Cold-Weather Performance of Polyisocyanurate

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Cold-Weather Performance of Polyisocyanurate

Researchers are beginning to understand the causes of the problem, but solutions remain elusive

Posted on Sep 4 2013 by Martin Holladay

The ability of insulation products to resist the flow of heat changes with temperature. Most insulation products — including fiberglass batts, 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.), and expanded polystyrene (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.) — perform better at low temperatures than high temperatures. At lower temperatures, there is less conductionMovement of heat through a material as kinetic energy is transferred from molecule to molecule; the handle of an iron skillet on the stove gets hot due to heat conduction. R-value is a measure of resistance to conductive heat flow., less convection, and less radiation — so insulation materials usually work better than they do at warmer temperatures.

However, one type of rigid foam, polyisocyanurate, doesn’t follow this pattern. At temperatures below 50°F, polyiso performs worse than it does at a mean temperature of 75°F.

Although many building scientists have known about this phenomenon for years, published data from recent testing by Chris Schumacher and John Straube — two researchers at Building Science Laboratories in Waterloo, Ontario — have sparked new questions about the performance of polyiso at cold temperatures. (See links to previous reports on the topic in the “Related Articles” sidebar, below.)

There are two ways that polyiso can disappoint

Some people confuse the problem of polyiso's poor cold-weather performance with a different problem — that of “thermal drift.” The phrase “thermal drift” refers to the gradual dissipation of gaseous blowing agents which are replaced by air as they exit the foam. This process takes several years.

Gaseous blowing agents are chosen for their thermal properties, so the escape of these gases causes the R-value of polyiso to decline. Polyiso manufacturers have faced decades of criticism from those who assert that thermal drift makes the R-value labels on polyiso unrealistic. Responding to these critics, polyiso manufacturers agreed in 2002 to adopt a new method of R-value testing, the “long-term thermal resistance” (LTTR) method. This testing method strives to come up with a more realistic R-value for polyiso — one that takes thermal drift into account. (For more on this issue, see Thermal Drift of Polyiso and XPS.)

Freon is a big fat molecule

In hopes of finding out more about polyiso performance at cold temperatures, I recently telephoned John Straube. In addition to being a principal at Building Science Laboratories — the lab responsible for recent cold-temperature tests of polyiso — Straube is a professor of building envelopeExterior components of a house that provide protection from colder (and warmer) outdoor temperatures and precipitation; includes the house foundation, framed exterior walls, roof or ceiling, and insulation, and air sealing materials. science at the University of Waterloo. Below is a transcript of our conversation.

Q. Manufacturers of polyiso have used several blowing agents over the years. Does the problem of cold-weather performance degradation happen with all different blowing agents?

Straube: In 2000 or thereabouts, polyiso manufacturers switched away from HCFC 141B, a blowing agent that causes ozone layer damage, to a combination of CO2 and pentane. After that change, people began noticing that the R-value at cold temperatures became a problem. Older samples of polyiso perform just fine at cold temperatures — we know this because of modern tests on old foam.

There are two things that can change the R-value of polyiso. The first issue is that over time, the blowing agent is gradually replaced by by nitrogen and other atmospheric gases. The reason the blowing agents give you a better R-value [than air] is that they have lower thermal conductivity. As the blowing agent is replaced with atmosphere, the R-value of the insulation goes down.

We’ve known about that for a long time, ever since we first switched away from Freon, also known as CFC-11 [in 1993]. The industry transitioned away from Freon to reduce the problem of ozone depletion. That raised a new worry — the gradual reduction in R-value. Freon is a larger molecule [than the newer blowing agents] so it has a lot of nice benefits. For one thing, it leaked out at a much slower rate than smaller molecules.

The discussion about the gradual reduction in R-value was a kind of pissing contest for a decade, until the industry came up with LTTR, which is a way of trying to predict the long term R-value of polyiso.

At the time [when the LTTR debate was raging] Honeywell was trying to convince people to switch to their chemical [Enovate 3000], a blowing agent with a low potential for ozone depletion. But manufacturers switched instead to pentane and CO2. The Honeywell blowing agent maintains a good R-value at low temperatures. By using pentane and CO2, polyiso manufacturers ended up with R-value problems at low temperatures. These people seem to have been caught off guard. But researchers like Mary Bogdan at Honeywell knew about these problems back in 2003.

Below 50°F, the thermal conductivity of polyiso blown with pentane (shown in the upper two lines on the graph) goes up — which is another way of saying that the insulating performance of the products goes down. On the other hand, the thermal conductivity of polyiso blown with Enovate 3000 (a rarely used Honeywell blowing agent) goes down when temperatures drop below 50°F (a shown in the lowest line in this graph) — which is another way of saying that its insulating performance improves. [Image credit: Mary Bogdan, Honeywell]

Straube: Manufacturers could have used the Honeywell blowing agent. But since then the industry has moved on. Now we have to a new push for blowing agents with a low global warming potential [GWP]. The latest solution from Honeywell is the company's new low-GWP product called HFO 1234yf. This is the gas used in some brands of closed-cell spray polyurethane foam. The same blowing agent can also be used to make polyiso, but it costs more money.

In Europe, there are impending regulations to switch to blowing agents that are low-GWP, while in North America, it is just a market-driven thing. Manufacturers are kind of getting it, because they believe that the market is headed there.

The blowing agent condenses

Q. So why is the performance of polyiso worse at cold temperatures?

Straube: We don’t have evidence as to why this is happening, but we have ideas as to why this is happening. The leading contender is that we have condensation of the blowing agent within the pores of the insulation. Chris Schumacher dug into testing at a finer level of detail, hoping to could find out when the condensation occurs.

Some samples of polyiso insulation, when tested, do significantly better than others. But we can’t presume that they are all blown with pentane. The manufacturers don’t tell you whether they are pentane-blown.

Differences from factory to factory

Q. You’ve been hinting that there are other chemicals — chemicals other than pentane and CO2 — in the blowing agents used by some manufacturers. Is the difference in performance you’re talking about — differences you see when you compare samples from one factory with samples from another factory — due to contaminants in the blowing agent or chemicals deliberately added to the blowing agent by the manufacturer?

Straube: I’m not sure. Both of those are possible. So maybe the manufacturers are jiving us. Maybe they are not telling us the truth. But in apparently honest conversations with several polyiso manufacturers, they’ve told me that they do not fully understand why this is happening.

PIMA [the Polyisocyanurate Insulation Manufacturers Association] doesn’t even believe the phenomenon is real. Some of their members don’t believe it is real. They say, ‘I am not 100% sure it is a real effect.’ Well, it is real.

Obfuscation from PIMA

Q. I didn’t find the on the topic to be convincing. It was all about averages — trying to convince builders to look at average temperatures instead of cold temperatures.

Straube: I agree. I think this is all obfuscation. They are trying to buy themselves time. You know, no one is willing to put their name on that PIMA report. Who is the person who wrote it? Why aren’t they willing to put their name on that report? That report is bullsh*t. It is all talking about averages.

That's proprietary information

Straube: Here’s the thing about polyiso manufacturers: They do not share their proprietary formulations. Manufacturer A may not know what is in Manufacturer B’s foam. So now, when we discuss cold weather performance, they are kind of going, ‘I’m really surprised.’ But it’s possible that they are jiving me.

A polyiso manufacturer has two plants. One plant produces polyiso that performs better at cold temperature than the polyiso from the other plant. One hypothesis was that the two plants used different gas mixes for the blowing agent. So we said, ‘Guys, will you let us understand your process better?’

Many manufacturers are in the business of manufacturing and not in the business of developing products or looking at marketing their products. I don’t know of any polyiso manufacturer who has a certain idea of what is going on or who has a clear path to a solution. They could go to a new blowing agent, but that approach is expensive. They will probably try to use small tweaks to their existing systems and to see if that gets them the performance they need.

We’re talking about this as if it were foam and a gas, but it is really a recipe. There are other ingredients in the recipe — fire retardants, for example. What that means is that there may be other chemical interactions that could be part of the explanation.

Polyiso R-values are all over the map

Straube: The National Roofing Contractors Association tested a lot of polyiso samples. If you look at the R-value chart, you realize that there isn’t a good one and a bad one. There is a range.

[Editor's note: For a thorough explanation of this graph, see Chris Schumacher's comment (Comment #13) below.]

The idea of calling polyiso a single product is a joke. From the data we have in front of us, that idea will never work. Even at room temperature, there is a significant variation in performance from one manufacturer to another.

You’d have to be naïve to think that all brands of polyiso perform at around R-6 per inch. With variations like the ones we see in this chart, the message is: don’t stress out over the decimal points.

Why doesn't one manufacturer try to make a better product?

Straube: A market leader [among polyiso manufacturers] might say, ‘I’m going to produce the best stuff and show data proving I make the best stuff.’ But that hasn’t happened.

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. Who is the consumer watchdog?

How much do these differences in R-value really matter?

Q. Maybe these variations from manufacturer to manufacturer aren’t that important, because of all of other ways that variations can occur. After all, with many types of insulation, installation errors probably account for more variations than manufacturing differences.

Straube: It matters. The reason it matters is twofold. First, we are asking for higher R-values than we used to. The second reason is that we are getting more sophisticated. People are trying to take into account thermal bridgingHeat flow that occurs across more conductive components in an otherwise well-insulated material, resulting in disproportionately significant heat loss. For example, steel studs in an insulated wall dramatically reduce the overall energy performance of the wall, because of thermal bridging through the steel. . A large percentage of people in the industry care about these issues, so now this becomes important. Thermal bridging through wood framing might amount to 25% of your wall, but the variation in performance from one brand of polyiso to another could be on the same order.

That is why this topic has some resonance. An increasing number of people care. And no one is checking on behalf of the consumer.

The polyiso industry deserves some credit for changing their marketing claims to say that polyiso is R-5.7 per inch. Everyone seems to agree now that you should use R-5.7 per inch. So let’s give them credit for that. That is close to the honest truth if you are talking about polyiso at room temperature. If you are selling to residential customers, that’s how you are supposed to report it. But how does it perform when it is cold? What about that? There is no legal requirement [in the Federal R-Value Rule] to report that performance.

The polyiso people have done the right thing from a legal perspective. They seem to be moving their marketing claims to be legally compliant.

Any advice for builders?

Q. What should builders do: switch to EPS? Use a sandwich of two types of foam, with polyiso toward the interior and EPS on the exterior? Or just use thicker polyiso?

Straube: One option is to stick with polyiso and just make it thicker. If we do that, let’s call polyiso R-5 per inch. I would stick with polyiso rather than a sandwich with polyiso plus EPS on the exterior. The problem with advising people to use 2 inches of polyiso covered with 2 inches of EPS is that now I have to have two types of material on the job site. That’s a pain to do on a small residential job. Maybe you can do that if it is a big school — for a big job, it isn’t so difficult. Also, it’s very painful to try to install a torch-down roofing membrane over EPS, because the EPS melts. Solvents can also melt EPS. Asphalt can melt EPS.

So normally, I would advise builders to just up the thickness of the polyiso.

I’m hopeful that the polyiso people will be able to make tweaks to make it better. They don’t want to have to tweak too much in their factories, though. We may see a manufacturer finally get it, finally understand that they need to do better. They may finally get their chemists to work.

Some of the polyiso manufacturers are aware of it. Some of them will try to address the issue. We are past the denial stage and moving to the grief stage.

Martin Holladay’s previous blog: “Hygrothermal Software Sometimes Yields False Results.”

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Image Credits:

  1. Marc Rosenbaum

Sep 4, 2013 2:56 PM ET

Which brands...
by Dana Dorsett

...of closed cell polyurethane have switched over to using HFO1234yf ???

Also, in the National Roofing Contractors Association graphs showing polyiso performance at different outdoor temps, what is the temp at the conditioned space side of the foam?

Showing just the performance relative to outdoor temp is useless unless it's presumed that polyiso is 100% of the insulation.

And if that's the case those graphs are meaningless for trying to assess performance as insulating sheathing on the exterior of framed wall or roof with cavity insulation, since the average temp through the foam won't bet the same as if one side were maintained at single temp.

The range of performance between samples/vendors is impressive though, even if not all the relevant parameters are spelled out.

Sep 4, 2013 4:22 PM ET

Edited Sep 4, 2013 4:53 PM ET.

Response to Dana Dorsett
by Martin Holladay

Q. "Which brands of closed cell polyurethane have switched over to using HFO1234yf?"

A. [Edited answer] I'm not sure. Looking into it.

Q. "In the National Roofing Contractors Association graphs showing polyiso performance at different outdoor temperatures, what is the temperature at the conditioned space side of the foam?"

A. I'm looking into that now. I'll post the information when I learn it.

Sep 4, 2013 4:43 PM ET

Per John Straube....
by Dana Dorsett

John Straube states:

"The latest solution from Honeywell is the company's new low-GWP product called HFO 1234yf. This is the gas used in some brands of closed-cell spray polyurethane foam."

Can he name even one brand using it, or is that conjecture on his part?

Sep 4, 2013 4:50 PM ET

Response to Dana Dorsett
by Martin Holladay

Sorry. I'm guilty of a hasty misreading of your question. My brain read "polyiso." I'll look into it.

Sep 4, 2013 5:35 PM ET

Edited Sep 4, 2013 5:38 PM ET.

Temperature question
by Martin Holladay

As you probably know, the Federal R-Value Rule requires that insulation manufacturers test insulation materials at an average temperature of 75°F according to the ASTM C518 procedure. ASTM C518 permits testing at a range of temperatures from 20°F to 120°F. David Yarbrough explained, "An average temperature of 75°F can be achieved by the hot plate at 100°F and the cold plate at 50°F -- a commonly used set of conditions. An average of 75°F can also be achieved by hot plate at 90°F and cold plate at 60°F. There are an infinite number of combinations that average to 75°F."

Is this test method satisfactory? Not really. Manufacturers are not required to report the temperatures at which they test their insulation materials, as long as they follow the Federal R-Value Rule and ASTM C518.

I just emailed John Straube with this question: “In the National Roofing Contractors Association graphs showing polyiso performance at different outdoor temperatures, what is the temperature at the conditioned space side of the foam?” Straube responded, "It varies. They [the people performing the test] don't have interior and exterior, just hot and cold. They likely choose a 50 F° difference, but the hot and cold side [temperatures] chosen to generate the mean they reported [varies]."

Sep 4, 2013 6:15 PM ET

Edited Sep 4, 2013 6:15 PM ET.

But the graph doesn't present the mean temp, it's outdoor temp.
by Dana Dorsett

Straube suggests:

"It varies. They [the people performing the test] don't have interior and exterior, just hot and cold. They likely choose a 50 F° difference, but the hot and cold side [temperatures] chosen to generate the mean they reported [varies]."

The axis on the graph reads "Outdoor Temperature", not "Mean Temperature".

BTW: My prior understanding was that for labeling purposes per the FTC insulation had to be tested under ASTM C518 at a 75F mean temp at a nominal 30F delta-T (90F hot side, 60F cool side). ASTM C518 doesn't specify any delta-T or the mean temp, but you can't just pick & choose when labeling the performance. ( That 30F nominal delta may have been a minimum, not a nominal. I haven't read the regs in quite awhile, I could be getting fuzzy on that.)

Sep 4, 2013 11:08 PM ET

Testing I care about.
by Andy Kosick

First off, thank you for this, it filled in some gaps in my knowledge of this issue, and I have been considering this as a factor on some projects lately.

More than anything though, this made me wonder why insulation is tested the way that it is. It seems to me to be tested under a condition that nobody cares about. I certainly don't (unless someone can explain why I should). So scrap the details of ASTM C518, etc., I want to know what its heat flow is with 68F on one side and 0F on the other. While we're at it let's do one for the folks down south at 75F and 100F. Those are numbers I care about. I realize that I'm over simplifying here but for the sake of conversation humor me. When is anybody counting on build insulation to perform at a mean temperature of 75? At 75F I have my windows open.

Maybe we should be pushing for different testing standards and let that force the hand of manufacturers.

Sep 5, 2013 6:20 AM ET

Edited Sep 5, 2013 6:53 AM ET.

Response to Andy Kosick
by Martin Holladay

One of the main aims of the Thermal Metric Project undertaken by the Building Science Corporation was to address the issues that bug you. Some data from that project have been published, but unfortunately researchers have released no information on the brand names of the tested polyiso samples. One of the reasons that brand names have mostly been kept secret is that much of the funding for the testing came from insulation manufacturers.

The Thermal Metric Project has been discussed in several GBA articles, including these:

A Bold Attempt to Slay R-Value

Air Leakage Degrades the Thermal Performance of Walls

The main political reason that it's hard to redefine R-value is that any change in the definition would create winners and losers. The losers all have lobbyists.

Your proposed changes are intriguing but open to challenge. For example, many cold-climate builders would note that there are relatively few hours per year when the outdoor temperature is at 0°F or colder. This is the argument used by those who favor the European method of measuring window U-factor (based on an outdoor temperature of 32°F) over the North American method of measuring window U-factor (based on an outdoor temperature of 0°F). For more information on this controversy, see Presumptive European Superiority Syndrome.

Many hot-climate builders, on the other hand, will note that roofing temperatures above 100°F are quite common, so your proposed hot-climate testing protocol should be changed to reflect a higher outdoor temperature.

These problems can't be solved easily, because no matter what testing temperatures you choose, someone will be dissatisfied.

Sep 5, 2013 6:42 AM ET

"The graph reports outdoor temperature"
by Martin Holladay

Thanks for your comments. I am awaiting more information from John Straube and Chris Schumacher on the graph reporting NRCA data.

Sep 5, 2013 8:08 AM ET

by Dana Dorsett

Unless the polyiso is the ONLY insulation in the assembly, only one side (or perhaps none) will be at either the interior or exterior design temperature. (Under the roofing it will be both above and below the outdoor design temps, due to solar gain and night-sky radiation.)

The performance at the mean temp through the foam layer is a more useful metric, since it doesn't vary rapidly with delta-T. It's performance at +25F on one side and 5F on the other (mean temp +15F, delta-T of 20F) isn't very different than with +20F on one side, and +10F on the other (mean temp of +15F, delta-T of 10F) or 30F on one side, 0F on the other. But if it's centered around +30F it can be pretty different than when centered around +15F.

That's what makes the graphs showing only outdoor temp sort of useless. The mean temp performance (at any realistic delta-T) would be something you could actually design around. But with only the outdoor temp you can't. At +15F on one side it's performance will likely be very different between a warm-side time of +30F, as compared to what it would do with the warm side at 70F or 90F.

Sep 5, 2013 11:16 AM ET

by Hobbit _

what I think Dana was saying, why don't they pick a *fixed delta* for
all this testing, with an easily discernible average, and just slide
the endpoints up and down together as needed?? That might yield a
far more uniform set of numbers. I'd be interested in seeing what
*that* curve looks like across different brands/blowing-agents.


Sep 5, 2013 9:56 PM ET

Edited Sep 5, 2013 9:58 PM ET.

Response to Martin and Dana
by Andy Kosick

Yes, my proposed changes are absolutely open to challenge. Dana, you're right of course about the location of the foam in the building assembly (no more pulling numbers out of the air at 11pm for me). I think the point I was trying to make is that the conditions under which the insulation is going to be expected to perform should be the conditions that it is tested under.

Also, I can see using average winter/summer temperatures to determine the testing conditions, this making the most sense for overall performance in something like energy modeling software, but there is a strong argument to be made for testing at temperature extremes. After all, it's at the extremes that R-value is counted on the most, with heat pumps being sized within a hair's breadth and sheathing needing to be kept above dew point temperatures.

Sep 6, 2013 4:26 AM ET

An explanation of the graph displayed here
by Chris Schumacher

It’s unfortunate that the graph shown in this article is the one that we have to talk about. It looks like the graph was lifted from the Thermal Metric presentation that I made at the 2012 Summer Camp. That graph is misleading without the accompanying verbal explanation (which I think we skimmed across at summer camp).

To my knowledge NRCA has only ever tested using the “standard” conditions described in ASTM documents: Mean temperatures of 25, 40, 75, and 110 F, all with temperature differences of 50 F between the hot and cold plates. The attached table (top image, below) summarizes the temperatures used in the tests.

Also attached (second image, below) is the original version of that graph. Notice the bottom axis and label are different! The graph summarizes the data (from testing at those 4 mean temperatures) that NRCA presented at the 2010 roofing conference.

Now, mean temperatures really confuse people. "What does mean mean? Is it an average over time? No? Oh it’s an average temperature across the wall.”

“How do we use these numbers to design our buildings? We don’t typically have buildings that operate at a mean temperature of 25 F with a cold side at 0 F and a hot side at 50 F.”

Back at the 2012 summer camp, someone (might have been Joe Lstiburek) asked, “What would the equivalent outdoor temperatures be?” So, we extrapolated the test temperatures to get “Equivalent Outdoor Temps.” For example, if you assume the indoor temperature is 72 F, and you have a mean temperature of 25 F. To get that mean temperature in a wall, the outdoor temperature would have to be 72-2x(72-25)=-22F. (See the table reproduced as the bottom image, below.)

You can use this approach with materials that have “normal,” near straight line R-value vs. Temperature relationships. This approach doesn’t work for materials like polyiso. I didn’t like THAT graph when we made it; I’m hating it now. At summer camp, I hope I prefaced the slide with something like “the data are developed at standard mean temperatures but we’re making a leap to extrapolate them to outdoor temperatures here.”

Almost immediately after summer camp I moved beyond the ASTM standard test temperatures and developed my “In service” test temperatures -- In service temperature test with room temperature on one side and outdoor temperatures on the other. Those were the ones we used for Info Sheet 502. I believe that was published / posted somewhere late 2013.

The problem with the in-service temperature test data is that the results are only valid for the thickness tested and the precise temperatures tested. In 2014 I moved beyond those to try to get a REAL R-value/in vs Temperature (i.e. a k vs T curve) that you could use to calculate the R-value and performance of any assembly under any conditions. We’ve mostly worked that method out, and hope to publish it at an ASTM Symposium.

When you see any of my data that says “convergent,” it was developed with that method. Now, our graphs will typically have an x-axis label that says “mean temperature” when the data was derived using the standard (ASTM) test temperatures; or they’ll have an x-axis label that says simply “temperature” when they use our convergent method.

John and I have tried not to overwhelm the average practitioners with all of this technical crap but I think it is important for you to have some knowledge of it for context.


Schumacher - Table 1 copy.jpg Schumacher - Graph 1 copy.jpg Schumacher - Table 2 copy.jpg

Sep 6, 2013 4:40 AM ET

Response to Chris Schumacher
by Martin Holladay

Thanks very much for taking the time to provide such a thorough explanation; it is very helpful.

During my phone conversation with John Straube -- the interview presented here -- John referred to the NRCA data, and sent me an email that included the graph he was referring to. I reproduced the graph which he sent me.

I'm grateful to both of you for your work and for your explanations.

Sep 8, 2013 2:00 PM ET

Response to Dana Dorsett's question about HFO1234yf
by Martin Holladay

Today I got an email from John Straube, responding to the question, "Which brands of closed-cell spray polyurethane foam now use HFO1234yf as a blowing agent?"

Straube answered, "I am pretty sure none are yet marketed. They are trying this out in the field with preferred contractors to test out the real world issues, if any, with the product. There has been lots of lab testing so we know it works there. Why don't you contact Mary Bogdan from Honeywell and ask what the status is? She can give you definitive information."

I have emailed Mary Bogdan and will share any information she provides when she answers.

Sep 8, 2013 4:09 PM ET

Thanks Chris, Martin!
by Dana Dorsett

I s'pose they should have labeled that axis for what is was: "Extrapolated equivalent outdoor temp, at an indoor temp of 72F", which would give it the proper context. That begs other questions ("Extrapolated how, from what?") but the fuller explanation with the other graph.

Clearly the mean temp and delta-T on the polyiso layer changes when it's being used as insulating sheathing on a framed wall with cavity insulation, but knowing the mean temp performance even at a big delta-T is better than nothing.

With 2" of polyiso a 2x6/R20 house when it's 0F outside and 68-72F inside the delta-T across the polyiso is WELL under 50F, and it has a mean temp somewhere around +10F. Performance at mean temp of +10F the performance with the cold side is 0F and the warm side is ~20F (a 20F delta-T) could be substantially different from when the cold side is -15F, and the warm side 35F, as the the standardized 50F delta-T case. The mean models for this type of application are prone to large errors from building design point of view unless the tested delta-Ts are more realistic for the application.

Even if the polyiso sheathing were fully half of the total center-cavity R, the only realistic data points would be at outdoor temps in the (70F - 50F - 50F= ) -30F range so that you get pretty much the full 50F delta-T across the polyiso with the other 50F across the cavity fill of roughly equal R. Everywhere else you'd never see anything even close to a 50F delta-T. The only places in the US that might see average mid-winter outdoor temps of -30F would be at altitude, in the interior of Alaska, say at a cabin at 5000' up in the Brooks Range or something. The mean temp performance curves at 20F or 30F delta-Ts would be more relevant for typical insulating sheathing applications in US zones 4-7, and given the nonlinearity of the performance with temp it really does matter.

Sep 9, 2013 3:54 PM ET

More on HFO1234yf
by Martin Holladay

I just received an email from Jeannine Sohayda, a marketing rep for Honeywell.

She wrote, "I am responding to a question you sent to Mary Bogdan. We are not aware of any spray foam manufacturers / systems houses in North America using HFO-1234yf as a blowing agent in spray polyurethane foam."

Sep 9, 2013 4:10 PM ET

I guess that's the answer then.
by Dana Dorsett

I'd think moving to a greener blowing agent COULD be a marketing coup, assuming the market for closed cell foam actually cared.

It may be that just as with other blowing agent changes, it won't happen until it's required by regulation. Potent HFC greenhouse gases are more tightly regulated in Europe than in the US, but we may get there eventually.

On the polyiso nonlinearity front, you can't really model the nonlinear aspects of it using mean temp with a large delta-T, since that would presume that the changes are fairly linear, and they are clearly NOT. Breaking it down across mean temp with much smaller temperature differences would be necessary to see what it's doing with reasonable resolution, since it's more likely to be sorta-linear across a 20F difference than across 50F, looking at the mean-temp performance curves at using a 50F delta. The performance of polyiso clearly changes dramatically and non-linearly over a range of 50F within some temperature ranges that matter.

For instance, if the performance drops to say, R3.5/inch at a mean temp of 20F at a 50F delta-T, it could easily be that the foam that's colder than 15F could be performing at less than R1/inch and the part over 30F of it could be performing at R6-7/inch, and near 20F it could be running R5/inch. But there's no way to tell without breaking it down into smaller ranges to know where the sharp knees in the curves are. Using a 50F delta-T makes it all blurry using a single number for the very large range.

Mar 25, 2016 7:57 PM ET

Correction to chart
by Michael Bluejay

In the first chart, the line labeled "0.417" should be "0.174".

Mar 25, 2016 8:27 PM ET

Response to Michael Bluejay
by Martin Holladay

Thanks. It's been fixed. GBA appreciates your correction.

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