Hugh Crowther
2021-01-22 08:55

The goal of building science is to create safe, comfortable places for people to live and work while minimizing the impact on the planet – in short, sustainable design.  Reducing energy consumption ( the holy grail is a Zero Energy Building) has long been a popular performance metric to reduce the impact on the planet.  Recently, North America has started to consider Carbon performance as a more holistic metric over just energy.  Something the Europeans have been considering for some time.

US Total Energy Consumption by End Use Sectors, 2018
Note: Sum of individual percentages may not equal 100 because of independent rounding

The built environment and climate change are closely linked. The chart above shows the 2018 total energy consumption in the United States of which 39% was used in the built environment (the highest sector).   Much of the energy used in buildings is for heating where burning fossil fuels (e.g. Natural Gas) provides the heat source.  Buildings use 72% of all electricity produced in the USA (EIA data) and around 60% worldwide (National Renewable Energy Lab (NREL)). 

Sources of US Electricity Generation

The chart above shows that more than half of electricity is produced by burning fossil fuels in the USA. US Emissions from power plants was 1.85 billion metric tons of CO2 in 2017 (EIA data). Other developed countries have similar ratios with coal providing 40% of the electricity worldwide (NREL).

Energy Sources for US Commercial Buildings

The above pie chart shows the energy sources for US commercial buildings (2012 EIA). With 62% of electricity produced by burning fossil fuels, around 77% of the energy consumed in buildings is produced with fossil fuels as the primary energy source. This makes the built environment the largest cause of CO2 emissions. There is no path forward on reducing CO2 emissions that does not include changes to how buildings are designed, constructed and operated.

Zero Carbon Buildings

Improving the building’s Energy Use Intensity or EUI (measured in Btu/ft2-yr) has been the drumbeat to reduce energy use and thus indirectly reduce carbon emissions.  An EUI is easily calculated using metered energy use data and building area.  The challenge with EUIs is twofold.  First, not all energy sources produce the same amount of CO2.  Hydroelectric power is zero carbon for example.  It is quite easy to design two buildings with the same EUI but quite different carbon footprints (e.g. gas boiler vs. electric heatpump). 

The second issue is that embodied carbon is not considered at all when looking at energy use and many consider that embodied carbon in building materials will soon be the bigger issue.

So, what is a zero carbon building?  Here are some current definitions from leading organizations.

Architecture 2030 defines Zero-Net -Carbon (ZNC) building as;

a highly energy efficient building that produces on-site, or procures, enough carbon-free renewable energy to meet building operations energy consumption annually.

The Canada Green Building Council  defines a Zero Carbon Building as;

one that is highly energy-efficient and produces onsite, or procures, carbon-free renewable energy in an amount sufficient to offset the annual carbon emissions associated with operations.

Other organizations have similar definitions and carbon reduction programs.  The basic goal is a low energy use building that sources the remaining required energy from zero carbon sources. This is not the same as a Zero Energy Building (ZEB). In a ZEB building it would be possible to burn fossil fuels to heat the building but offset the energy usage with onsite power generation (e.g. photovoltaic panels) to average out to zero energy but having released CO2.

The Pembina Institute created the following chart that shows five types of green buildings which is very good in explaining the difference between zero energy and zero carbon.

A practical challenge for evaluating Carbon use vs. (site) energy use is converting energy use to Carbon emissions.  It requires a conversion table, typically government supplied and is sensitive to the location (how local electricity is generated).

Building Project Energy SourceCO2e (lb/kwh)CO2e (kg/kWh)
Grid delivered electricity and  other fuels not specified in this table1.3870.630
LPG or propane0.6000.272
Fuel oil (residual)0.7510.341
Fuel Oil (distillate)0.7060.320
Natural Gas0.4830.219
District chilled water0.3320.151
District steam0.8120.368
District hot water0.7670.348
CO2e Emission Factors

The Table above shows emission factors from ASHRAE Standard 189.1 – Standard for the Design of High-Performance Green Buildings. It is based on US national averages.

Building Electrification

The path to a zero carbon buildings is often linked to using electricity as the energy source.   Using electricity to heat a building avoids onsite fossil fuel use.  This can be down with ground source heat pumps (GSHPs), Variable Refrigerant Flow (VRF) or heatpump – chiller plants. 

Shifting the building heating load to electricity assumes that generating the electricity used less carbon based fuels that directly heating the building with fossil fuels would have.  This will depend on how the electricity used for the building is generated.

Consider the following example:

The building is in a location where  heating in winter will be a requirement. Traditionally the heat would come from a natural gas heater such as a boiler or gas heat section in a packaged HVAC unit.  This type of equipment can have a 90%  efficiency (COP=0.9) based on source energy. An all-electric solution would use heat pumps that extract the heat from the environment (COP~ 2.5). 

Natural Gas Boiler COP = 0.9Electric Heatpump COP = 2.5
100 kW capacity 100/0.9 = 111 kW input Using 0.483 lb/kWh 111 x 0.483 = 53.6 lb CO2/h100 kW capacity 100/2.5 = 40 kW input Using 1.387 lb/kWh 40 x 1.387 = 55.5 lb CO2/h
Comparison of Gas Boiler to Electric Heatpump

Using the national average values from Standard 189.1, the natural gas boiler has a lower carbon usage.  Now relocate this project to  Montreal, Canada where all the electricity is hydroelectric and the heatpump not only wins but is zero carbon. 

While much of the electricity today is produced by fossil fuels it does not show how fast this is changing. The grid in North America is undergoing a rapid change. Renewables are expected to produce 30% of the electricity by 2023 (International Energy Agency. “Renewables 2018: Market Analysis and Forecast from 2018 to 2023.” A report by International Renewable Energy Agency (IRENA) showed onshore wind power generation at $0.03-0.04/kWh,  photovoltaic (PV) generation can be as low as $0.03/kWh compared to gas plants at $0.05 to 0.15/kWh. Costs for wind power and PV dropped 13% predicting that wind and PV power generation will be consistently cheaper than traditional fossil fuels by 2020. Buildings designed to operate on electricity will be comparable today to dual (Natural gas heat, electric cool) fuel designs and will de-carbonize as the grid de-carbonizes over the next decade.

Embodied Carbon

Share of Global Energy Related CO2 Emissions by Sector, 2015
Derived with EIA (2017), World Energy Statistics and Balances, IEA/OCED, Paris,

Embodied Carbon refers to the greenhouse gases that are emitted during building construction.  The building sector is the largest contributor to CO2 emissions with 28% from building operations and 11% from embodied Carbon. The “Construction Industry” is an estimate of the portion of the overall industry sector that applies to the manufacturing of materials for building construction, such as steel, cement and glass.

Embodied Carbon in Common Construction Materials

The table shows embodied carbon levels for common construction materials from ICE database at Concrete gets particular attention because of the cement used in concrete. By some estimates, Portland cement represents 5% of global CO2 emissions. 

Considering embodied carbon adds a whole new dimension to building design.  As buildings become more efficient and the electric grid de-carbonizes, the embodied carbon issues will become dominant. 

It is early days on how best to evaluate buildings from a embodied carbon point of view.  The leading building carbon standards are addressing embodied carbon as the tools become available.  Environmental Product Declarations can help provide embodied carbon data.   Architecture 2030 is creating the Carbon Smart Materials Palette. The Canada Green Building Council’s Zero Carbon rating program requires embodied Carbon information based  on Life Cycle Assessment (LCA). Athena Impact Estimator and Tally are two tools that can be used to estimate LCA for embodied Carbon.


CO2 from burning fossil fuels is the major cause of increased greenhouse gases in the atmosphere which leads to increased global temperatures and climate change.  Buildings are the number one driver of greenhouse gases both in operation and construction.  By focusing on zero carbon buildings rather than zero energy buildings (ZEBs) carbon emissions can be greatly reduced.   This means shifting building design towards  all electric buildings utilizing heatpump technology for heating.  While today the carbon savings may be limited over high efficiency fossil fuel equipment,  the pace of renewable electric power generation will mean electric  buildings will be low carbon buildings.  To truly achieve a zero carbon building, embodied carbon must also be considered.  

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