Polyiso vs. XPS: correcting a few common myths about R-value, LTTR, temperature, moisture, and UV

We recently received a note from a reader that raised important points—and also repeated a few oversimplifications that can mislead homeowners, builders, and DIYers. Insulation performance is real engineering: it depends on temperature, moisture exposure, and how R-value is measured and reported. There isn’t a single “always wins” foam.

Here’s the reader’s message (quoted), followed by the most accurate interpretation of the standards and building-science evidence.

“All rigid insulation with blowing agents off gas to the LTTR state.”
“The ISO R value, regardless if temperature, fluctuates by 10–12%.”
“Polyiso board has a higher LTTR factor than XPS and is more UV resistant and absorbs about the equal per volume % of moisture.”
“Polyiso board outperforms XPS board overall.”

1) “All rigid insulation with blowing agents off gas to the LTTR state.”

This is partly true, but too broad.

LTTR (Long-Term Thermal Resistance) is a standardized way to estimate the “aged” R-value of gas-filled, closed-cell foams whose thermal performance changes as the blowing agent diffuses over time. ASTM describes this explicitly: the LTTR method applies to “rigid gas-filled closed-cell foam insulations” and is designed to predict performance by accelerating aging through specimen preparation and testing. ASTM International | ASTM+1

What’s missing from the claim is that not every rigid insulation is meaningfully “gas-filled” in the same way. For example, EPS is mostly air-filled and doesn’t undergo the same “thermal drift” mechanism that is central to LTTR discussions for some closed-cell products.

Also, “LTTR” isn’t one magical number that makes all comparisons easy. It’s a useful metric—especially when understood as an aged estimate and used appropriately for the product type. (Industry groups like PIMA have published primers on LTTR and R-value reporting for polyiso.) Polyiso

2) “R-value… regardless of temperature, fluctuates by 10–12%.”

This statement is not correct as written.

R-value is temperature dependent for many insulation materials, and polyiso in particular has extensive documentation showing non-linear changes across service temperatures. Building Science Corporation explains that label R-values are typically tested at a mean temperature of 75°F, and polyiso’s R-value can decrease as temperatures drop—often not in a simple linear way. Building Science+1

Independent and manufacturer-adjacent research also shows meaningful temperature dependence in polyiso performance (sometimes large enough that “10–12% regardless of temperature” is not a defensible generalization). For example, a research summary on aged polyisocyanurate reports a sizable spread in measured R/in across temperatures, underscoring that temperature can materially change effective R-value. rockwool.com

Practical takeaway:

• Label R-value is not a constant.

• Mean temperature matters.

• Any claim that temperature only changes performance by ~10–12% “regardless of temperature” is almost certainly oversimplified.

3) “Polyiso has a higher LTTR factor than XPS.”

This is often true at standard rating conditions, but it does not mean polyiso is always higher in real-world conditions.

Polyiso is widely marketed with a higher R per inch at common rating conditions (again: typically around a 75°F mean). That advantage is real in many above-grade and roof assemblies where the foam’s mean temperature stays closer to the rating point.

But cold-weather performance is where polyiso can lose some of that edge—hence why building-science guidance stresses temperature dependence and why designers sometimes choose materials differently depending on climate zone and assembly position. Building Science+1

4) “More UV resistant…”

As a blanket statement, this is not reliable.

• Owens Corning explicitly states that polystyrene-based products (including XPS) deteriorate if exposed to UV for long periods and recommends covering FOAMULAR promptly. Owens Corning

• Polyiso can also degrade if exposed for prolonged periods, especially if unfaced; and even faced products should not be treated as “permanently UV-proof.” Building Enclosure Online

• It’s true that facers (foil or glass facers) can help protect polyiso foam cores, and industry literature often highlights that benefit. YMAWS

Practical takeaway: treat UV as a jobsite handling and sequencing issue. The safest assumption is: don’t leave any foam board exposed longer than manufacturer guidance—cover it.

5) “Absorbs about the equal per volume % of moisture.”

This is generally not supported by standard specifications.

For XPS, the most commonly cited benchmark is ASTM C578 water absorption limits (by volume). Many XPS types are required to meet a maximum of 0.3% by volume water absorption under the standard’s short-term immersion test. XPSA+1
Manufacturer data sheets also commonly reflect the same order of magnitude (e.g., 0.3% by volume in product literature). DuPont+1

For polyiso, allowable water absorption depends on the specific ASTM C1289 type/class and facer system, but published polyiso technical bulletins and classification tables frequently show higher allowable absorption values than typical XPS limits (though there are exceptions for certain cover boards and specific constructions). ASTM International | ASTM+1

There is one nuance worth stating clearly: standard water absorption tests do not perfectly predict long-term field behavior. Industry and enclosure-science groups have pointed out that ASTM C578 is not a “guarantee” of in-service water uptake (and that real assemblies involve wet soils, drainage planes, freeze-thaw cycling, cut edges, fasteners, etc.). IIBEC+1
So, while XPS is typically specified for lower absorption in short-term tests, good design still matters.

Practical takeaway: below grade or in wet exposure, you should select insulation as part of a system (waterproofing, drainage, protection board, detailing)—not by one lab number alone.

6) “Polyiso outperforms XPS overall.”

This is too broad to be “correct.” The honest answer is: it depends on the application.

A more accurate way to think about it:

Polyiso often shines when:

• You need high R per inch in an above-grade wall or roof assembly near typical rating temperatures.

• You’re building a compact assembly where thickness is constrained.

• You’re using products designed for the specific application (roof polyiso vs wall polyiso, etc.). ASTM International | ASTM+1

XPS often shines when:

• The insulation will see high moisture exposure, especially below grade, where low absorption and durability are prioritized.

• You want a product family commonly specified for foundations/under-slab conditions, with published low absorption values in standard testing. DuPont+1

Temperature matters for both, but especially polyiso:

• Polyiso can be temperature-sensitive; label R-values may not reflect cold-weather in-service behavior. Building Science+1

A clearer way to choose: ask these three questions

1. What is the likely mean temperature of the foam in service? (Roof above deck in winter is not the same as interior side of a wall.) Building Science+1

2. Will the insulation be exposed to bulk water or ground moisture? If yes, treat moisture management as a system design problem, not just an R-value discussion. IIBEC+1

3. Are you comparing labeled R, aged R (LTTR), or in-service R? Those are not the same thing. ASTM International | ASTM+1

Bottom line

The reader is right that aging/LTTR matters for some foam products—but the message goes too far by claiming temperature effects are always small, moisture absorption is “equal,” and polyiso “wins overall.” The more accurate statement is:

• Polyiso and XPS are both excellent materials in the right assembly.

• Temperature and moisture exposure determine which is “better” for a specific job.