Sandwich Panel Selection Guide: EPS vs Rock Wool vs PU

Which Sandwich Panel Is Right for Your Steel Building?

If you're building a steel structure warehouse, workshop, or office, one of the most important decisions you'll make is choosing the right sandwich panel. The panel affects insulation, waterproofing, fire safety, and your total budget. Get it wrong, and you could face condensation issues, high energy bills, or even fire code problems down the road.

At Derun, we've supplied sandwich panels for over 200 steel projects across Africa, Southeast Asia, and the Middle East. In this guide, we break down the three most common types — EPS, rock wool, and PU — so you can make the right choice for your building.

What Is a Sandwich Panel?

A sandwich panel is a three-layer composite material: two metal sheets on the outside and an insulating core in the middle. The outer sheets are usually color-coated steel or aluminum, and the core is what makes the difference in performance.

These sandwich panels are used for both walls and roofs of steel buildings. They're fast to install, lightweight, and come with built-in insulation — which is why they're the standard choice for prefab steel structures.

EPS Sandwich Panels: Cost-Effective and Lightweight

EPS (expanded polystyrene) is the most common and affordable sandwich panel core. If you're building on a tight budget, EPS is usually your first choice.

Pros of EPS

• Lowest cost among the three types

• Lightweight, easy to handle and install

• Good resistance to moisture — EPS doesn't absorb water easily

• Decent thermal insulation for general use

• Wide availability from most panel manufacturers

Cons of EPS

• Poor fire performance — EPS melts and drips when exposed to high heat

• Not suitable for buildings with strict fire codes

• Lower R-value compared to PU for the same thickness

• Can be damaged by rodents over time

Best Use Cases for EPS

EPS panels work well for warehouses, agricultural buildings, and general workshops where fire safety is not a top priority. If you're building in a region with mild climate and low fire code requirements, EPS will give you the best value for money.

Rock Wool Sandwich Panels: Fire Safety and Noise Control

Rock wool (also called mineral wool or stone wool) is made from basalt rock spun into fibers. It's the best choice when fire safety is your main concern.

Pros of Rock Wool

• Excellent fire resistance — can withstand temperatures above 1000°C

• Non-combustible, meets Class A fire rating

• Great sound insulation — reduces noise from outside

• Doesn't melt or produce toxic fumes in a fire

• Good thermal insulation performance

Cons of Rock Wool

• Higher cost than EPS

• Higher water absorption — needs proper waterproofing

• Heavier than EPS and PU, requires more structural support

• Fibers can irritate skin during installation — workers need protective gear

• Can settle over time, reducing insulation effectiveness

Best Use Cases for Rock Wool

Rock wool panels are ideal for office buildings, factories with fire hazards, residential buildings, and any structure that needs to meet strict fire codes. If you're building in a dense urban area or storing flammable materials, rock wool is the safe choice.

PU Sandwich Panels: Best Insulation, Higher Cost

PU (polyurethane) foam is a closed-cell insulating material. It offers the best thermal performance among the three, but it also comes with a higher price tag.

Pros of PU

• Highest R-value per inch — best thermal insulation

• Excellent air and moisture barrier — almost zero water absorption

• Lightweight, similar to EPS

• Good for cold storage and temperature-controlled buildings

• Longer service life with proper installation

Cons of PU

• Most expensive option

• Moderate fire performance — PU can burn and release toxic fumes

• Not suitable for buildings with strict fire codes

• Quality varies a lot between manufacturers — cheap PU can degrade quickly

• Can be damaged by UV exposure if not properly coated

Best Use Cases for PU

PU panels are perfect for cold storage warehouses, food processing plants, clean rooms, and buildings where temperature control is critical. If you need to maintain a constant indoor temperature and budget allows, PU is the best choice.

Side-by-Side Comparison

How to Choose the Right Panel for Your Project

Here's a simple decision framework we use with our clients:

Step 1: Check Your Local Fire Code

This is the most important step. Some countries and regions require Class A fire rating for commercial buildings. If that's the case where you're building, rock wool is your only option.

Step 2: Consider Your Climate

If you're in a hot climate and need to keep the interior cool, PU panels will save you money on air conditioning over time. In mild climates, EPS is usually enough.

Step 3: Calculate Your Budget

As a rough guide, rock wool costs about 20-30% more than EPS. If you want to know the total cost of your building, check our guide on steel warehouse cost., and PU costs about 40-60% more. But remember: the panel is only 10-15% of your total building cost. Don't skimp on insulation if it means higher energy bills for years.

Step 4: Think About What You're Storing

If you're storing chemicals, food, or other temperature-sensitive materials, PU panels are worth the extra cost. For general storage and workshops, EPS or rock wool will do fine.

Common Waterproofing Mistakes

One of the most common questions we get is about sandwich panel waterproofing. Even the best panel will leak if installed poorly. Here are the mistakes we see most often:

1. Improper Lap Joints

Roof panels need to overlap by at least one rib. If the overlap is too small, water will seep through during heavy rain. Make sure your installer follows the manufacturer's instructions exactly.

2. Wrong Sealant

Not all sealants work with metal panels. You need a neutral-cure silicone sealant that's compatible with both the metal sheet and the panel core. Cheap sealant will crack within a year and cause leaks.

3. Ignoring Wall-Flashings

The junction between wall panels and roof panels is where most leaks happen. Proper flashing and gutters are essential — don't let your installer skip this step to save time.

4. Poor Drainage Slope

Roof panels need to have enough slope to drain water. If the slope is less than 5 degrees, water will pond on the roof and eventually find its way inside.

Why Panel Quality Matters More Than Type

Here's something most people don't tell you: a high-quality EPS panel will outperform a cheap PU panel every time. The thickness of the metal sheets, the density of the core material, and the quality of the factory bonding all make a huge difference.

When you're comparing quotes, always ask for:

• Steel sheet thickness (0.4mm vs 0.5mm vs 0.6mm)

• Core material density (kg/m³)

• Brand of paint coating (PVDF vs polyester)

• Factory test reports for fire rating and thermal conductivity

What We Recommend at Derun

For most of our clients in Africa and Southeast Asia, we recommend:

• Warehouses and general workshops: EPS panels — cost-effective and good enough for most uses

• Office buildings and factories: Rock wool panels — better fire safety and sound insulation

• Cold storage and food processing: PU panels — best insulation for temperature control

We can supply all three types with your steel structure building package. If you're not sure which one is right for your project, send us your requirements and we'll help you calculate the cost difference and make a recommendation.

Ready to Get Started?

Choosing the right sandwich panel doesn't have to be complicated. Start with your fire code requirements, then think about your climate and budget. If you're still unsure, ask your supplier for a sample panel and compare the weight and thickness in person — it's surprising how much difference there is between cheap and quality panels.

If you'd like a quote for your steel structure building with sandwich panels included, contact Derun today. We'll get back to you within 24 hours with a detailed quotation.