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Building Science

More About Global Warming and Insulation

It’s not as straightforward as many seem to believe

The global warming impact of insulation seems pretty clear from this graph. But what are the complexities behind these lines?
Image Credit: Alex Wilson

Well, I really stirred things up with my last article on insulation and global warming. My intention was to explain why Alex Wilson’s results could be doing a disservice to the green building community. In the end, I was rightly accused of have done a disservice myself.

So, here goes with Part Three of my take on the global warming impact of insulation. Let’s see if I can get closer to the truth this time.

A public apology to Alex Wilson

First, I need to apologize to Alex Wilson. I apologized privately at the time of my last article. Now I do so publicly. I wrote things that went too far, saying his results were bogus and he was engaging in pseudoscience. I regret those comments and have removed them from that article.

As I’ve said before, Alex Wilson has done great work in his career. He’s taken green building further than just about any other person in the field. His Environmental Building News has set the standard for green building news and analysis for decades. He also has a background in science and takes science seriously.

Wilson’s insulation and global warming study

Since that last article, I’ve done more reading of Wilson’s 2010 article, Avoiding the Global Warming Impact of Insulation, and related works, as well as discussing it with others. Briefly, what he did was to calculate the amount of time a highly-insulated wall assembly would have to be in service to “pay back” in reduced carbon emissions the amount of global warming created by the insulation itself. That insulation impact, he stated, comes from two things: the embodied global warming potential (GWP) and the GWP of the blowing agents used in foam insulation. His conclusion was that extruded polystyrene (XPS) and closed-cell spray foam had long payback periods and should be avoided.

To do the calculations, he had to assume certain things and set the parameters for his model. As I’ve written before, some of his assumptions were:

  • The manufacturers used high GWP blowing agents.
  • The offgassing profile is uniform.
  • The lifetime of the product is somewhere between 50 and 500 years, though the article doesn’t say what numbers they used.

Those were the main things I focused on in my two previous articles. After doing more reading, discussing, and thinking, I now see I was looking at the wrong things. Here are the assumptions of his model that are more relevant:

  • He looked only at walls.
  • He calculated GWP payback from various insulation materials after the 2×6 cavities were insulated with cellulose.
  • He didn’t give any credit to the air-sealing properties of any insulation.
  • Energy savings and carbon reductions were based on heating with a 90% efficient gas furnace.

In the next two sections, I’ll discuss these other assumptions.

Looking at the whole system rather than individual materials

Wilson’s study looked at the contribution to global warming from additional insulation on a 2×6 wall after the stud cavities were filled with cellulose or fiberglass. So the starting point was a wall with a whole-wall R-value of 14, which results from putting R-20 in the cavities and factoring in the effects of the framing.

As I discussed in my article on the diminishing returns of adding more insulation, most of the energy savings are due to the first few inches of insulation. As you add more and more insulation, the amount of energy you save keeps diminishing. If you’re calculating the global warming impact of insulation based on the amount of energy saved after you’ve already got a wall with R-20 in the cavities, the results are guaranteed to be worse than if you take the initial energy savings from the insulation in question.

Since Wilson was looking at highly-insulated walls with R-values up to 60, the results don’t tell you what the payback would be if you used closed-cell spray foam insulation or extruded polystyrene (XPS) by themselves in a wall that just meets code. In that case, it wouldn’t look nearly as bad for those two insulation materials because they’d get credited with all of the energy savings.

Also, Wilson’s study excluded any air-sealing benefits of the insulation materials they looked at. But if you’re using closed-cell spray foam, airtightess is one of the biggest benefits you get. If you’re going to compare the global warming impact of fiberglass, mineral wool, or cellulose to closed-cell spray foam, you need to account for the global warming impact of the air sealing materials used for the fibrous insulation materials. And you’d need to give credit for the savings due to airtightness.

The bottom line here is that I don’t think you can look only at the global warming impact of specific materials here. You have to look at the complete system. If we want to get serious about understanding the global warming impact of buildings, we need to model all the impacts together and see how those results compare.

Consider the source

Another really important component lacking from this study is a consideration of the source of the energy being saved. In their calculations, Wilson used a 90% efficient natural gas furnace. That’s an easy one to do. Natural gas doesn’t vary much in its carbon emisions from one area to another.

But what if the house you’re building is going to be all-electric? And what if your electricity comes from one of the utilities with the highest carbon emissions? My electricity here in the Atlanta area comes from the Southern Company, which is reported to have the highest carbon emissions in the U.S. That makes anything I do to save energy very good at reducing carbon emissions.

Or let’s say you’re going to build a net-zero house in the Pacific Northwest? You’ve got lots of photovoltaics on the roof, and when you draw from the grid, you’re getting some of the lowest-carbon electricity in the country. In that case, you’re going to have a hard time reducing carbon emissions at all. (Keep in mind, though, that insulating homes does a lot more than just helping with our global warming problem.)

The fact is that you’ve got to be able to account for the source of the energy being saved if you want to know something about the real global warming impact of your insulation.

The real takeaway from Wilson’s study

After spending so much time thinking about this recently, I’ve discovered that what bothered me the most about Wilson’s 2010 article. He simplified a complex calculation and drew a conclusion that seemed to define a static property for various insulation materials. He wrote:

These differences are dramatic enough that, even if our assumptions are off by a significant factor, we can draw some general conclusions about sensible choices. If we’re building highly insulated buildings and doing so in part to mitigate global warming, we should use insulation materials other than XPS or SPF…

I believe he underestimated that “significant factor” of uncertainty. That all-electric, code-minimum house using high-carbon electricity would have payback that is perhaps only a tenth of what Wilson found. Would you feel the need to avoid using XPS or closed-cell spray foam if the payback were only 10 years instead of 100?

The real takeaway from Wilson’s study, however, is something most of us seem to have missed. What he showed is that we can calculate the global warming impact of buildings. It just needs to include more inputs than he used. It needs to be based on assemblies at a minimum, although it really should be done for a whole enclosure. That way you can take into account the airtightness. You can also look at different combinations of insulation in the walls, floors, and ceilings. The calculations also need to include the actual carbon data about the energy being saved.

The good news is that this process has already begun. David White put together a spreadsheet in 2011 that is a bit more nuanced than Wilson’s calculations. Martin Holladay wrote about it and included a link so you can download it and use it. It still lacks inputs for carbon emissions from the source of electricity and for modeling different assemblies and enclosures. It’s a good start, though.

I’ve been ranting about Wilson’s article since it first came out, but now I see its true value. He took L.D. Danny Harvey’s academic paper on the subject, which is inaccessible to most, and started the process of creating a useful tool. Eventually, designers and builders will be able to run what-if scenarios to minimize the carbon emissions of a particular building. Kudos to Alex Wilson for that.

Allison Bailes of Decatur, Georgia, is a speaker, writer, energy consultant, RESNET-certified trainer, and the author of the Energy Vanguard Blog. Check out his in-depth course, Mastering Building Science at Heatspring Learning Institute, and follow him on Twitter at @EnergyVanguard.


  1. Alex Wilson | | #1

    Thanks Allison
    These are exactly the sorts of issues that I had hoped would be raised by my original article. Your pushback on some of my assumptions is justified and, as you correctly note, the conclusions one draws will vary depending on the assumptions used. It's a complicated issue. Nice job with this blog.

  2. GBA Editor
    Allison A. Bailes III, PhD | | #2

    Response to Alex Wilson
    You're welcome, Alex. Thank you for commenting. Sorry it took me so long to get to this point. I'm a bit slow at figuring things out sometimes.

  3. charlie_sullivan | | #3

    Between a rock and a soft place
    If our only option for good insulation was high GWP foam, we'd be between a rock and a hard place--the rock being using lots of fossil fuel to heat a building that had no* or little insulation, and the hard place being the use of substantial amounts of high GWP foam. We'd then need to fret about making the right choice, which would require doing this kind of analysis, and making sure we had the right assumptions built into it. David White's spreadsheet would be an essential tool.

    But that's not the world we live in. We not only have the rock and the hard place as options--we also have a soft place: piles of fluffy cellulose. Or mineral wool, EPS, neopor, polyiso, open-cell spray foam, and now even Lapolla's 4G closed cell spray foam blown with low GWP HFO blowing agent. So we can all relax and stop losing sleep over exactly how bad HFC blowing agents are. They are clearly much worse than the alternatives in terms of GWP, and you can use other materials that don't have that problem to achieve the same result.

    For example, if you prefer to avoid a separate step for air sealing in addition to insulation (e.g. if you don't have a crew or contractors you can trust to do air tight drywall or membranes, but you do have contractors you trust to do the spray foam and to mix the chemicals right), you can use open cell foam in most places and Lapolla 4G where you need low vapor permeability.

    When Alex wrote his original article, there were a few situations, such as a field-stone foundation wall, where it was truly hard to find a good-performance low GWP option. But that's no longer the case. There are low GWP options for any requirement.

    *Footnote: A comparison with no insulation as the alternative is a red herring as no GBA reader would build that way, but that seems to be what Allison suggests, so include that for completeness.

  4. GBA Editor
    Allison A. Bailes III, PhD | | #4

    Response to Charlie Sullivan
    Good points, Charlie. But the main takeaway should be that we can't talk about GWP of individual materials in isolation. They're part of the bigger system of the building they're incorporated into.

    Also, I certainly am not suggesting comparing any scenario to a scenario with no insulation. That was true in my article on the diminishing returns of insulation and it's true here.

    What I said was that, for example, if you were building a house and using Lstiburek's Perfect Wall, you'd have insulation only on the outside and no cavity insulation. All of the energy savings would accrue to the exterior insulation, whether it be XPS, mineral wool, or something else. The payback in that case would be much more favorable for the high-GWP insulation materials. If you can get a 10 year payback with XPS or closed-cell spray foam, even with the HFC blowing agents, then you can't eliminate them from consideration on the basis of GWP. You might have other reasons not to use them, but the GWP argument would fail.

  5. lutro | | #5

    Fainting with damp praise
    I appreciate your willingness to revisit this topic, and recognize that some previous comments had validity. However, by your count, this is your third article on the topic, and pretty much all of it is focused on disagreements with a six year old article. I think it's time to hear from Allison on Allison. Suggested first sentence, "I disagree with some things Alex wrote six years ago." Next 100 sentences expand on: "Here is how I would do the analysis today, what I see as the important factors, and what the data shows when we run different scenarios for common situations in this country." You give us hints at that in this article, but I would love to see you present your stand-alone coverage of this topic for 2016, start to finish from your viewpoint and priorities.
    Rather than getting just a taste of your approach, presented in reaction to Alex's previous article.

  6. GBA Editor
    Allison A. Bailes III, PhD | | #6

    Response to Derek Roff
    The article today concludes with where I think this needs to go. And indeed I'm looking into helping to take it that next step along the path. I've just submitted an abstract to speak on this topic at the North American Passive House Conference in Philadelphia. Once I have something to show, I'll be sure to publish it here, too.

  7. user-4524083 | | #7

    Allison - Thank you for revisiting this topic. Some of your arguments are good for pointing out the limitations of Harvey's paper and Alex Wilson's covering of it. Some are not so good. For one, The" whole wall might be different from the added to wall" argument matters little. In either case one is adding insulation to achieve a certain R-value. The fact that these foams emit a lot of global warming potential gases, compared to other choices that could do the same thing with significantly less GWP is still a valid point. Secondly, you're confusing two separate but important issues when you start talking about economic payback. An insulation may have a great payback but still be horrible for the planet. Any mention of the payback periods of different insulations has no significant place in this discussion, which is about GWP. Third, the type of energy running the building also has no effect on the GWP of these insulation choices. It doesn't matter whether you have photovoltaics, hydropower or burning old tires. The production of these foams uses products that in and of themselves cause their own GWP, irrespective of the energy source of the home. That would be a separate and interesting article. How much energy is saved at the house, or how many dollars are saved, again, another issue. A valid point you raise is the GWP of the whole building assembly. Double wall houses need more framing, and often have more layers of sheathing, to be used as air barriers. Other wall systems would need less embodied energy in the form of, say, sheets of plywood. (Another variable is the GWP of the disposal of various products at the end of there useful life.) As far as I am concerned, the bottom line is to not use these products when there are other choices available. Thanks again for keeping this discussion alive, It's an important one.

  8. gusfhb | | #8

    The reason money is important
    The reason money is important in the discussion is that foam can be used to build an inexpensive assembly. In a retrofit one cannot just add R10 of cellulose and put siding or roofing over it. The cost of labor and supporting structure can easily exceed the cost of the actual insulation. So if foam was not available, no insulation would be added, and so it is the GWP of insulation not added, if you will. This is the same when consider the cost based on heating with oil and coal fired electricity.

    The foam insulation[and other improvements] save at least 600 gallons a year of heating oil in my house, and insulating it otherwise would have been impossible both economically and practically, so the insulation would not have been installed.

    What is the GWP of that?

    The reason Allison's article, and continued discussion is important is that, as in any scientific endeavor, we must be our own harshest critics.

    "foam is bad" is simply not a useful statement

    "Encouraging the building of low GWP building assemblies" is a useful statement.

    1. sustainble9 | | #12


      Alex didn't say foam is bad. His paper focused on XPS and SPF, both of which (at the time) used blowing agents that had massive emissions of greenhouse gases. When we consider foam boards, there are alternatives: namely EPS and Polyiso. EPS in particular has other beneficial properties, such as being vapor open and having a stable R-value over time and outdoor temperatures, that make it preferrable when using as exterior insulation. His point is that there are more climate-friendly options, and if your motivation for building green is climate, then an informed choice would recommend avoiding XPS and SPF.

  9. user-4524083 | | #9

    Foam is bad ?
    Keith - There are certainly places for foam, both board and spray, especially when it comes to retrofits. But with new construction there are a lot of other choices, and the use of these products should probably be more judicious. Money is important in most discussions on insulation. Trust me, I'm extremely frugal. But in regards to this GWP arena, it clouds the argument of GWP - R value of various insulations. I certainly did not say "Foam is bad."

  10. AntonioB | | #10

    Hi Folks. Three years have lapsed since this article and I would be curious to know what is the status of research on this topic now. Have foam insulation manufacturers changed their formulas to reduce damage due to blowing agents? Have useful design tools been developed based on whole-building analyses?

    Current references would be very welcome to this reader.

    Looking around me in the world, and in many of the articles and recommendations here at GBA, I see us wrapping our buildings in more and more plastic. Inches and inches of foam surrounding everything. Somehow I can't help feeling there's got to be a better way.

  11. Jon_R | | #11

    You can get some GWP=1 closed cell spray foams. XPS should still be avoided in the US. I believe that more changes are mandated for December 2020.

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