XPS vs. Polyiso Insulation: Understanding the Key Differences
XPS vs. Polyiso Insulation: Understanding the Key Differences
When choosing insulation for a roofing or wall project, two popular materials often come up: XPS (Extruded Polystyrene) and Polyiso (Polyisocyanurate). Both offer strong thermal performance and moisture resistance, but they differ in cost, R-value, and applications. Knowing these differences helps you choose the right insulation for your project.
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What Is XPS Insulation?
XPS insulation is a rigid foam board made by extruding polystyrene into a closed-cell structure. It delivers consistent thermal performance and moisture resistance, making it a reliable choice for below-grade, foundation, and wall applications.
Benefits of XPS
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Durable and moisture-resistant: The closed-cell structure prevents water absorption, which helps XPS maintain its R-value even in damp environments.
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Strong compressive strength: Ideal for heavy-duty applications like under concrete slabs or rooftop assemblies.
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Stable R-value: Provides around R-5 per inch of thickness and maintains performance over time.
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Easy to cut and install: Lightweight boards simplify installation on both vertical and horizontal surfaces.
Because XPS resists moisture so well, it performs best in foundation walls, basements, and below-grade insulation systems where exposure to water is likely.
What Is Polyiso Insulation?
Polyiso insulation is a rigid foam board made with a thermoset plastic core between two facers, often fiberglass or foil. It’s known for its high R-value per inch and is widely used in commercial roofing and wall assemblies.
Benefits of Polyiso
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Higher R-value: Polyiso delivers about R-6 to R-6.5 per inch, offering more thermal resistance than XPS at the same thickness.
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Energy-efficient: Its higher R-value allows thinner walls or roofs without sacrificing insulation performance.
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Environmentally friendly: Polyiso often contains recycled materials and uses blowing agents with low global warming potential.
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Fire-resistant: The thermoset foam structure adds built-in fire resistance, making it a safe option for roof systems.
Polyiso shines in above-grade and roofing applications, especially where insulation thickness matters or energy codes require high R-values.
Comparing XPS and Polyiso
| Property | XPS | Polyiso |
|---|---|---|
| R-Value (per inch) | ~R-5 | ~R-6 to R-6.5 |
| Moisture Resistance | Excellent | Moderate |
| Compressive Strength | High | Medium |
| Fire Resistance | Moderate | High |
| Cost | Slightly lower | Slightly higher |
| Best Use | Below-grade, foundations | Roofing, walls, above-grade |
If your project involves ground contact or potential moisture exposure, XPS insulation performs better. If you’re insulating a roof or need maximum thermal efficiency in limited space, Polyiso insulation delivers more value.
Which Insulation Should You Choose?
Your choice depends on where and how you plan to use it.
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Choose XPS for below-grade or damp environments where water resistance matters most.
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Choose Polyiso for roofs or walls that need a higher R-value in a thinner profile.
Both materials offer long-term performance, and combining them in different areas of the same building can provide the best of both worlds.
Final Thoughts
XPS and Polyiso both offer reliable, energy-efficient solutions for modern construction. XPS stands out for moisture resistance and durability, while Polyiso delivers superior thermal performance and sustainability. The right choice depends on your project’s climate, design, and budget—but both options help you build with better energy control and long-term value.
