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Musings of an Energy Nerd

Calculating the Global Warming Impact of Insulation

If you insulate with certain types of foam, thick insulation puts the planet at greater risk than thin insulation

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A graphic display of global warming potential. Graphs like the one shown above are automatically generated by David White's new calculator for comparing the global warming potential of different types of insulation. The blue line on top is XPS, while the red line on the bottom is cellulose.
Image Credit: David White
A graphic display of global warming potential. Graphs like the one shown above are automatically generated by David White's new calculator for comparing the global warming potential of different types of insulation. The blue line on top is XPS, while the red line on the bottom is cellulose.
Image Credit: David White
Covering the outside of a house with foam is a great way to lower energy consumption. But if you care about global warming, it's important to select the right type of rigid foam.

In June 2010, Alex Wilson published a ground-breaking article, “Avoiding the Global Warming Impact of Insulation,” in Environmental Building News. In the article, Wilson examined the implications of the fact that the HFC blowing agents used to make extruded polystyrene (XPS) and most types of closed-cell spray polyurethane foam have a much greater global warming impact than CO2. As it turns out, the negative global warming impacts of escaped blowing agents from some types of foam insulation can sometimes outweigh the positive climate impacts attributable to energy savings.

Needless to say, the article generated a great deal of discussion. (A shorter version of Wilson’s article appeared as a blog on GBA.) As a direct result of this article, XPS became the pariah of the rigid-foam world — at least for green builders. (The problem of HFC blowing agents should not be confused with questions surrounding the flame retardant HBCD, another problematic component of some types of foam insulation. Because both XPS and EPS contain HBCD, the “greenest” type of rigid foam is polyisocyanurate.)

How insulation materials influence climate change

Insulation materials can affect our climate in at least three ways: one positive way and two negative ways.

  • When properly installed, all types of insulation have the potential to slow climate change. When a builder installs more insulation than the legal minimum, the building where the insulation is installed burns less fuel and uses less electricity than it would otherwise — and burning less fuel is good for the planet.
  • All insulation materials have embodied energy, because fuel must be burned to power the factory where insulation is manufactured and to deliver the insulation to its final location; burning fuel contributes to global warming.
  • Extruded polystyrene foam and most types of closed-cell spray polyurethane foam contain hydrofluorocarbon (HFC) blowing agents that harm the atmosphere when they are gradually released. (The blowing agent used for open-cell spray foams is water, which does not cause any climate-change concerns.) HFCs are very potent greenhouse gases — 1,430 times more potent than carbon dioxide.

When you calculate the embodied energy of an insulation material combined with the effects on the atmosphere of escaped blowing agents, you can determine the “embodied global warming potential” of an insulation.

More insulation isn’t always better

There are many reasons to install thick insulation on your house: to improve comfort for the occupant, to reduce energy costs, and to lower your contribution to global warming.

According to Wilson’s article, it’s possible to install “too much” XPS on your walls — that is, too much from a perspective of global warming. Of course, even if you install “too much” XPS, the extra insulation will still help keep you comfortable, and it will still help lower your energy costs. But if your XPS is too thick, your wall will actually have a higher global warming potential (GWP) than a wall with less XPS.

As David White, a Passive House consultant from Brooklyn, explained at the March 2011 Building Energy conference hosted by NESEA in Boston, extra XPS can negatively affect the climate. “Let’s say I have a wall insulated with an inch of something,” said White. “We know the CO2 emissions of the building, based on the heat loss through that wall. As I add insulation, the heat loss drops but never goes to zero. The first 1-inch investment in additional insulation saves more energy than 2 inches of insulation installed later. In other words, the relationship between insulation thickness and energy savings is not linear. But there is a linear relationship for the embodied carbon dioxide and the effect of the blowing agents. If you look at the total global warming impact, including embodied energy and the effect of the blowing agents, then for some insulation materials you eventually get not just diminishing returns, but negative returns.”

Although it’s possible to estimate the global warming effects of blowing agents used to make foam insulation, the effects can’t be pinned down precisely — because we don’t know that rate at which blowing agents escape from foam and evaporate. It’s well established that gas bubbles in cured foam will escape from the edges of the foam, but the exact release rate is unknown. In his article, Alex Wilson used the best available data to estimate blowing agent release rates.

A new calculator

To help designers understand the global warming impact of HFC blowing agents, David White has developed an Excel spreadsheet that automatically creates line graphs to compare the global warming impact of different insulation types and thicknesses — for any climate. “This tool emerged out of work done by Danny Harvey, who wrote a paper on the global warming potential of insulation, and work by Alex Wilson and John Straube,” White explained. “The tool was meant to help you more specifically understand the way these concerns apply to your particular building.”

White’s spreadsheet is being distributed at no cost; here’s the link if you want to download it.

Adjusting your assumptions

White’s calculator allows users to adjust several variables, including:

  • Climate (heating degree days);
  • Heating fuel;
  • Heating appliance efficiency;
  • Expected life span of insulation (most users assume 50 years);
  • The blowing agent release rate (White suggests using the default rate, “high”); and
  • The types of insulation you want displayed on the graph.

White is forthright about the calculator’s limitations. “We are not talking about any environmental impacts other than global climate change,” said White. “Nor are we talking about costs — either the cost of insulation or the cost of energy.”

White’s calculator reveals:

  • For most types of insulation, including cellulose, more insulation is always better, because more insulation reduces the GWP of a building by reducing the building’s energy use. Of course, there are diminishing returns as the insulation gets thicker — but thicker insulation always reduces a building’s GWP, as long as the insulation doesn’t include HFC blowing agents.
  • The exceptions to the above rule are XPS and closed-cell spray polyurethane foam. When you plot the GWP of these two types of insulation on the Y axis of a graph, with the thickness of the insulation on the X axis, you get a U-shaped curve: at first, the GWP of the insulation decreases with increasing insulation thickness, but then the curve bottoms out and starts climbing. Beyond the bottom of the curve, thicker insulation actually causes more planetary climate damage than thinner insulation.

White explained, “In many climates, if you add XPS to an existing cellulose-insulated 2×6 wall, even the first inch of XPS increases the global warming impact of the wall.”

Better blowing agents are now being developed

Most industry experts expect HFCs to be phased out within a few years. “This is really a temporary problem until the industry shifts to different blowing agents,” said Wilson. “In Europe, XPS is already made without HFCs — they use CO2 as a blowing agent — but the R-value of European XPS is only R-4 per inch instead of R-5 per inch.”

In the meantime, concerned green builders should try to avoid the use of XPS and closed-cell spray polyurethane foam with HFC blowing agents. “Polyiso has a very low global warming potential, and is a good alternative in an above-grade application,” said Wilson. “For below-grade applications — for example, under a slab — EPS is an option. Rigid mineral wool, which is very common in Europe, has zero global warming potential. Its embodied energy is similar to fiberglass, and it can be used as a drain board on the outside of foundation walls. Roxul TopRock, a mineral wool product, is now being tested under slabs.”

Last week’s blog: “Nailing Window Flanges Through Foam.”


  1. Doug McEvers | | #1

    Greener Foam

    I thought there was talk a while back of a greener blowing agent for XPS made in the US (Dow).

  2. User avater GBA Editor
    Martin Holladay | | #2

    Response to Doug McEvers
    I think you are confusing the problem of ozone depletion with the problem of a chemical's global warming potential.

    Foam manufacturers have made great strides in the last 30 years at reducing the ozone depleting potential of their blowing agents. You may have read Dow's press release on this issue:

    However, just because a blowing agent doesn't deplete the ozone layer, doesn't mean that the blowing agent doesn't have a high global warming potential. Here's what Alex Wilson had to say on the issue in his article:
    "Two common foam insulation materials are produced with hydrofluorocarbon (HFC) blowing agents that are potent greenhouse gases— extruded polystyrene (XPS) such as Dow Styrofoam of Owens Corning Foamular, and standard closed-cell spray polyurethane foam (SPF). ... Manufacturers are working to develop and deploy fourth-generation blowing agents that have zero or very low GWP while still providing the various performance and safety properties that are required. Such developments would alter the conclusions of this article. Both Dupont and Honeywell are working on hydrofluoroalefin (HFO) blowing agents. At the 2010 NESEA Building Energy Conference, Gary Loh of DuPont described a zero-ODP, low-GWP (less than 10), nonflammable, low-conductivity blowing agent, for SPF, FEA-1100, that is currently undergoing toxicity testing and should be introduced (if toxicity testing continues to demonstrate safety) sometime between 2013 and 2015. Honeywell already has a similar product on the market in Europe for one-component polyurethane foams (HFO-1234ze) and expects to broaden its family of HFO blowing agents. When these new products replace the HFC-blown insulations in the coming years, the argument for avoiding SPF and XPS on the basis of lifetime GWP should largely disappear."

    In Europe, manufacturers of XPS have already switched to blowing agents with a low global warming potential. But these blowing agents result in XPS with an R-value of R-4 per inch. If your Dow Styrofoam has a label that says R-5 for 1 inch or R-10 for 2 inches, it's a safe bet that you've got the type of "bad" Styrofoam with a damaging blowing agent and a high global warming potential.

  3. Rock Termini | | #3

    Recycled Rigid Foam Panels
    I have been able, through vigorous searching, to always find once used rigid foam insulation panels. Besides the significantly lower cost, my take on the above issue is that the greatest majority of HFC release occurred with the manufacture and is part of the 'inherent global warming value" of the original structure where it was used. What I am doing is the "highest and best use" of the C&D waste when the original structure was re-done. I am getting the benefit of the insulation value with only some attached transportation 'deficit', and saving land fill space for some other poorly conceived disposal option. That sounds too smarmy - I think re-use of the original item with no additional manufacturing requirement should be a goal. The lower upfront cost is an added benefit. Am I missing something in my appraisal?

  4. User avater GBA Editor
    Martin Holladay | | #4

    Reply to Rock Termini
    Your question is similar to the one posed by someone who opposes the use of animal fur to make clothing, but who nevertheless buys a mink coat at the local Goodwill store because after all, "the animal is already dead."

    Your question is one for philosophers and moralists rather than engineers; there is no simple answer. Personally, I admire your decision to re-use building materials that might otherwise end up in a landfill.

  5. Jim Merrithew | | #5

    Reuse and diversion
    I agree with your philosophy. There are many benefits to reusing and diverting. You save money by not purchasing a new product. The rigid foam was created and used by another builder. The original manufacturing process has already contributed to the global warming effect. By diverting and reusing the rigid foam, you reduce the need to manufacture and transport another new product. There is less waste transported to the landfill and buried. The insulation reduces your consumption of fossil fuel for heating and cooling. Seems like a winning proposition. Jim

  6. Hugh Driscoll | | #6

    composite assemblies
    If I understand the the excel tool properly, it compares global warming potential between given assemblies where each assembly is entirely constructed of a single material. In a typical application I might specify 2" XPS outside of a wood stud wall. Assuming R10 for XPS and R8 for EPS, 6000hdd, and "high" agent release, the graph shows 1.35kgsCO2/sfyr (extruded) vs 1.4kgsCO2/sfyr (expanded). Advantage XPS because of it's superior insulating properties. This advantage is increased if the release is assumed "low". Am I misunderstanding the tool?

    Also, I am interested in data on the off-gassing of the blowing agents. Not that I trust the manufacturers information, but NCFI Polyurethane claims that in it's closed cell spray foam there is no off gassing of HFC245a "at any appreciable level under ordinary conditions".

  7. Hugh Driscoll | | #7

    composite assemblies
    I see now that the excel tool allows input for insulation in addition to the foam in question. For a 2x6 or 2x8 frame wall with cellulose, the curve begins to rise immediately for XPS and continues to dip for EPS.

  8. Tyler Dotten | | #8

    Impacts of Open Cell Foam (w/ Water Based Injection).
    This is some great information and that chart does a great job shedding some light on the GWP vs R-Value relationship. If this chart were to include the air sealing properties of the various insulations, I imagine the GWP of water based injection spray foams would be very low.

    I understand that insulating with plastic has it's drawbacks, but it's inherant air sealing properties provide some serious benefits as well. Clearly, avoiding using XPS and HFC injection SPF makes sense from both a global warming and ozone depletion stand-point. But given that open cell foam uses 1/4 as much plastic as closed cell and is widely available in water based injection methods, what are some of your thoughts on the overall impacts of using open cell foam for insulating? Are there any studys that you can bring to my attention?


  9. User avater GBA Editor
    Martin Holladay | | #9

    Response to Tyler Dotten
    Q. "What are some of your thoughts on the overall impacts of using open cell foam for insulating? Are there any studies that you can bring to my attention?"

    A. The Excel spreadsheet described in this blog addresses your question, at least in terms of global warming potential. The use of open-cell spray foam does not contribute to global warming, even when installed to a high R-value. The same cannot be said of closed-cell spray foam.

  10. Tyler Dotten | | #10

    Re: Response to Tyler Dotten

    Thanks for responding to my post. That spreadsheet simply said Spray P/U Water/CO2. I wasn't sure if this was refering to open cell foam or closed cell. The R-Value per inch that I usually see associated with Open Cell is 3.8. The excell spreadsheet assigns an R-value per inch of 5 with the
    said PU. For that reason I assumed it wasn't open cell (or low density) foam. From what I understand the amount of plastic used for an open cell foam is about 1/4 the amount used for a closed cell product, so they would obviously score quite differently on this GWP calculation.

    Additionally, this spreadhseet and article are exceptionally helpful when it comes to comparing Rvalue to GWP, but it deliberately takes air sealing out of the equation. One of the main benefits of spray foam lies in it's air selaing qualities.

    There are seemingly endless ways to look at the +/- of different insulation products (environmental impacts of production, fire saftey, off-gassing, r-value air sealing etc.), all insulation products will score high in some of these areas, but fall down in others. As much as I understand there is no way to quantify all of these impacts, I am trying to figure out if open cell spray foam is a "best-practice" type of product. As a homeperformance company, we do a lot of insulating and our current policy is to avoid foam insulations. For obvious reasons I embrace this policy as it applies to most closed cell spray foams and XPS rigid. My question is, do all of the negative impacts of open cell foam w/ water blown projection, outweigh the great air-sealing/r value benfits the product provides? If there are any further studies or educated people who can weigh in on this, it would sure help us (and a lot of other people) out.

    Thanks for your time,

  11. User avater GBA Editor
    Martin Holladay | | #11

    Another response to Tyler
    I agree that David White's default value for open-cell spray foam (R-5 per inch) is too high. I have sent David an e-mail asking him to clarify that value on his spreadsheet. Thanks for pointing it out.

    While David's spreadsheet is useful, he never claimed that all of the environmental virtues and drawbacks of an insulation product could be caught in a single formula. All the spreadsheet does is help clarify the GWP of released blowing agents, comparing that effect to CO2 reductions due to the insulating value of the products under consideration.

    The analysis does not address air leakage.

    Here's the thing: it's possible to build a very tight house without any spray foam, so it's hard to know whether to credit spray foam with improvements in airtightness. Even if one wanted to, the thermal reductions attributable to that particular quality of the spray foam would be very hard to quantify.

    So, here's the bottom line: there is no mathematical formula to weigh these effects. It all boils down to judgment.

  12. User avater GBA Editor
    Martin Holladay | | #12

    More info from David White
    David White just e-mailed me. His spreadsheet does NOT include open-cell spray foam in his listed insulation options. The reason for the omission, according to David, is that he "had no data on embodied GWP" for open-cell foam.

    The option titled "Spray P/U Water/CO2" is actually a brand of closed-cell spray foam that happens to be rated at R-5 per inch.

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