A Primer on Reducing Embodied Carbon

Dee Yonker

KPMB Architects are known for making good buildings: Critic Alex Bozikovic said the firm’s work is “a contemporary expression of architectural modernism, that are not easily summarized.” And whereas American architect Peter Eisenman once said “‘Green’ and sustainability have nothing to do with architecture,” KPMB takes them both very seriously. […]

KPMB Architects are known for making good buildings: Critic Alex Bozikovic said the firm’s work is “a contemporary expression of architectural modernism, that are not easily summarized.” And whereas American architect Peter Eisenman once said “‘Green’ and sustainability have nothing to do with architecture,” KPMB takes them both very seriously. The firm’s KPMB LAB, an interdisciplinary research group, recently looked at what the best insulation was for reducing embodied carbon in a study published in Canadian Architect magazine.

It is a deceptively simple study, designed to tell a much bigger story. Geoffrey Turnbull, director of innovation at KPMB, tells Treehugger it was an attempt to “have a conversation that is relatable” – an attempt to explain the fundamentals and importance of the concept of embodied carbon. While reviewing past KBMB work, he found it had been dealt with inconsistently—the data available are vague with “astonishing variation”—so he decided to go back to first principles.

In that spirit, and after a term teaching the concept of embodied carbon to my sustainable design students at Ryerson University, I am going to go back to the really basic concepts before we dive into the KPMB report. Some of this has been said on Treehugger before, but the KPMB work clarifies so much that I am hoping that this will be a useful consolidation.

Operating Energy vs Embodied Energy

Building Science

It is important to understand that this is a relatively new concept. Architects, engineers, and building code writers have been trained since the energy crisis of 1974 to address the issue of operating energy—the energy used to heat and cool and operate homes and buildings, the vast bulk of which came from fossil fuels. Embodied energy was the energy used to make the materials and build the building. Twenty-five years ago, as the graph notes, “embodied energy was swamped by operational energy in almost all building types.” So everyone has this in their DNA today, operating energy is what matters.

credit: John Ochsendorf/MIT

But as can be seen in this famous graph from 2009 by John Ochesendorf, as buildings got more efficient, the embodied energy assumes much greater significance. With a high-efficiency building, it takes decades before the cumulative operating energy is greater than the embodied energy. He was more worried about embodied energy from a full life-cycle point of view.

MIT Energy Initiative reports:

“Conventional wisdom says that the operating energy is far more important than the embodied energy because buildings have a long life—maybe a hundred years,” says Ochsendorf. “But we have office buildings in Boston that are torn down after only 20 years.” While others may view buildings as essentially permanent, he views them as “waste in transit.”

Embodied Energy vs Embodied Carbon

All of this started with an energy crisis, at a time when most of our energy came from fossil fuels. But over the last decade, it has turned into a carbon crisis where greenhouse gas emissions have become the defining issue of our time.

Fossil fuel energy is currently cheap, local. and plentiful—the original issues in the energy crisis—so that’s not a problem anymore. The issue now is what happens when you burn them?

Renewable, carbon-free alternatives are becoming more common. Many who think about the issue at all are still using embodied energy and embodied carbon interchangeably, but as will become obvious when we get to the KPMB research, they are fundamentally very different issues requiring different approaches.

Embodied Carbon vs Upfront Carbon

World Green Building Council

Embodied carbon is defined as the “carbon emissions associated with materials and construction processes through the whole lifecycle of a building or infrastructure.” It is a terrible and confusing name because carbon is not embodied in anything—it is in the atmosphere now.

What we are really talking about here is what I have called “upfront carbon emissions,” and which the World Green Building Council has adopted as upfront carbon—”the emissions caused in the materials production and construction phases of the lifecycle before the building or infrastructure begins to be used.” I defined it earlier more simply as “the carbon emitted in the making of building products.”

There are subtle but important distinctions; some industries will stress the embodied carbon’s full lifecycle definition because their materials last over the long run. But as economist John Maynard Keynes noted, “In the long run we are all dead.”

Under the terms of the 2015 Paris Accord, we have a carbon budget ceiling and are supposed to be reducing our carbon emissions by almost half by 2030. So what matters are the emissions happening now, what architect Elrond Burrell called the carbon “burp” and other less attractive terms.

What’s the Best Insulation for Reducing Embodied Carbon?


Turnbull and his team ask this question about the best insulation, but that isn’t actually what they are trying to do here, starting with the statement that “like many architects, we have begun to pay much closer attention to the embodied carbon associated with the materials we are specifying.” This study is more about explaining how it works than it is about comparing materials. Insulation is relatively straightforward and homogenous, the data on it are comparatively trustworthy, and its purpose is to reduce operating energy, so one can see the tradeoffs being made.

Turnbull and his team write:

“We performed a study to compare the embodied carbon values for nine commonly used types of insulation with the goal of presenting the results in a relatable way…Insulation is somewhat unique among building materials in that one of the primary reasons it is incorporated in buildings – to reduced energy flow through the building envelope – has a significant direct impact on the operational emissions produced by the building.”

KPMB doesn’t do house renovations but modeled a simple scenario: an uninsulated bearing masonry wall where a homeowner wants to increase the insulation level from R-4 to R-24 in a home heated with natural gas.


They calculated the embodied carbon for each type of insulation for the same insulation value, and plotted “how long it takes for the operational savings (reduced operational emissions) to exceed the investment (embodied carbon) in the insulation.” Although this is titled “Carbon Payback Analysis,” Turnbull acknowledges the term payback makes no sense—it is about money and we are talking about carbon, and probably shouldn’t mix up the terminology. This becomes an important point.

Note how the blue line representing Dupont XPS, or extruded polystyrene, takes almost 16 years before the cumulative savings in emissions from burning natural gas are actually greater than the upfront carbon emissions from making the XPS insulation. That’s because hydrofluorocarbon (HFC) blowing agent has a Global Warming Potential (GWP) of 1430 times that of carbon dioxide (CO2).

After years of pressure from Europe, where they have been taking the issue of embodied carbon far more seriously, new blowing agents have been introduced with far lower GWP. That’s why Dupont’s new XPS has a GWP of about half that of the standard stuff.

Owen-Corning’s XPS is even better, as can be seen on the table:


These are ranked according to the GWP of the greenhouse gases released producing a square meter of R-5.67 (RSI-1) insulation. Commenters on Linkedin have complained there are no spray foams or regular EPS insulation, but to reiterate, the point of the exercise is to “have a conversation that is relatable,” not to be a definitive guide.


When one zooms in on the detail, the blown-in cellulose is doing its job in about six weeks, while Owen-Corning’s new XPS digs out of its carbon emission hole in about 18 months and starts doing something positive. Any insulation that doesn’t make it into the zoom window here shouldn’t even be considered when we are worried about carbon emissions now.

KPMB concludes:

“Polyiso, Rockwool, and GPS are all board or semi-rigid batt products, and all have GWPs that are significantly lower than XPS. In situations where blown cellulose insulation is not a suitable choice, these products – Rockwool and GPS in particular – offer considerable flexibility in terms of suitable installations and quite good embodied carbon values.”

Natural Gas vs Heat Pump


KPMB ends the study with this graph where they change the heating system from natural gas to an electric heat pump powered by Ontario’s very low-carbon hydro and nuclear electricity. They don’t dive deep into it, simply concluding: “The study also underscores the significant differences in operational emissions resulting from the two heating systems contemplated.” In fact, I might call this “The Graph of the Year,” because it has profound implications.

Because the operating carbon emissions from the heat pump are negligible, the three XPS foams, including two of the new reduced GWP ones, never get to dig out of their hole. In fact, from an operating carbon point of view, when you have such low-carbon heating and cooling, what the insulation is made of becomes more important than how much there is.

As researcher Chris Magwood has pointed out in his version of this exercise, you actually emit less CO2 by going back to 1960 levels of insulation than you are using these foams. According to this KPMB chart, from a carbon emission point of view, you would be better off not insulating at all, you are 200kg below zero and are stuck there.

However, you wouldn’t be very comfortable, and electricity is far more expensive than gas; in Ontario at peak times, 5.67 times as much per unit of energy. Heat pumps stretch that a lot further, but blended with off-peak lower rates, it still costs well over twice as much. That’s why operating energy is a very different issue from operating carbon, why each needs its own solution, and why decarbonization of our energy is so important.

The real lessons from Chart 2:

  • Electrify everything to reduce operating carbon.
  • Insulate everything to reduce operating energy.
  • Build everything from materials with low upfront carbon.
  • Measure everything, like Geoffrey Turnbull is trying to do at KPMB.

This is all doable. As inventor Saul Griffith notes, it doesn’t need magical thinking or miracle technology. And as architect Stephanie Carlisle pointed out in another discussion of embodied carbon: “Climate change is not caused by energy; it’s caused by carbon emissions… There is no time for business as usual.”

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