Polyiso vs XPS vs EPS: Which Foam Board Insulation Is Best?

If you’ve started looking into foam board insulation, you’ve probably run into three main types:

• Polyiso (ISO)

• XPS (Extruded Polystyrene)

• EPS (Expanded Polystyrene)

And at first… they all kind of look the same.

Rigid boards. Different colors. Different prices.

So what’s the actual difference—and more importantly, which one should you use?

Let’s break it down in a way that actually makes sense.

The Quick Answer (If You Just Want the Basics)

• Best overall performance: Polyiso (ISO)

• Best moisture resistance: XPS

• Best budget option: EPS

But the real answer depends on what you’re using it for.

What Is Polyiso (ISO) Insulation?

Polyiso, short for polyisocyanurate, is one of the most widely used insulation types in commercial construction—especially roofing.

Why people use it:

• High R-value (usually around R-6 to R-6.5 per inch)

• Great for large-scale insulation projects

• Common in flat roofs and exterior walls

Downsides:

• Performance can drop in very cold temperatures

• Usually more expensive (unless reclaimed)

You’ll often find a lot of reclaimed polyiso on the market from commercial roof tear-offs—which is where major savings come in.

What Is XPS Insulation?

XPS (extruded polystyrene) is the dense, usually pink or blue foam board you’ve probably seen at hardware stores.

Why people use it:

• Strong and durable

• Excellent moisture resistance

• Consistent R-value (around R-5 per inch)

Best uses:

• Basement walls

• Foundations

• Under concrete slabs

• Anywhere moisture is a concern

Downsides:

• More expensive than EPS

• Less eco-friendly than reclaimed options

What Is EPS Insulation?

EPS (expanded polystyrene) is the lightweight white foam board that looks similar to packing foam.

Why people use it:

• Most affordable option

• Lightweight and easy to handle

• Widely available

Best uses:

• Interior walls

• DIY projects

• Budget-focused builds

Downsides:

• Lower R-value (around R-3.5 to R-4 per inch)

• Less moisture resistance than XPS

Side-by-Side Comparison

R-Value (Insulation Performance)

• Polyiso: ⭐ Highest

• XPS: ⭐ Medium

• EPS: ⭐ Lower

Cost

• EPS: 💰 Cheapest

• XPS: 💰💰 Mid-range

• Polyiso: 💰💰💰 Highest (new), but cheaper when reclaimed

Moisture Resistance

• XPS: ⭐ Best

• Polyiso: ⭐ Good

• EPS: ⭐ Lowest

Strength & Durability

• XPS: ⭐ Strongest

• Polyiso: ⭐ Strong

• EPS: ⭐ Lightweight

Which One Should You Actually Choose?

This is where it gets practical.

For Garage or DIY Projects

Go with:

• EPS (budget)

• XPS (if moisture is a concern)

For Roofing Projects

Go with:

• Polyiso (ISO)

This is the industry standard, especially for commercial roofing.

For Basements or Foundations

Go with:

• XPS

It handles moisture much better than the other two.

If You’re Trying to Save Money

Look for:

• Reclaimed Polyiso

• Reclaimed XPS

• Surplus EPS

This is where you can cut your costs significantly without sacrificing performance.

What About Reclaimed vs New?

Here’s something most people don’t realize:

The type of insulation matters less than where you buy it

You can often get:

• Polyiso

• XPS

• EPS

…for 30–70% less if you buy reclaimed or surplus.

That’s why many contractors:

• Use reclaimed for large areas

• Use new material only where needed

How to Choose the Right One (Simple Checklist)

Before buying, ask yourself:

• Is moisture a concern? → Choose XPS

• Do I need maximum insulation? → Choose Polyiso

• Am I on a tight budget? → Choose EPS

• Can I use reclaimed materials? → Save money across all types

Where to Find These Insulation Types

Here’s the tricky part:

You won’t usually find the best deals at big box stores.

Instead, you’ll want to look for:

• Insulation resellers

• Surplus material suppliers

• Reclaimed insulation yards

The easiest way to compare options is to browse suppliers by insulation type and location, so you can see what’s actually available near you.

Final Thoughts

Polyiso, XPS, and EPS all have their place—it’s not about which one is “best,” but which one is best for your project.

If you’re trying to:

• Maximize performance → go Polyiso

• Handle moisture → go XPS

• Keep costs low → go EPS

And if you really want to stretch your budget…

Start by looking at reclaimed and surplus suppliers first.

That’s where the biggest savings usually are.

Browse suppliers near you to compare Polyiso, XPS, and EPS options currently available