.039–0.041	Medium-Low	Cheapest, lightweight	Poor fire resistance; highly combustible and prone to melting and dripping

 

Professional Tip: Polyurethane PIR (Polyisocyanurate) offers slightly better fire resistance than standard polyurethane-potentially reaching Class B1-but it does not qualify as Class A. Do not be misled by the "Class B1" rating into believing it can serve as a substitute for rock wool. 

 

III. Fire Rating: A "Matter of Life and Death," Not Just a "Bonus Feature"

If a fire breaks out in a steel-structure building, the steel beams and columns will rapidly soften under high temperatures. At this critical juncture, whether or not the insulation core material contributes to the combustion or releases toxic fumes directly determines the time available for occupants to escape and the risk of structural collapse.

• Class A (Non-combustible): Meets the requirements of all fire safety inspections; does not burn or produce toxic fumes when exposed to fire. Rock wool and glass wool fall into this category.
• Class B1 (Flame-retardant/Difficult-to-burn): Self-extinguishing once the fire source is removed, though it will still burn and emit smoke while exposed to flames. Suitable for projects that do not require mandatory Class A fire ratings.
• Class B2 (Combustible): Once ignited, this material will actively participate in combustion; it is not recommended for use in permanent structures intended for human occupancy.

As a professional steel structure builder, I offer you a word of advice: Never-under any circumstances-use Class B2 core materials for a factory building that requires Class A fire acceptance testing, simply to save a few tens of thousands of yuan. We have had clients attempt this, only to find that the subsequent costs for rectification and panel removal were three to five times the original material cost-and that doesn't even account for the losses incurred due to work stoppages.

 

 

IV. Moisture Resistance and Durability: Special Requirements for Steel Structure Companions

 

The greatest enemies of steel structures are condensation and corrosion. If the core material absorbs water, it not only loses its own thermal insulation effectiveness but also allows moisture to remain in prolonged contact with the inner surface of the steel panels, thereby accelerating rust and corrosion.

Preferred choices for high-humidity environments: Polyurethane and XPS (due to their closed-cell structure, which prevents moisture absorption).

Exercise caution with Rock Wool/Glass Wool: Unless paired with an exceptionally effective vapor barrier and a well-designed roof ventilation system, these materials are ill-suited for the high-humidity conditions found in southern China during the "return-to-south" season (spring dampness) or within livestock breeding facilities. Under such conditions, water absorption causes them to sag, clump together, and lose their effectiveness, while simultaneously causing the steel panels to corrode from the inside out.

Cold Storage Facilities, Seafood Processing Plants, and Livestock Farms: These applications require-almost exclusively-the use of polyurethane sandwich panels; there is virtually no viable alternative.

 

V. Construction and Long-Term Performance: Easily Overlooked "Pitfalls"

 

1. Settlement Issues

After a few years of service, the low-density glass wool or rock wool used in roof sandwich panels may gradually sag and settle toward the eaves due to the combined effects of gravity and structural vibration. The result is a loss of thermal insulation in the upper sections of the roof, leading to the formation of "thermal bridges" and subsequent condensation and dripping during the winter months.

Polyurethane and XPS, with their rigid, closed-cell structures, are not susceptible to this particular issue.

 

2. Construction Irritation and Environmental Impact

During installation, the fibers in rock wool and glass wool can cause skin irritation; consequently, workers are required to wear full protective gear. Furthermore, the formaldehyde emission levels of certain low-cost products warrant careful scrutiny.

While the environmental impact of blowing agents used in polyurethane production has improved significantly (most manufacturers now utilize pentane-based foaming agents), it remains essential to select products supplied by reputable, large-scale manufacturers. 3. Composite Panel Manufacturing Processes

Rock Wool / Glass Wool: Primarily produced as manual or machine-made panels; the core material and steel sheets are bonded together using adhesives, resulting in relatively lower long-term bonding strength.

Polyurethane: Produced via continuous high-pressure foaming lines; the core material and steel sheets form an integral chemical bond during the foaming process, offering the highest strength and effectively achieving "integrated structural insulation."

 

 

VI. Direct Reference for Typical Scenarios

 

Building Type	Primary Core Material	Secondary/Alternative Options	Key Rationale
Standard Factory (No Strict Fire Safety Regulations)	Glass Wool	Rock Wool	Cost-effective; meets basic thermal insulation requirements
Strict Fire Safety Factory / Logistics Warehouse	Rock Wool (Class A)	Glass Wool (Class A)	Strictly Class A; no compromises
Cold Storage / Seafood Processing / Breeding Farm	Polyurethane (PIR)	None	The only solution for combined moisture resistance and thermal insulation
Sports Stadium / Supermarket / Hospital	Rock Wool (Class A)	None	High-occupancy environment; safety is paramount
Temporary Prefabricated Structure / Construction Site Office	EPS (Class B2)	Polyurethane (Class B1)	Cost-driven priority; ensure separation from ignition sources

 

Summary: Choosing a Core Material Means Choosing Safety and Longevity

Selecting an insulation core material is not merely about comparing data sheets to see which product has the lowest thermal conductivity; rather, it is about finding the optimal balance-among fire resistance, moisture resistance, cost, and service life-that best suits your specific project.

As a professional builder, my recommendation is as follows:

• For any permanent building that requires fire safety acceptance and involves human occupancy, prioritize Grade A non-combustible core materials (Rock Wool / Glass Wool).
• For buildings in high-humidity environments-such as cold storage facilities or livestock farms-prioritize Polyurethane, even if it possesses a lower fire rating (as this can be compensated for through other fire safety measures).
• For roof sandwich panels, exercise caution when using low-density Glass Wool to avoid core material settlement years later, which could lead to roof failure.

 

If you are currently planning a steel-structure factory, warehouse, cold storage facility, or livestock farm, please provide the following information, and I will be happy to offer expert recommendations on the most suitable insulation core material for your needs:

• Building purpose and fire safety classification (specifically, whether a Grade A rating is required).
• Local climate conditions (average annual humidity and temperature fluctuation range).
• Intended application: Roof or wall panels?
• Target service life (5 years / 10 years / 20+ years).

Choosing the right core material ensures your steel-structure building remains safe, durable, and worry-free-sparing you the regret of a poor choice five years down the road.