Choosing Insulation for Carbon Value – Why More is Not Always Better Part 2

In Part 1 of this blog post, we highlighted two of the most commonly used insulations in the U.S.– XPS board and closed-cell polyurethane spray foam – and noted that they are produced with blowing agents (HFC-based) that are putting more carbon into the air during construction than they save during building operation for many decades. We left you with a question: if we don’t use these insulations, how can we make up for the loss of the helpful qualities that has made us dependent on them?

Insulation Alternatives

One part of the answer comes from the development of new materials. In Europe over the last decade, Honeywell developed a new blowing agent, a hydro-fluoro olefin (HFO), which claims a global warming potential (GWP) of less than one, which is less than that of carbon dioxide.  First in Europe, and now in the U.S., manufacturers such as Demilec and Carlisle are coming to market with a closed-cell polyurethane spray foam that uses this blowing agent instead of the HFCs that carry a GWP of well over 1,000. These spray foams have a slightly better R-value  than their high-carbon predecessors, and otherwise have the same qualities that make them useful in multiple contexts – air/vapor barrier capability, conformance to irregularities and penetrations, etc.  However, they also have many of the same downsides – high flammability, potential (and not completely understood) off-gassing post-application, and the basic fact that they are petroleum products.

Soy-based plastic spray foams, both open- and-closed cell, have recently been developed as lower-carbon, less toxic alternatives to HFC polyurethane foam. These are water-blown (GWP=0), and manufacturers claim they do not off-gas because they do not incorporate urea/formaldehyde. However, soy foams are still made with urethane as a bonding agent and can be as much as 85% urethane, so in the end, they are still a petroleum product with the carbon footprint and flammability that entails. The closed-cell versions have the benefit of acting as air/vapor barriers, but they have a significantly lower R-value (~R5 as opposed to R6.7-6.9) than the HFO/HFC blown polyurethane foams –.  For this reason and overall scalability, manufacturers of these foams are competing in the residential/single-family market, not the urban high-rise market. There is also some controversy over the long-term dimensional stability of water-blown foam insulations.

Several expanded plastic insulations that have been on the market for decades are blown with low- or zero-GWP agents. For example, an expanded polystyrene (EPS) board (R3.9/inch) and polyisocyanurate foam board (R6/inch) is blown with pentane (GWP 7). Foil facing or vacuum wrapping has been used with these (as well as with mineral and glass fiber insulation) to provide a vapor barrier. So far, however, there is nothing on the horizon that replicates the combination of beneficial qualities of HFC-blown XPS insulation board, particularly for subgrade applications. Will it be possible to develop an HFO-blown version of the XPS board? Can mineral or organic fiberboard materials be wrapped or treated in such a way as to perform as effectively as XPS, particularly in a subgrade application?

Mineral fiber (wool), available in board, batt, and sprayable form, has become a common choice for sustainably-minded commercial construction in NYC and beyond. Aside from a serviceable R- 4, these products are fire-resistant to the point that they are routinely used as part of required firestopping assemblies.  Mineral wool boards can achieve the same compressive strength as XPS board and are unaffected by water (they don’t slump or lose R-value), so they can be used successfully in similar applications, most importantly in creating thick layers of continuous insulation on the exterior of a building structure (below cladding or stucco). However, unlike SPF and XPS, mineral fiber insulations are not air or vapor barriers.


Image of mineral wools

Mineral wool board insulation, blown in mineral wool, and mineral wool batt insulation


Image of fiberglass batt insulation

Fiberglass batt insulation

Fiberglass batt, the inorganic fiber insulation that not too long ago was omnipresent in both commercial and residential applications, has become less attractive as sustainability and more stringent code requirements increasingly drive insulation choice. Fiberglass batt’s lower R- 3.3, combined with its vulnerability to water or humidity, make mineral fiber batt the better choice thermally. Batts, in general, are becoming less and less useful because their intended use in interior stud framing cavities is now recognized to be a highly inefficient use of material. In metal framing in particular, batt insulation garners 40% of its nominal R-value at best and is liable to promote internal convection around the edges of each stud cavity. This is not a responsible or sustainable use of embodied carbon or operational carbon.

Mineral fiber is made from rock, and fiberglass is silica-based – both are therefore energy-intensive to manufacture and require removing material from the ground. Therefore, they can have high embodied carbon costs, which must also be weighed when using them.

Image of wood fiberboard insulation

Wood fiberboard insulation

Organic fiber-based insulations (wood, hemp, straw) have been more easily scalable to and used in single-family and smaller commercial or residential buildings, although there is excellent ongoing research and potential. The El Dorado behind organic fiber is the idea that with responsibly managed forestry or crop growth and low- or no-fossil fuel burning processing and transportation, many of these materials could be carbon negative in construction, meaning that they will ultimately result in removing carbon from the atmosphere (sequestration).  There is every reason to pursue this line of development as vigorously as possible, but a complex array of factors makes organics not yet easily applicable to much commercial urban construction. Insulation developed from recycled/reused materials, such as post-consumer cellulose, denim and plastic are also in this category.

Lots of options, but designers must still understand the critical implications of various insulation qualities in addition to carbon footprint and R-value. For example, without accounting for air barrier and water barrier effectiveness, there is a potential for disaster, primarily in the form of condensation and consequential mold growth, but also in other ways. On the Enclosure Team here at SWA, one of our chief tasks is to understand the complex interaction of all the factors surrounding insulation (or any material) choice and to advise architects and owners of the hidden dangers of certain choices and the best options for any condition.

Doing the Math

Carbon Footprint Analysis (a sub-set of LCA) takes into account all the carbon emissions (greenhouse gases put in the atmosphere), associated with a building or material from cradle to grave, i.e., to the point of recycling or disposal, and includes both embodied and operational carbon emissions. There is a growing number of firms that are now specializing in this type of analysis, which is increasingly available to building industry professionals, government officials, owners and manufacturers. Furthermore, as the concepts and facts upon which LCA and carbon footprinting are based are becoming more familiar, building professionals can now take advantage of the several computer applications that have been developed to assess and compare the carbon value of building materials and processes. Examples are Athena, LCA One-click, Tally, and most recently EC3. Our buildings are complex, and so is the mathematics behind them. Getting a big picture sense of what’s hot and what’s not, of relationships and proportions, will allow us to make much better decisions at design conception, inception and throughout design and construction, which will ultimately impact the course of climate change.

The Greenest Building is the One That’s Already Built

Whenever we build new, we have an opportunity to design any assembly and choose from a variety of materials. If we decide that certain materials are no longer acceptable options, we have other options. With design forethought, we can eliminate HFC-blown XPS and SPF from new construction.  Insulation is the tip of the (melting) iceberg; however, the structure and cladding of a new building accounts for roughly 80% of its embodied carbon.

In the larger picture, most new buildings – even operational net-zero buildings – rack up a huge carbon debt, directly adding to the carbon load in the atmosphere before the lights are turned on. And, this is compounded if an existing building is destroyed to make way for the new.  Consequently, there is growing realization and acceptance that the building that already exists is the greenest building, amply demonstrated in the groundbreaking study with that title published a decade ago by The Green Lab of the National Trust for Historic Preservation. From an LCA/embodied carbon point of view, the logical conclusion is that the conservation and reuse of existing buildings should be prioritized over new construction.

Because the embodied carbon debt of older buildings has already been paid down or off, older buildings represent an unparalleled opportunity for carbon savings, especially if they are retrofitted to lower their operational carbon, so there may still be a role for HFC-blown XPS insulation products. Such products may be the best solution for particular conditions because of the unique combination of thermal, moisture and air management benefits of these insulations. Then the savings from not demolishing a building and not building new may offset the proportionally small carbon expenditure of the insulation material itself. The new and improved tools for quantifying embodied carbon coming online will increase our understanding of embodied carbon and our decision-making processes.

We Can Do It!

It is easy to become overwhelmed by the enormity of climate change and the complexity of our operations that feed it, but there is an athlete’s maxim we should remember, resistance is greatest when closest to the goal.  We may feel that we have been making an effort for decades, yet still there is so much waste, and the carbon line continues to goes up. Perhaps we should take a moment to recognize how far we have come, what we have achieved, how much we already have put in place, and how close we really may be to our goal. It is a matter of persistence, of heart and will, and we can do it.

All hands on deck!


Image of Catherine Paplin



Written by Catherine Paplin, Senior Building Enclosure Consultant

5 replies
  1. Avatar
    Melissa Kops says:

    Thanks for the post. This is an issue I have been tackling for a long time.
    I use Type IX EPS below grade. It has a similar R-value to XPS and has been shown to perform just as well if not better. Unfortunately on most projects even though I have specified EPS in multiple locations – Architectural and Structural Specifications – XPS always shows up on the job site. Best scenario is I catch it early before much of the insulation has been installed.

    • Jayd Alvarez
      Jayd Alvarez says:

      On behalf of Catherine:
      Thanks for your interest and your helpful comment, Melissa. A colleague brought Type IX EPS to my attention recently, and I intend to look into the building science behind it to see if I can become convinced it’s a good substitute for XPS. I have to overcome bad memories of finding disintegrated soggy EPS in old roofs. Of course in the end, EPS is still plastic, and I keep thinking there ought to be a way to encapsulate mineral wool or other boards with fluid applied wp membrane or something in order to create a subgrade-worthy insulation less reliant on polymers. I’d also like to find out more about the science of aging blown insulations. It’s great that you and others are striving to turn the tide on this. Best, – Catherine

  2. Avatar
    Patrick McKenna says:

    Thanks for adding to the conversation on this.
    I have been weighing the pros and cons of various insulation materials to use in historic building rehabs, and have yet to find a suitable, cost effective and sustainable material for insulating stone basement walls.
    I’m curious if you have thoughts on the performance advantages of a close cell spray foam versus its GWP, off gassing and carbon foot print, (even using a soy based foam) compared to the more expensive option of framing an interior wall with a smart membrane, with mineral wool, which potentially is less effective, still has a high carbon footprint and possibly off gases formaldehyde.

    • Jayd Alvarez
      Jayd Alvarez says:

      On behalf of Catherine:

      Dear Patrick,
      To be honest, I don’t have an easy answer for your thoughtful and important question, and I am in the midst of ongoing research and thought on this exact topic. Insulating basements needs a deep rethink based on embodied carbon and net zero considerations. Up till now my go-to has typically been XPS, but aside from the astronomical embodied carbon – per my research to date – there is also the question of performance longevity – due to aged R value, effects of longterm saturation, off-gassing, etc. High density EPS has been suggested – and some people swear by it – but more research is needed. I am re-looking into closed cell foam now as well – specifically the HFO blown foam, which claims an R value of over 7 per inch (Demilec) and a GWP value of <1. In reality, I and my team here at Steven Winter Associates develop assemblies on a case by case basis after careful assessment of the particular conditions for each project - and so come up with almost as many answers as there are projects. I'll be interested to hear what experience teaches you, and will continue to share my experience as well - the best thing we can do. Best regards, Catherine


Trackbacks & Pingbacks

Leave a Reply

Want to join the discussion?
Feel free to contribute!

Leave a Reply

Your email address will not be published. Required fields are marked *