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Embodied carbon in a Passive House

One of the questions that I am often asked about passive buildings is whether the carbon emissions from manufacturing their extra insulation cancel out the climate benefits of all that energy efficiency. Let's take a look...



The answer will vary from one project to another, depending on the choice of materials and the climate. That being said, I live and work in a cold-climate Certified Passive House, so I’ll use my own house as a case study to explore this question further.


This is fundamentally a question about embodied versus operational carbon, so let’s start off with a few definitions:


Embodied carbon refers to the emissions related to the materials used in the construction of buildings. This includes the extraction of raw materials, manufacturing processes, transportation and the installation of materials on site.


Operational carbon refers to the emissions that come from the energy consumed when the building is in use. The energy efficiency of a building and the fuel source of its heating and hot water systems will have a big influence on operational carbon.



There has been lots of debate in recent years about which is more important, embodied or operational carbon. The short answer is both:

  • Embodied carbon emissions will occur in the very-near present, at a time when rapidly and drastically reducing emissions is critical to keeping global warming below 1.5°C.

  • Operational carbon will be emitted over the many years that the building will be in use: decisions that we make today about energy efficiency and fuel sources will result in a certain amount of carbon emissions being emitted, year after year.

 

Embodied Carbon Analysis


Now, to come back to our original question: does the embodied carbon in the extra insulation of a Passive House cancel out the operational carbon savings of a more efficient building envelope?


To answer this question, I used Builders for Climate Action’s BEAM Estimator to assess the embodied carbon of my house as it was designed, then made a version of the same model where I downgraded the assemblies so that they just barely met the minimum performance required in the Quebec Construction Code. Here are the assemblies from the real and “code minimum” versions, along with their effective U & R-values:



Both sets of assemblies use relatively low-carbon standard construction materials, and I used the same types of insulation in the Code Minimum version to make it a fair comparison. Here’s how they compare in terms of embodied carbon:


The Actual Design of the Passive House has a 14% reduction in embodied carbon compared to the Code Minimum version.


Surprised? The magic here lies in the use of cellulose insulation. Since cellulose is made from recycled paper, we are diverting the carbon from this plant-based material out of the waste stream, essentially storing carbon in the building enclosure.


There has been a lot of enthusiasm in the construction industry lately for using wood as a carbon-storing material, however BEAM’s methodology does not attribute carbon storage to virgin timber products (2x4s, plywood, etc.) because of "uncertainty about the amount of carbon released from soils during logging operations; the amount of carbon returning to the atmosphere from roots, slash and mill waste; the amount of carbon storage capacity lost when a growing tree is harvested; and the lag time for newly planted trees to begin absorbing significant amounts of atmospheric carbon dioxide.".

I wholeheartedly agree with their approach, and you can read more about it here, in the section titled "BEAM counts carbon stored in bio-based materials". BEAM's methodology estimates that virgin timber is a net source of carbon emissions.


Even though the actual design's double-stud walls use more lumber than a traditional single-stud wall, the negative emissions attributed to the carbon-storing cellulose more than offsets the emissions from the extra lumber.


Images: Cellulose installed behind mesh netting in the Meadow House.


Materials Matter.


These are great results, but I cannot overstate the importance of material choices when it comes to minimizing embodied carbon. When I created a version of the Passive House BEAM model that used fibreglass insulation instead of cellulose, the embodied carbon jumped from 18,225 kg CO2e to 29,789 kg CO2e, an increase of 63%!



 

Operational Carbon Analysis


To compare the energy consumption of the project as designed versus the code minimum version, I made a copy of the project’s Passive House Planning Package (PHPP) energy model with the following changes:

  • Building assemblies were downgraded as shown above;

  • High-performance triple-glazed windows with wood-aluminum frames were changed to more ordinary double-glazing in wood window frames;

  • Other building systems were unchanged (energy recovery ventilator, domestic hot water, appliances...);

  • Airtightness was unchanged (I was feeling charitable).

Here’s how the two versions of the building compare:



Now, let's convert that annual energy consumption into annual operational carbon, using the average GHG emissions intensity for electricity generation in Canada (the building is 100% electric, which makes these calculations easier!):


 

Total Carbon Emissions


Now that we have calculated the embodied carbon and annual operational carbon emissions, we can graph the total carbon emissions for our scenarios. The embodied emissions mostly occur in the lead-up to construction, so they are the starting point for our timeline, then we add the annual operational carbon every year:



This graph makes it clear that, in these scenarios, the operational emissions savings of the Passive House options more than make up for any increase in embodied carbon in the long term. However, it's important to remember that the next decade is critical for keeping global warming under 1.5°C, just as the overall timeline is important for long-term climate stability.


What happens to these projections if we consider that electrical grids worldwide are aiming for decarbonization over the coming decades?


The simplified calculations above assume that the greenhouse gas emissions (GHGs) per kWh of energy from the electrical grid remain constant over 50 years, but this is unlikely since utilities everywhere are looking to transition away from fossil fuels in favour of renewables. I live in a version of that decarbonized future: in Quebec, 99.77% of electricity is generated from renewable sources, primarily hydropower. I used the Canadian electrical grid's average GHG emissions for the calculations above to make them more widely applicable, but the GHG emissions from Hydro-Québec’s grid are a minuscule 0.0006 kg of CO2e per kWh.


What does an almost-decarbonized electrical grid do to our graph? The slope on those lines gets a lot flatter. Since the design of my Passive House had lower embodied carbon than its code-minimum version, it's still a clear winner. On the other hand, the Passive House variation that used insulation with higher-embodied carbon has higher total carbon emissions, even after 50 years.



Given that I live in a place with an almost-decarbonized electrical grid, you might be wondering why I spend so much of my time trying to minimize the energy consumption of the buildings that I design.


It's true that the operational carbon of an all-electric building in Quebec is quite low, even if the building isn't particularly efficient, thanks to our relatively "clean" grid. This is where context matters: freeing up some of the electrical grid’s capacity by making buildings more efficient makes it possible for other sectors, such as transportation or industry, to switch from fossil fuels to electricity. In places that are still working to decarbonize their electricity system, energy efficient buildings can help to make the challenge of scaling up renewables and grid capacity more manageable. Even Hydro-Québec has recently been considering building new hydro dams or reactivating an old nuclear plant to meet growing demand. That puts the cost of more efficient buildings into a whole other context.


 

Conclusion


Embodied carbon is an important design consideration. It is possible to design a building that achieves exemplary energy efficiency with lower embodied carbon than an ordinary building designed to the code minimum, but we need to be careful about our material choices. We can have our cake and eat it, too (yay!), but we need to choose its ingredients carefully.


Even in a future when the electrical generation is 100% renewable, efficiency will still be important to avoid overbuilding expensive infrastructure. Achieving energy efficiency through a Passive House approach also provides lots of other benefits, such as thermal resilience, excellent indoor air quality and year-round comfort.


Cake by the extraordinarily talented @theresagoodbakeontherise

All images & text copyright of Tandem Architecture Écologique.


 

Information sources & additional resources:

Provincial and Territorial Energy Profiles - for information about electrical grid emissions

Passive House Canada - for training on how to design & build to the international Passive House Standard and how to use the PHPP energy modelling tool. They're also currently developing a course on how to use PHribbon, the embodied carbon plug-in for PHPP.




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