why truly net zero buildings can still be decades away

Why Truly Net Zero Buildings Can Still Be Decades Away

Carbon dioxide (CO₂) emissions come from two sources: using things (also called operational emissions) and making things (embodied emissions). To cite a well-known example of the first kind, your home probably burns natural gas for heating and uses electricity that may have been generated from fossil fuels. Governments plan to eliminate these emissions by making buildings better insulated and airtight, so they use less energy and ensure that renewable sources, such as wind and solar, generate enough power.

But what about the second kind—those embodied emissions from making thermal insulation, solar panels, bricks and concrete blocks in low-energy buildings?

A typical cubic meter of brick has an emission of about 357 kg CO₂. This is due to the extraction of the raw material from quarries, processing, shaping into raw stones and firing in a kiln at a very high temperature. One cubic meter of concrete can emit about 3,507 kg of CO₂, ten times more than brick.

These and other materials add many tons of embodied emissions to the construction of a home. An alternative material based on a mixture of hemp and lime, called hemp concrete, has according to research published in 2008, negative embodied emissions of -108 kg CO₂ per cubic meter. This is because the hemp crop absorbs more CO₂ during growth than is released when making hemp.

New buildings

A typical three-bedroom semi-detached house, newly constructed of brick, insulation and concrete blocks, will contain about seven tons of CO₂ in the exterior walls alone. An additional 19 tons of embodied CO₂ will be incorporated into the ground floor floor slab, which is typically made of concrete, the roof, glazing and other materials, bringing the total to 26 tons of embodied CO₂.

Thermal insulation reduces operational emissions, but has its own embedded emissions.
Bilanol / Shutterstock

If engineers fit the same house with solar panels on the roof, it will have negative operational emissions of -0.6 tons of CO₂ per year. That’s because when solar panels generate and supply electricity to a home, they not only avoid emissions that would otherwise have been generated by fossil fuels, they also reduce them by the same amount by supplying new clean energy to the grid. This is why the official way to account for these so-called net-displaced emissions is to use them with a minus sign.

These negative operational emissions will offset the incipient embodied emissions from year to year. At this rate, the total embedded and operational emissions of the semi-detached house built with conventional materials in 2021 will be reduced to zero by 2065.

If the same house is built of hemp concrete, with walls thick enough to achieve the same level of insulation as in the house, made of conventional materials, the built-in emissions in the exterior walls will be -7.3 tons. Adding the remaining 19 tons of embodied CO₂ stored elsewhere produces 11.7 tons of embodied CO for the entire house.

Since the starting point for embodied emissions is much lower in the hemp-walled house, the solar panels on the roof will eliminate the house’s total emissions by 2041 – 25 years earlier than the house made of conventional materials.

Existing buildings

However, new buildings are only a small part of the problem. About 80% of the buildings that will be there in 2050 already exists and have high operational emissions.

Existing buildings must be retrofitted with thermal insulation and renewable energy systems, such as solar panels and heat pumps or other renewable heating systems, to achieve net zero. Tight doors and windows and mechanical ventilation systems are used to keep these buildings airtight and to prevent cold wind from entering or warm air coming out.

A Modern House With Wall-To-Wall Windows.

The Zero Carbon House in Birmingham, UK.
Martine Hamilton Knight/Arcaid Images/Alamy Stock Photo

For more than ten years, our laboratory has been monitoring the energy consumption and CO2 emissions of the Birmingham zero carbon house. The house was built in 1840 and expanded and renovated in 2009 with solar panels and thermal solar collectors for heating water. In 2010, the house won an award for architectural excellence from the Royal Institute of British Architects.

Emissions in the original building materials have been present in the atmosphere since 1840. The house has been retrofitted with materials that required little energy to make, such as unfired clay blocks, bricks from demolished buildings, recycled newspaper insulation, lime plaster with recycled soil glass, tamped earth floors and reclaimed 200-year-old wood from the floor of a silk factory .

Even then, the embodied emission after the retrofit amounted to more than 40 tons of CO₂. The renewable energy systems, including solar panels that generate electricity and solar thermal collectors that heat water, reduce the house’s total emissions year over year, but our calculations showed that the house will not reach net zero until 2030.

A House With White Walls And A Red Tile Roof Covered With Blue Solar Panels.
Solar panels can compensate for the embodied emissions in buildings over several decades.
Smileus/Shutterstock

Failure to account for embodied emissions could mean exceeding carbon emissions targets by decades. Even a home retrofitted with numerous recovered materials and negative operational emissions will take 21 years to reach net zero. This puts into perspective the magnitude of the effort needed in the UK and beyond to reach net zero by 2050.

Fully 75% of British municipalities strive to make their activities carbon neutral by 2050, including the social housing stock. These targets, which in many cases are brought forward to 2030, cannot be achieved without taking into account the embodied emissions in construction materials. The net-zero target by 2050 may seem a long way off, but taking embodied carbon into account means recognizing that countries may have already exceeded it.


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