The Paris Agreement and almost all climate scientists call for the world economy to get to zero net carbon by 2050 in order to prevent the worst of the projected climate disruptions. To reach this goal all buildings and their associated transportation will need to achieve zero net carbon. Until recently, most of the discussion and effort has been focused on reducing carbon emissions resulting from energy use to operate buildings. While there is still far to go to reach zero operational energy, it’s now time for builders, designers, and homebuyers to set their sights on the ultimate goal and start down the path to zero net carbon.

Start with Zero Energy

If you are a builder or designer and have not yet built a zero energy home, we suggest you start now. Zero energy homes are straightforward and use standard strategies and off-the-shelf technologies that are readily available. Any skilled builder or contractor can build them following the steps outlined here. Designing and building a zero energy home is an important first step to building zero net carbon homes. These not only have zero operational carbon but also eliminate or offset the carbon emissions embodied in a building’s materials, construction practices, maintenance, and decommissioning.

Four Steps to Zero Carbon

Building on basic zero energy design, there are four steps to getting to zero carbon. The first is to make the home all electric. The second is to select building materials and processes that have the least embodied carbon or may even sequester carbon. The third step is to assess the embodied carbon in your design. And the fourth is to add sufficient solar panels to not only become net zero energy, but to also offset the embodied carbon in the building. Basically you are creating a positive energy building that exports enough renewable energy to both meet all its energy needs and offset the embodied carbon in the building. This additional energy production creates an opportunity to charge electric vehicles in homes and offices removing carbon emissions from building-associated transportation.

Go all Electric

The most effective step to take to reduce carbon from buildings is to make them all electric with the electricity coming from renewable sources. The renewable sources can be rooftop solar, community solar, or electricity coming from a utility that uses all renewables. This strategy will eliminate natural gas use – the main source of direct carbon emissions in buildings. High efficiency electric appliances, heat pump water heaters, driers, and HVAC systems are many times more energy efficient than natural gas equipment. In conjunction with building insulation and air sealing, going all electric will dramatically cut overall energy use and carbon emissions while increasing comfort and durability. And though the initial costs are higher for electric than for gas, electric equipment can be cheaper than that required for natural gas over the equipments’ lifetime. Electrifying all homes and buildings between now and 2050 could result in cumulative emissions reductions of up to 87%. To learn more read our post, The Future is Electric.

Reduce Embodied Carbon in Buildings

Embodied carbon includes all the carbon emissions involved in the processing, transport, utilization, and end-of-life disposition of building materials. According to Architecture 2030, almost half of the carbon impact from new construction by 2050 will come from embodied carbon. And according to the New Buildings Institute, 11% of all carbon emissions worldwide come from the embodied carbon in buildings. Addressing how to reduce embodied carbon in buildings leads to several questions. What building materials have the most embodied carbon? Which have the least? Which if any actually can sequester (absorb and store), carbon from the atmosphere?

High Embodied Carbon Building Materials

In a study of 600 buildings, Thornton and Tomasetti found that the structural elements of a building contributed 55% of its embodied carbon, with concrete, including floor slabs and foundations, accounting for 64% of that amount. They found that steel structures had more embodied carbon than concrete buildings. Even so, far more concrete is used making it the single largest source of greenhouse gas emissions in building construction and accounting for more than 8% of global emissions. It’s not necessary or desirable to completely eliminate these materials from building designs. Instead, select low carbon versions along with low carbon and carbon sequestering alternatives and then install or purchase enough renewable energy capacity to offset the reduced amount of embodied carbon that remains.

Low Embodied Carbon and Carbon Sequestering Materials

Low embodied carbon materials and some carbon sequestering materials are already on the market and should be rapidly integrated into new construction and remodeling projects. Here are some examples to consider:

Replace standard cement with available low carbon alternatives. Concrete made with fly ash or slag has the potential to reduce total emissions from concrete by about 40% and is now available in many areas of the country. Hollow core slabs or wood floor structures are another option that can be used to replace standard concrete slabs. Carbon sequestering cement that absorbs CO2 is in development and may be available soon, as will carbon negative concrete that absorbs CO2 from power plants during manufacture.

When it comes to using steel, reuse structural components from existing structures or use recycled steel fashioned with electric arc furnaces, which reduce the carbon footprint by 50%. Use sustainably grown wood to construct wood engineered commercial buildings with cross laminated timbers for structural support. Sustainably grown wood reduces embodied carbon due to the carbon sequestered during forest growth. Multistory wood-based commercial buildings such as this Passive House Demonstration Project, in Boston are being built around the world.

Look for and select lower carbon options in insulation, such as denim, sheep’s wool, dense-pack cellulose, cork, hemp, and straw. Cellulose, wool, and straw actually sequester carbon. Straw-based, wool-based, bamboo-based, and cellulose-based structural insulated panels (SIPs) are now available in the U.S. If you choose to use gypsum board, use the thinnest profile feasible. Finally, modern carpeting is highly carbon intensive, so be sure to select carpeting made from recycled materials or bio-based materials, such as wool. Alternatively, consider finishing the slab for flooring or using natural wood flooring.

Architecture 2030 has developed a Carbon Smart Materials Palette for architects, engineers, and builders that identifies the attributes of specific high carbon materials and suggests some alternative products and strategies for reducing embodied carbon. Lastly, it is important to design so that you get the most function out of smaller spaces. Doing so will take fewer materials for construction while providing the same benefit.

Assess the Embodied Carbon in your Building Design

The important third step in the design process is to conduct a carbon Life Cycle Assessment (LCA). Assessing the embodied carbon in your building design has two important functions. First, it helps you identify the embodied carbon of specific materials you are considering for use so you can identify and select those that have the least carbon impact. Second, it identifies the full life cycle carbon burden of your building that will need to be offset by producing and exporting renewable energy or, if needed, by purchasing carbon offsets. There are several assessment tools available for this purpose.

The Embodied Carbon in Construction Calculator links whole building lifetime carbon assessments to actual products. It uses data from nearly 15,000 certified Environmental Product Declarations, or EPDs®, that disclose environmental data over the life cycle of a product. The calculator helps users easily compare the carbon impacts of the most common building materials used in North America, including materials such as concrete and steel, metal framing, insulation, glazing, aluminum, gypsum board, ceiling tiles, and carpet.

Tally is a plug-in application for architects and engineers working in Revit design software. The program allows design teams to compare the carbon footprints of various materials early in the modeling process. Tally users can conduct whole building life cycle analysis during design and use the data to compare the differing carbon impacts of different design and material options.

The Athena Impact Estimator for Buildings is a spreadsheet tool that helps firms carry out carbon life cycle analysis in the early phases of the design process. It is the only free U.S. software tool for evaluating whole buildings based on internationally recognized life cycle assessment methodologies. Users can easily evaluate and compare the environmental impacts of industrial, institutional, commercial, and residential designs for new buildings and major renovations.

One Click LCA is a life cycle assessment software that will evaluate the carbon in building designs. It allows architects to import an existing design and energy model from a variety of software programs, and select materials from the largest life cycle analysis database of manufacturers’ EPDs. Then the design can be analyzed to find the hotspots of carbon intensity, discover opportunities for lowering the carbon footprint of the design, and compare the carbon footprint of different design options.

Solar Collectors: The Final Step

Once you have used an LCA to determine the carbon footprint of the life cycle of your building expressed in kg or lbs, you are ready for the final step: installing sufficient solar panels to offset the embodied carbon in the building. We discuss this last step in more detail in a separate post.