Thermal Drift of Polyiso and XPS

musingsheader image

Thermal Drift of Polyiso and XPS

As these rigid foam products age, their R-value decreases

Posted on Jun 3 2016 by Martin Holladay

Most insulation materials have an R-valueMeasure of resistance to heat flow; the higher the R-value, the lower the heat loss. The inverse of U-factor. lower than R-5.6 per inch. As David Yarbrough, a nationally known insulation expert, explains, “At 75°F, the theoretical maximum R-value of a product is 5.6 per inch. That represents the maximum R-value if there is no convection and no radiation — it represents the pure conductivity of air. That’s as high as you can go unless you are talking about a product that incorporates encapsulated gas, or a vacuum, or nano-scale powders.”

Since this is a theoretical maximum, it isn’t surprising to learn that cellulose, mineral wool, fiberglass, and EPSExpanded polystyrene. Type of rigid foam insulation that, unlike extruded polystyrene (XPS), does not contain ozone-depleting HCFCs. EPS frequently has a high recycled content. Its vapor permeability is higher and its R-value lower than XPS insulation. EPS insulation is classified by type: Type I is lowest in density and strength and Type X is highest. generally have somewhat lower R-values (in the range of R-3.5 to R-4.2 per inch).

Why have many manufacturers of polyisocyanurate, at least until recently, been touting R-values that are higher than this theoretical maximum? Because polyiso (like extruded polystyrene) includes encapsulated gas; as Yarbrough noted, insulation products with encapsulated gas can exceed the theoretical maximum R-value of R-5.6 per inch.

If the bubbles of air found in foam insulation are replaced with bubbles of a different gas — a gas with lower vapor conductivity than air — it’s possible to achieve a higher R-value. The blowing agents used to produce polyiso and extruded polystyrene (XPSExtruded polystyrene. Highly insulating, water-resistant rigid foam insulation that is widely used above and below grade, such as on exterior walls and underneath concrete floor slabs. In North America, XPS is made with ozone-depleting HCFC-142b. XPS has higher density and R-value and lower vapor permeability than EPS rigid insulation.) fall into this category.

Thermal drift

Although the blowing agents used to produce polyiso and XPS successfully increase the R-value of newly manufactured rigid foam, they have an Achilles’ heel: over time, these blowing agents dissipate. The gases gradually diffuse through the rigid foam, and as they dissipate, they are replaced by air. (Covering the rigid foam with a vapor-impermeable facing like aluminum foil slows, but does not prevent, this process.) Because this process results in a gradual reduction in the foam’s R-value, it’s often referred to as “thermal drift.”

The blowing agents used to make expanded polystyrene (EPS) are replaced by air very quickly after the EPS is manufactured. (The R-value of EPS is measured after the blowing agent has dissipated.) That’s why the R-value of EPS (unlike the R-value of polyiso or XPS) is stable for the life of the rigid foam.

An EPS manufacturer touts a “zero thermal drift guarantee”

A lot of information about “thermal drift” that appears on the web has been posted by EPS manufacturers, who point out that the R-value of EPS stays constant for decades. For example, one EPS manufacturer, Neopor, offers a “zero thermal drift guarantee,” bragging, “The use of air as an insulating gas allows Neopor to maintain its outstanding R-value performance over time and contributes to sustainable building practices.”

Similarly, another EPS manufacturer, ACH Foam Technologies, disparages the competition, noting that “polyiso and XPS are affected by aging and temperature.” ACH also brags that one of its products, Foam-Control EPS, “is stable and the R-value will not change with time. Foam-Control EPS comes with a warranty for 100% of its original R-value.”

Of course, we can’t necessarily expect EPS manufacturers to provide a balanced presentation of the thermal drift issue. We need to dig a little deeper to determine whether these statements provide a complete picture of thermal drift.

A short history of polyiso labeling

While there’s been a lot of attention to the problem of thermal drift in polyiso over the years, less has been written about thermal drift in XPS. Aware of this knowledge gap, I decided to track down more information on thermal drift in XPS. But to understand how thermal drift affects XPS, we first need to look at thermal drift in polyiso.

For most of the last 30 years, polyiso manufacturers have provided misleading R-value information. The main impetus behind the effort to require polyiso manufacturers to provide more accurate R-value labels has not come from government, code bodies, academic researchers, ASTMAmerican Society for Testing and Materials. Not-for-profit international standards organization that provides a forum for the development and publication of voluntary technical standards for materials, products, systems, and services. Originally the American Society for Testing and Materials. , or polyiso manufacturers — although all of these agents played a role in the drama. Much of the impetus came from a contractors’ organization: specifically, the National Roofing Contractors Association (NRCA). Mark Graham, executive director of technical services at NRCA, has spent decades spearheading a campaign for more accurate polyiso labels.

In the 1980s, when polyiso manufacturers were claiming that their products had an R-value of 7.5 per inch, the NRCA tested polyiso samples and found that most samples had lower R-values than shown on the insulation labels. (The R-7.5 value only occurs for a brief period of time, immediately after the foam is manufactured.) In 1987, the NRCA issued a technical bulletin advising roofers that “an R-value of 5.6 per inch thickness is a reasonable value to be used when calculating thermal performance [of polyisocyanurate] over the anticipated life of the roof.”

The campaign for more accurate polyiso labels has taken almost 30 years — not only because this type of lobbying takes time, but because the development of new test methods takes time. Within the last few years, polyiso manufacturers finally conceded that Graham was right.

There were many steps on this long journey. Until 2003, the R-value labels on polyiso were based on a testing protocol developed by the Polyisocyanurate Insulation Manufacturers Association (PIMA) known as the PIMA 101 test method. Although PIMA 101 values accurately reflect the thermal performance of polyiso during its first few weeks of life, in-service R-values are significantly lower ten years down the road.

In 2002, polyiso manufacturers and XPS manufacturers agreed to use a new test method that more accurately reflects the long term thermal performance of their rigid foam products. The new test method and labeling protocol, dubbed “long-term thermal resistance,” or LTTR, was implemented in the U.S. by industry consensus on January 1, 2003 for polyiso insulation that is either unfaced or that has a gas-permeable facing (Type II polyiso), and for all types of XPS. The precise steps for the LTTR testing procedure — which is sometimes called the “slicing and scaling method,” since the procedure involves the testing of thin slices of foam — are detailed under ASTM C1303, and form part of the ASTM standard for reporting thermal resistance as specified by ASTM C1289.

As John Geary explained to me in 2003, when Geary was the marketing services manager at Firestone Building Products (a polyiso manufacturer), “The LTTR value is a 15-year time-weighted average. If you were to take the R-value every year for 15 years, and average those 15 numbers out, the result should equal the LTTR.”

As implemented in 2003, the LTTR method resulted in polyiso R-values ranging from 6.0 to 6.25 per inch. When I interviewed Graham in 2003, he wasn’t quite satisfied. “NRCA is not backing away from our recommendation concerning R-5.6 per inch,” he told me. “PIMA and the whole polyiso industry have come a tremendously long ways, and we have made a substantial step in the direction of achieving a consensus on how to rate the thermal resistance of polyiso … but I don’t think the book is closed yet.’”

In 2006, Mark Graham reported on further polyiso testing. In an article called “Testing LTTR,” Graham wrote, “This research consisted of testing the thermal resistance of aged, stored, plastic foam insulation and comparing the results with published LTTR values. … Seventeen of the 20 samples tested exhibited R-values less than their established LTTR values. All these [polyiso] samples were less than five years old, the relative age the LTTR methodology is intended to represent. Four of the samples with R-values less than the established LTTR values were less than one year old. On the basis of this data, a positive bias in the LTTR methodology clearly is apparent — that is, the LTTR methodology appears to overstate a product’s actual R-value at five years of relative aging.” With this evidence in hand, Graham continued to lobby for more realistic polyiso labeling.

Further changes finally came in January 2014, when polyiso manufacturers agreed to abide by an updated version of the ASTM C1289 test protocol. The updated version (ASTM C1289-11 2014) was changed to note than when ASTM C1303 is used, only the core slices of polyiso should be tested in the prescriptive method. The new version of ASTM C1289 — surprise, surprise — resulted in polyiso labels of R-5.6 or R-5.7 per inch — in line with NRCA’s decades-old recommendation.

In a recent document posted on the PIMA website, the manufacturers’ organization describes polyiso R-values in a way that is more accurate than ever. “The PIMA QualityMark certification program is now reporting LTTR values in accordance with ASTM C1289-11 2014. To participate in the certification program, a Class 1 roof is suggested to have a design R-value of 5.7 per inch.”

Whether the polyiso manufacturers’ 30-year journey from R-7.5 to R-5.7 has been admirable or appallingly slow depends on one’s perspective. When I discussed polyiso R-values with building scientist John Straube last year, he was generous in his praise for polyiso manufacturers. “The polyiso industry deserves some credit for changing their marketing claims to say that polyiso is R-5.7 per inch,” Straube said. “So let’s give them credit for that. That is close to the honest truth if you are talking about polyiso at room temperature.” (At mean temperatures of 50°F or lower, polyiso perfoms even worse than its R-value label implies; however, this issue is separate from the thermal drift issue.)

XPS guarantees

As I wrote earlier, most published reports about thermal drift focus on polyisocyanurate. What about XPS? Most builders know that XPS is rated at R-5 per inch. How much lower will the R-value of XPS be in 20 years?

One manufacturer of XPS, Owens Corning, provides a guarantee that its XPS will retain 90% of its R-value after 20 years. In other words, if the R-value of the XPS drops to R-4.5 per inch in 20 years, Owens Corning considers that drop in R-value to fall within its warranty terms.

Dow also warrants that its Styrofoam XPS will retain 90% of its R-value (as long as the XPS is at least 1 1/2 inch thick), but Dow's warranty is for 50 years rather than 20 years. (Of course, the warranty notes that Dow will simply supply new insulation if the customer can show that the rigid foam has lost more than 90% of its R-value; Dow will not pay for installation.)

The trade association for XPS manufacturers, known as the Extruded Polystyrene Foam Association (XPSA), has published a document titled “Long Term Thermal Resistance.” The document notes, “Extruded Polystyrene Rigid Foam Insulation products (XPS) have a closed cellular structure and are manufactured with a blowing agent specifically selected for its ability to facilitate the XPS extrusion manufacturing process and which enhances the thermal performance of the foam. Over a long period of time (50 to 75 years), the blowing agent slowly diffuses through the thickness of the foam, and air slowly diffuses into the closed cellular structure replacing the blowing agent. The result of this gas movement is the overall thermal resistance (R-value) of the XPS insulation changes over time. … The Canadian standard, CAN/ULC S770-03, defines the long-term thermal resistance (LTTR) of foam insulation products as the 5-year R-value after aging in a laboratory environment.”

The XPS industry, according to the document, recommends that designers assume that the aged R-value of XPS is R-5 per inch. No surprise there. But how long will the foam retain an R-value of 5?

The S770 test

The XPSA refers to the Canadian standard for determining LTTR, standard S770-03. But the document failed to note that the standard is controversial when applied to XPS.

Jessica Robinson, a marketing representative from Dow Building Solutions, shared the following statement provided by an unnamed “technical expert” from Dow: “The out-of-date CAN/ULC S770-03 method specified that LTTR is calculated as the average of the initial thermal resistivity times the effective aging factor, where the effective aging factor could be the higher of the aging factor for surface slices and aging factor for core slices. For XPS, this generally resulted in higher LTTR than an actual 5 year aged value.”

Two researchers who looked into the issue, Sachchida N. Singh and Paul D. Coleman, reported on their findings in a paper called “Accelerated Aging Test Methods for Predicting the Long Term Thermal Resistance of Closed-Cell Foam Insulation.” Singh and Coleman concluded that “The current S770 method is a reasonable method for measuring the LTTR for polyiso boards.” But for reasons they explain in the paper, “The current S770 method does not appear to be an appropriate method to measure LTTR for XPS boards.”

An Owens Corning document titled “Technical Bulletin: Long Term Thermal Resistance” seems to share the reservations expressed by Singh and Coleman. The document declares, “Owens Corning does not promote the use of LTTR-values generated per the CAN/ULC S770-00 thin slicing method.”

The problem seems to stem from the fact that S770 was developed for polyiso rather than XPS. RCI, Inc., a nonprofit association of professionals who specialize in roofing and waterproofing, published an article called “Long-Term Thermal Resistance: Five Years Later.” The article noted, “Since diffusion rates for XPS are at least an order of magnitude higher than that for polyiso, researchers have found it difficult to establish a test method appropriate for both materials.”

To address the deficiencies in CAN/ULC S770-03, the standard was updated. The unnamed Dow expert explained, “Beginning with CAN/ULC S770-09 and also in the current CAN/ULC S770-15, the LTTR is calculated as the average of the initial thermal resistivity times the effective aging factor, where the effective aging factor is calculated from actual percentage of contribution from skins and from cores. ... This results in a more accurate LTTR and generally a lower LTTR than the CAN/ULC S770-03 version.”

In short, the improved (more accurate) S770 test method results in lower R-values than the older version of the test.

Measuring the R-value of old samples of XPS

Perhaps the best available research paper discussing thermal drift in XPS is “An Evaluation of the Thermal Conductivity of Extruded Polystyrene Foam Blown with HFC-134a or HCFC-142b” by Chau V. Vo and Andrew N. Paquet.

Vo and Paquet measured the R-value of aged samples of XPS. “The results confirm the slow diffusion rate of CFC-12, HCFC-142b and HFC-134a through extruded polystyrene foam, and demonstrate that HFC-134a or HCFC-142b-blown extruded polystyrene (XPS) foams meet the requirement for applications requiring excellent long-term insulation performance,” the researchers wrote.

Vo and Paquet report the thermal performance of rigid foam in mW/m-K, a unit that most American builders are unfamiliar with. Before we can discuss Vo and Paquet’s findings, it’s probably useful to take a temporary detour to discuss lambda (λ).

What is lambda?

In the S.I. system, thermal conductivity, also known as lambda (λ), is measured in W/m-K or mW/m-K.

If one is measuring an insulation material, a high thermal conductivity is bad; a low thermal conductivity is good. A low thermal conductivity corresponds to a high R-value per inch; a high thermal conductivity corresponds to a low R-value per inch.

Thermal conductivity is a material constant. Unlike U-factorMeasure of the heat conducted through a given product or material—the number of British thermal units (Btus) of heat that move through a square foot of the material in one hour for every 1 degree Fahrenheit difference in temperature across the material (Btu/ft2°F hr). U-factor is the inverse of R-value. or R-value, it does not vary with the thickness of a material.

To get a sense of the typical range of thermal conductivities for insulation materials, it helps to know that a vacuum-insulated panel has a thermal conductivity of 0.008 W/m-K (equivalent to 8 mW/m-K). That’s an excellent insulation material.

Mineral wool has a thermal conductivity of 0.033 W/m-K (equivalent to 33 mW/m-K). That’s a higher thermal conductivity than a vacuum-insulated panel, of course, but is typical for an insulation material.

If you want to convert thermal conductivity (λ) in W/m-K to North American R-value per inch, here’s what you do: (1) Multiply the lambda number by 6.9336, and (2) Calculate the reciprocal of the result (in other words, divide 1 by the product you calculated in step 1).

Back to Vo and Paquet

According to Vo and Paquet, the final lambda values for XPS after 50 years range from 29 mW/m-K (equivalent to R-4.9 per inch) for foam blown with CFC-12 to 34 mW/m-K (equivalent to R-4.2 per inch) for foam blown with HFC-152a, HCFC-22, or CO2. Foams blown with HCFC-142b and HFC 134s end up with lambda values of 30 mW/m-K (equivalent to R-4.8 per inch). These values are shown in the graph below.

[Image credit: Chau Vo and Andrew Paquet.]

Measurements of samples show lower R-values

So, are the values shown in the graph prepared by Vo and Paquet the last word on this issue? Perhaps not.

In a March 2007 article called “Long-Term Thermal Resistance: Five Years Later,” author Richard Roe describes tests of XPS that seem to raise doubts about the conclusions reached by Vo and Paquet. Roe wrote, “Ten samples of XPS, ranging in thickness from 1 to 4 inches, were collected from the field and submitted — first to an R & D laboratory for testing over time, and then to a third-party materials testing laboratory for independent corroboration. The samples submitted to the industry laboratory were 2.5 to 12.5 months in age. By the time they were submitted to the third-party laboratory, they were approximately 20 to 30.5 months in age. … In only two cases, the measured R-value met the 6-month reported value (R-5.0), which is also the S770 LTTR-value recommended by XPS manufacturers in their marketing literature. In at least two cases, the measured R-values at 2.5 to 5 months in age failed to meet the 6-month minimum value in the ASTM C 578 polystyrene material standard. In the majority of cases, the measured R-value was below the published R-5.0, especially as the samples aged.”

A table accompanying the article (see Image #2, below) showed that the measured R-values of the tested samples ranged from a low of R-4.42 per inch to a high of R-5.35 per inch. The average R-value of the XPS samples (all of which were less than 3 years old) was only R-4.74 per inch. These values are lower than the values that might be expected by referring to the graph prepared by Vo and Paquet.

Dave Yarbrough sums up

While researching this article, I sent some questions to Dave Yarbrough, a research engineer for R & D Services in Cookville, Tennessee. “The XPS will age due to changes in the cell gas concentration,” Yarbrough responded by email. “As the case with all unfaced or permeable faced cellular plastics, the cells will eventually be filled with air (just air). The open-cell insulations are filled with air — so you have at least an approximate answer [to the question of the eventual R-value of XPS]. Compare Type X [XPS] with Type II [EPS].” The label on Type X XPS shows R-5 per inch. Yarbrough implies that the R-value of XPS will eventually degrade to the R-value of Type II EPS — namely, R-4.1 per inch.

Yarbrough provided one more twist to this analysis. He noted, “Blowing agents have been evolving in recent years, so there is not much data that are valid for current products.”

Answers from an XPS trade group

When I reached out to representatives from the Extruded Polystyrene Foam Association (XPSA), a trade group representing XPS manufacturers, I was invited to participate in a conference call with Barbara Fabian, chairperson of XPSA's Environmental and Technical Affairs Committee, and John Woestman, XPSA codes and standards director. (While Fabian was speaking for XPSA during the conference call, it's worth noting that she also works as the R&D laboratory leader at Owens Corning.) Fabian summarized the inherent difficulties facing the development of a single test method to determine LTTR for XPS, polyiso, and spray foam.

The thin-slice method was originally developed as a tool for researchers. “The thin-slice method is not consistent across all material types and material thicknesses, and it isn’t appropriate for both faced and unfaced materials,” Fabian told me. She concluded, “LTTR is not appropriate as a design value.”

In that case, I wondered, what's a designer to do? Fabian, noting that my emailed questions discussed a possible “floor value” for the R-value of XPS, implied that such a floor value might be of interest to designers.

Picking up the thread, I asked Fabian, “Are you willing to suggest a floor value for XPS?”

“No,” she answered.

Fabian seemed to be implying that there isn't much point in turning to laboratory researchers or LTTR values for answers to design questions. In other words, designers who need to know the long-term R-value of rigid foam should develop their own approach rather than depending on the LTTR value of polyiso or XPS shown on manufacturers' labels.

Here’s my advice:

  • If you live in a warm climate, assume that polyiso has an R-value of R-5.6 per inch. If you live in a cold climate, assume that polyiso has an R-value of R-4.5 per inch or R-5.0 per inch.
  • Assume that XPS starts out at R-5 per inch and gradually loses R-value. Owens Corning guarantees that the R-value of its XPS won't drop any lower than R-4.5 per inch during the first 20 years of its life. After 40 or 50 years, the R-value of XPS is likely to drift down to about R-4.1 or R-4.2 per inch.
  • If you are bothered by the idea that some rigid foams lose R-value as they age, specify EPS — and if your project requires a high R-value per inch, choose graphite-enhanced EPS like Neopor.

Martin Holladay’s previous blog: “How Much Insulation is Too Much?”

Click here to follow Martin Holladay on Twitter.

Tags: , , , , , , ,

Image Credits:

  1. Image #2: Richard Roe

Jun 3, 2016 7:44 AM ET

Edited Jun 3, 2016 11:31 AM ET.

Thanks, and a stable way to boost R
by Charlie Sullivan

Thanks for digging into this! It's disappointing that the XPS industry isn't able or willing to say much useful. I hope that by asking the questions and publishing this you are helping to make them realize they need to do more. And thanks for boiling it down into practical assumptions to make for now, in the absence of better information.

A useful footnote is that if you want to have higher R-value than EPS, but don't want drift, you can get ~R4.5/in with graphite-loaded EPS. It's branded Neopor by BASF, and rebranded under various other names by insulation companies, such as "insulfoam platinum." Unless carpenter ants carry away bits of it, the R-value won't change over time.

In Europe, the XPS industry offers CO2 blown graphite loaded XPS. Stable R value and negligible global warming impact. Too bad they don't offer that here.

Jun 3, 2016 7:58 AM ET

Response to Charlie Sullivan
by Martin Holladay

Thanks for catching my typo (which I have corrected).

We must have been thinking along the same lines this morning. Ten minutes ago, I made a few last-minute edits to my piece, including a mention of Neopor in my final bullet point. After making the edit, I read your comment, which also noted the advantage of Neopor. So, needless to say, I agree with you. Thanks.

Jun 3, 2016 10:00 AM ET

Does size matter?
by stephen sheehy

Martin: does the rate of R-value loss differ with the thickness of the foam? Would a meter thick foam end up at R 4 at the same time as a millimetre thick f oam?
Does the location matter? Under slab v. In a wall?

Jun 3, 2016 10:09 AM ET

Edited Jun 3, 2016 10:29 AM ET.

Response to Stephen Sheehy
by Martin Holladay

Thickness definitely matters -- which is why Dow's guarantee only applies to XPS that is at least 1 1/2 inch thick. (The obvious implication is that the R-value of 3/4-inch XPS or 1-inch XPS drops off faster than for thicker types of XPS.)

I don't think that anyone can answer your question about subslab XPS vs. wall-mounted XPS -- but we can all speculate.

Jun 3, 2016 10:45 AM ET

Thanks for digging into this!
by Dana Dorsett

Nice piece of investigation, Martin- well worth the effort!

I would expect 2lb polyurethane blown with HFC and HFO agents to have the same issues, but not water blown 2lb polyurethane, since the water dissipates quickly.

Icynene's 2.0 and 2.1lb water blown foams run about R5/inch, and is probably an indication of where most 2lb polyurethane would end up (but over how long is still an open question.)

The smaller regional player Aloha Energy's water blown polyurethane claims as much as R6.5/inch at some unspecified density, but third party ASTM C518 testing data doesn't seem to be available online.

* 4 Foam Density Products available 0.5lb, 1.0lb and 1.8lb and 2.5lb
* Provides an R-Value of 3.6 to 6.5 per inch, depending on foam density "

Jun 3, 2016 10:57 AM ET

Edited Jun 19, 2016 10:30 PM ET.

Response to Dana Dorsett
by Martin Holladay

John Straube told me, "You know, installers of closed-cell spray foam sometimes brag, ‘We’re spraying foam at R-7 an inch.’ Maybe it’s R-7 instantly, right after you spray it, but after just a little bit of aging, you won’t getting anything near that. We’ve never seen any foam that achieves R-7. You may be spraying it, but it doesn’t make it to my lab. Lots of people make claims but no one is checking."

Jun 3, 2016 11:29 AM ET

That sounds right to me.
by Dana Dorsett

Some 2lb foam vendors are claiming R7/inch, but I'm a skeptic on that too.

I'm a bit surprised at just how poorly HFC-blown XPS can perform at the tender age of 3 years (or even 3 months!). I had been assuming that it would probably still be in the ~R5-ish range or higher at least to the ripe age of 5, but clearly that can't be counted on. I've long held the notion that assuming anything more than R4.2/inch for reclaimed or well-aged XPS isn't appropriate from a design perspective, but given the Richard Roe's third party tested performance table included in this article the age & condition no longer factors into my thinking. Even brand-new XPS cannot be assumed to be higher than R4.2/inch long enough to matter. (And I thank you again for that!)

Jun 4, 2016 7:14 AM ET

"Regular" EPS vs. Neopor

Martin finishes the blog with the reference to graphite infused Neopor EPS. I am planning to use 6 1/2" EPS SIPs on my new home for the walls. (EPS core is 5 1/2" thick) Manufacturer rates the EPS panels at R-23, Neopor at R-27. Is it likely to be worth a cost premium for the Neopor over the EPS for an added R-4? Or is that bump in R negligible for the added cost?

Jun 4, 2016 8:23 AM ET

It depends...
by D Dorsett

Whether the additional cost of graphite loaded EPS is "worth a cost premium" depends a lot on the limitations of how thick the wall can be. A 7.5" - 8" EPS SIP would have comparable (or better) performance to a 6.5" Neopor SIP.

I haven't personally priced Neopor in $/R-ft^2 (let alone Neopor SIPs vs. standard EPS), but anticipate that it's going to be more expensive per unit-R. Do the math. Where you can tolerate the additional thickness a standard EPS is probably going to be more cost effective.

If you are constrained to 6.5", a bump from R23 to R27 is a double digit percentage reduction in heat flow- it's not nothing. But whether the net performance gain for the whole house is cheaper at a given wall thickness by bumping up to Neopor vs. better windows or some other change is something the designer needs to work out.

Jun 4, 2016 8:59 AM ET

Response to James Kreyling Re whether Neopor is worth it.
by Charlie Sullivan

Like Dana says, we'd have to know the cost premium to know whether it's worth it. You can, as he says, compare that to increasing the thickness, or to improving the envelope elsewhere. We can also quickly and easily calculate the annual heat loss through the walls in the two cases and calculate the heating system energy use difference, if we know the type of heating system and the heating degree days for your location. And we can calculate how much PV you'd need to buy to cover that difference.

Under a slab, making the insulation thicker is possible for only the cost of the insulation. A thicker wall, however, increases other costs such for the details needed at windows and doors. So even if a thicker SIP is a cheaper way to get an R-27 rated wall panel, the total project cost might still be lower with the thinner Neopor.

But that's all just speculation until we have numbers for what they are charging as a premium for the Neopor.

Jun 5, 2016 5:12 AM ET

Edited Jun 5, 2016 5:17 AM ET.

Neopor Payback
by Andrew Bater

I was interested in using Neopor, researched it when we were planning our SIP home. Looking back through my emails to our GC reminds me that I had studied BASF's independent lab testing reports etc. I recall being convinced it was the way to go. Ultimately we didn't use it; the payback wasn't there for us, much the same as it wasn't for triple pane windows.

Found a 4 year old email to our GC that has a screen shot where I used a simple online energy calculator to determine whether Neopor would be beneficial for our roof. (Ignore the actual chosen materials, they were picked to provide the correct R value differential. We also don't use fuel oil; I assume I either entered it as a worst case assumption, or this tool had no provision for ground source heat pump HVAC.)


Jun 5, 2016 6:33 AM ET

At 37cents/R-ft^2 at high-R I'm not suprised it doesn't work.
by D Dorsett

The calculation shows 2160 square feet of R4 at a cost of $3205.44. That's $1.484 per square foot for R4, or $0.371 per square foot per R. That's pretty expensive performance enhancement ...

...~3.5x as expensive per R as standard EPS ...

...~2x as expensive per R as closed cell spray polyurethane...

...~10x as expensive per R of open blown cellulose in an attic.

Jun 6, 2016 10:32 AM ET

Foil faced XPS?
by Dillon Vautrin

Should the manufactures not produce a foil faced XPS? It sounds like a foil face could reduce the thermal drift by reducing the blowing agents from off gassing through the two faces of the panels. It would also make a nice surface for taping panels compared to how the panels are currently manufactured. Of course this would increase the cost of manufacturing and hence the panel. For most customers price means a lot, especially when they may only be looking at the R-value when comparing products.

I wonder what the internal pressure of the foams individual cells is before and after off gassing. I would think that the pressure would drop. Could this be the cause of the foam panel shrinkage? Is it a direct result of the off gassing, and would a foil face help reduce this?

Oh yeah, and how about a question for the manufactures. How would I go about having my foam tested for an R-value to possibly cash in on the warranty, and is the warranty transferable? I think it is convenient for the manufactures that it is not probable you would ever be able to test your foam board to make sure it retains 90% of the R-value 20 to 50 years later. If there are local labs for testing R-values of foam and the warranty is transferable, my friend Craig has a list with a lot of reclaimed foam board for sale that could be swapped for new.

Jun 6, 2016 10:41 AM ET

Edited Jun 6, 2016 10:44 AM ET.

Response to Dillon Vautrin
by Martin Holladay

First of all, several Chinese manufacturers produce foil-faced XPS. Here is a link to a web site describing such a product:

Second, here is contact information for a lab that can determine the R-value of an XPS sample or other types of insulation:

R & D Services
102 Mill Drive
Cookeville, TN 38501

Jun 7, 2016 2:52 AM ET

Offgassing toxicity
by Adam Liberman

This is slightly off-topic of R-value, but I'm wondering: If the blowing agent is slowly being released, does this create any air-quality concerns when used inside a structure, say for insulating basement walls.

Jun 7, 2016 5:14 AM ET

Response to Adam Liberman
by Martin Holladay

The rate at which the blowing agent diffuses is quite slow -- the process takes decades -- so the concentration of blowing agent in the indoor air is likely to be very, very low. Moreover, I don't know of any reports that these blowing agents are toxic. (I'm not saying they aren't, but I've never heard that they are -- especially at the remarkably low levels under discussion).

If the idea of blowing agents diffusing into the air is worrisome to you, choose EPS.

Jun 10, 2016 11:10 AM ET

HFC blowing agents aren't toxic
by Dana Dorsett

The biggest safety hazards with HFCs are frostbite from contact with liquids. At concentrations of over 500,000 ppm HFC134a has been shown to cause cardiac symptoms in dogs, but that's a concentration only achievable by opening up a can of automotive refrigerant in an unventilated room.

The fire retardents used in rigid foam are probably a bigger human-health issue than slow bleed down of the HFCs over years/decades, and even those might be tough to measure. In most houses you're probably at bigger risk from the VOCs in cleaning solutions and scented products.

Jun 23, 2016 11:44 AM ET

Extruded Polystyrene Association (XPSA) Response
by Laura Meditz

Determining R-value by focusing on thermal drift is not a comprehensive way of evaluating long-term thermal performance (LTTR) of foam insulation. Thermal drift is just one part of several heat transfer mechanisms that should be considered when accurately determining the thermal performance of foam insulation.

The three main heat transfer mechanisms to consider when determining an R-value are convection, conduction, and radiation. Heat transfer through these mechanisms varies based on the physical properties of different foams (e.g. density, cell size, polymer composition, gas phase composition), as well as environmental factors (e.g. aging, temperature, climate). Over the years, the foam insulation industry has never ceased to improve upon and make advancements in the thermal performance of foams with regards to all three mechanisms of heat transfer.

In light of the number of variables, and variety of methods to determine LTTR values through accelerated aging, and variety of types of insulation, the foam insulation industry’s consensus has been to evaluate R-value at 180 days at a 75 degree mean temperature in order to determine R-values for comparative performance, which gives you the average R-value in the second stage of aging. The industry evaluates R-values by ASTM C518 or C177 standards because they are uniform test methods that can be applied to all foam plastics.

Think of it like when you consider buying a car and ask how it is on gas mileage. You’re given a number in average miles per gallon that the industry has also calculated based on a set of variables. If those variables change (e.g. gas octane level, city traffic, condition of the carburetor, going up or down hill, age of the car), the average miles per gallon may change.

Much like the gas mileage analogy, the LTTR for foam insulations is not a fixed value due to the physical and environmental variations. Blowing agents are required in the foam plastic extrusion process to create desirable thermal performance qualities. And, while it is scientifically accurate that the slow diffusion of a blowing agent out of and diffusion of air into XPS foam minimally decreases its R-value over an extended period of time, the closed-cell structure of XPS rigid foam insulation allows XPS to retain significant thermal efficiency throughout its lifecycle, even after the 15-year weighted LTTR testing period. This is important to note because while other types of foam insulation claim to not have “thermal drift” because of differences in the manufacturing process, their products still allow penetration and absorption of moisture, reducing the foam’s insulation power and, ultimately, its R-value.

XPS insulation contributes significantly to a building’s energy efficiency and performance durability due to its very high compressive strength and moisture resistance, making it a sustainable and energy efficient choice for designers. When considering recommendations for foam insulation, make sure to do your research and consider all of the variables to make an informed decision that is right for your specific need.

The Extruded Polystyrene Association is happy to serve as a resource. Additional information can also be found at


Laura Meditz, Communications Manager, XPSA

Jun 23, 2016 6:30 PM ET

Edited Jun 23, 2016 6:38 PM ET.

Response to Laura Meditz
by Martin Holladay

Thanks very much for your comments, which seem to confirm the facts reported in my article.

One of your sentences is a little puzzling: "Thermal drift is just one part of several heat transfer mechanisms that should be considered when accurately determining the thermal performance of foam insulation."

Thermal drift is not "part of a heat transfer mechanism". As you correctly point out, there are just three heat transfer mechanisms relevant to insulation performance: convection, conduction, and radiation. Thermal drift is a shorthand phrase to describe the gradual reduction in insulation performance for some types of rigid foam. It is not "one part of a heat transfer mechanism."

The heat transfer mechanisms that govern heat flow through XPS after thermal drift reduces the R-value of the foam are exactly the same heat transfer mechanisms that govern heat flow through XPS before thermal drift reduces the R-value of the foam.

Register for a free account and join the conversation

Get a free account and join the conversation!
Become a GBA PRO!