I’ve been fortunate to work on several Passive House projects through my architectural practice, Tandem Architecture Écologique. When I embarked on the design and construction of my own home in Quebec, the prospect of having unlimited access to project data was an opportunity to go beyond Passive House design principles and energy modeling to dig into some fundamental questions that had been on my mind, namely:

1. How do we strike a balance between embodied carbon and operational carbon in a Passive House?

2. How closely does the energy model used during the design phase reflect the building’s real-world energy consumption?

Question #1: How do we strike a balance between embodied carbon and operational carbon in a Passive House?

For starters, here are a few definitions.

Embodied carbon, also known as “upfront emissions”, refers to the emissions associated with the materials used in the construction of buildings. Timing is everything; most upfront emissions are released just before or during construction, and we are increasingly aware of how critical it is to rapidly and drastically reduce emissions to keep global warming below 1.5°C.

Operational carbon refers to the emissions that come from the energy consumed when the building is in use. Decisions that we make today about energy efficiency and fuel sources will result in a certain amount of carbon emissions, year after year, for the lifespan of the building.

Embodied Carbon Analysis

I used Builders for Climate Action’s BEAM Estimator to assess the embodied carbon of my house as it was designed. To have a basis for comparison, I also made a version of the model where I downgraded the assemblies so that they just met the minimum performance required in the Quebec Construction Code. Both models use the same types of insulation to make it a fair comparison.

The assemblies from the real and code minimum versions, along with their effective U- and R-values, can be found in Figure 1.

Figure 2 shows 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. Because 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.

It is worth noting that 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."¹ I wholeheartedly agree with this approach, and I was glad to see that the extra lumber for the Passive House’s double-stud walls was more than offset by the carbon-storing cellulose.

Materials Matter

These are great results, and 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 fiberglass insulation instead of cellulose (see Figure 3), the embodied carbon increased by 63%!

Operational Carbon Analysis

To compare the energy consumption of our house’s actual design to a more conventional 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 to the same code minimum versions as the embodied carbon comparison.

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

• Other building systems were unchanged (ERV, domestic hot water, appliances...).

• Airtightness was unchanged (I was feeling charitable).

Based on the two models, our Passive House consumes 89% less energy for space heating and 72% less energy overall than the conventional construction version of itself (see Figures 4 and 5). As the house is heated with electricity, we can determine the annual operational carbon emissions by applying the greenhouse gas (GHG) emissions intensity for electricity generation. In Quebec, where the house is, 99.77% of electricity is generated from renewable sources, primarily hydropower. To demonstrate how this project would fare elsewhere in Canada, I have included the national average GHG emissions intensity as well (see Figure 6).

Total Carbon Emissions

Figures 7 and 8 show the house’s total carbon emissions over a 50-year period for the different design variations. The embodied carbon sets the starting point, as it mostly occurs in the lead-up to construction, and then the annual operational carbon adds its impact to the total carbon emissions in each and every year that follows.

In areas where the electrical grid has a higher GHG emissions intensity, the operational emissions savings of a Passive House can make up for any increase in embodied carbon. In the two Passive House scenarios studied here, the embodied carbon payback period ranged from zero to five years, depending on the choice of insulation.

In the graph for Hydro-Québec’s almost-decarbonized electrical grid, the slope gets a lot flatter due to lower operational carbon. Because the design of my Passive House had lower embodied carbon emissions than its code minimum version, my house is still a clear winner. On the other hand, the Passive House variation that used insulation with higher embodied carbon has higher total carbon emissions than the conventional house, even after 50 years.

Embodied Carbon in a Passive House: Conclusion

It is possible to design a Passive House with lower embodied carbon than a conventional building, but only if we carefully consider our material choices. As there is much more insulation in a Passive House project, the type of insulation that we use has a tremendous impact on a project’s overall embodied carbon. Luckily, there are already carbon-storing insulation options on the market, as well as new materials currently in development. Although design fees rarely cover an embodied carbon analysis, at the very least we should pay careful attention to our choice of insulation, since that is where the embodied carbon of a Passive House diverges the most from conventional construction.

Embodied carbon becomes an increasingly large proportion of total carbon emissions as electrical grids transition to 100% renewable energy generation. Nevertheless, energy efficiency remains essential, even in decarbonized grids, because it frees up capacity for other sectors, such as transportation or industry, to switch from fossil fuels to electricity. Energy efficiency also makes it possible for electricity generated from renewable sources to be exported so that it can displace more carbon-intensive electrical generation. As one example, there is a transmission corridor under construction that Hydro-Québec will use to export 10.4 terawatt-hours per year to New York State.

Space Heating Demand

Let’s begin with the energy used for space heating, as it accounts for the lion’s share of energy consumption in Canadian buildings, whether residential, commercial, or institutional (see Figure 9).

The Space Heating Demand (SHD), also referred to as Thermal Energy Demand Intensity (TEDI), is the annual energy demand for space heating per square meter of floor area. It represents the heating required to balance the energy gains and losses to maintain a stable interior temperature.

• The average SHD for residential buildings in Canada is 111 kWh/m² per year.

• The standard limit for SHD set by the Passive House Institute is 15 kWh/m² per year.

SHD Data Collection

Our house is fully electric, except for a small wood stove that is rarely used. Occasionally we'll light a fire during a festive dinner or a power outage, but this is infrequent enough that I felt it was reasonable to exclude the wood stove from our calculations to keep things simple. Measuring the energy consumption of the electric baseboards was easy, because they are connected to Sinope smart thermostats that collect consumption data.

SHD Results

When comparing the annual energy consumption for space heating, the real measured performance tracks quite closely to the PHPP model. There is a slight offset, which is unsurprising since weather varies from one year to the next, but we're within 3 kWh/m² of the modelled result. For context, I've included the space heating demand from the PHPP model for the conventional version of our house that was described in the embodied carbon study in Figure 10.

Things get even more interesting when we look at space heating demand on a monthly basis (see Figure 11). Our heating season is three months shorter than the conventional house model, which corresponds with anecdotal evidence from when we lived in our old leaky house. In the Passive House, we only need heating from November to March, which is an unusually short season for our climate.

The actual monthly consumption between 2022 and 2023 matches up quite closely with the modeled consumption, with the biggest discrepancy occurring in November 2023. It was an unusually mild winter, but it was also very grey and cloudy. When the sun is out, it provides all the heat we need, even when it is very cold out (-20°C or colder), but in the absence of sun, the baseboards run more frequently.

Total Energy Use Intensity (TEUI)

Total Energy Use Intensity (TEUI) is the sum of all energy used in the building, divided by the floor area to simplify comparisons between different buildings.

• The average TEUI for residential buildings in Canada is 181 kWh/m² per year.

• For our house, the PHPP energy model estimated TEUI to be 49 kWh/m² if we relied entirely on the electric baseboards for heating. (For certification, the project modeled space heating as 50% electric + 50% wood stove to be more conservative, but this does not match our living habits, so I modified it to 100% electric for this study to get a more apples-to-apples comparison.)

TEUI Data Collection

Normally, it is possible to add up your utility bills, divide by floor area and, presto, you've got a real-world TEUI value! In our case, things were more complicated because we live on a farm, so there are several non-residential energy uses that I had to subtract from our utility bills to try to isolate the house's energy consumption from the rest, namely:

• charging the electric car;

• the barn & workshop, which are on the same utility bill as the house;

• the water trough de-icer, a heating element that ensures that the horses and sheep have a source of drinking water all winter; and

• the block heater for our old tractor, a Soviet-era behemoth that is used to move 500-lb hay bales once a week, and the occasional odd job.

TEUI Results

Once I had subtracted the energy consumption attributed to farm and transportation uses, I was left with the residential energy consumption, which I broke down into three categories:

1. space heating, as measured by the smart thermostats;

2. domestic hot water, as measured by a Sinope "smart switch" connected to our hot water tank; and

3. all other residential uses.

The overall annual energy consumption tracks quite closely to the modeled value, as shown in Figure 12. The "other" category is where we observe the greatest discrepancy between the real and modelled consumption. We're still trying to figure out why this might be the case, and have the following (unproven) hypotheses for where that energy might be going:

• the ERV preheater;

• the pump for the well that provides us with drinking water; and

• the pump for the septic leach field.

The higher-than-anticipated other loads are partially compensated by a reduction in energy use for domestic hot water, so the overall energy consumption remains close to the modeled estimate (see Figure 12).

The Model vs the Real World: Conclusion

Although there are slight differences between the modeled and measured consumption, with variations from year to year depending on weather patterns, the house is performing much as expected overall. The thermal resilience has been the most surprising aspect of the design: even though we knew what to expect in theory, it's still extraordinary to be in a cozy warm house where the heat doesn't need to turn on when it's bitterly cold outside, as long as the sun is shining.

References

[1] https://www.buildersforclimateaction.org/beam-estimator.html