To meet the goals of the Paris Climate Agreement, we must make decisions that will result in the greatest near-term carbon savings. This means taking into account both embodied carbon—those upfront emissions associated with the extraction, manufacture, transportation, and assembly of building materials—as well as the carbon that’s emitted over the course of the building’s operational phase.
We can build a high-performance building with very low operational emissions, but if its embodied emissions are so high that even if it’s a net-zero energy building (meaning it has net-zero operational energy consumption) it would take decades for the building to reach net-zero carbon (meaning it has net zero whole-building lifetime carbon emissions), we’re not actually helping to solve the critical issue of near-term carbon.
Imagine a growing community needs a facility in which to locate a new school. Will they construct a new building, or adapt an existing building for reuse?
To reduce the project’s operational carbon impact, the goal is a net-zero energy level of performance. A tried-and-true methodology for this is to reduce loads and maximize efficiency.
- Reduce internal loads.
- Reduce lighting load with networked, BAS-integrated LED fixtures with embedded controls.
- Reduce plug load with ENERGY STAR appliances, equipment with power management features, and smart plug strips.
- Reduce envelope heat transmission rate with high-performance, thermally broken windows, doors, cladding, and insulation systems. Use air sealing techniques to minimize air leakage.
- Use heat recovery ventilation, which can typically recover 60-95% of heat in exhaust air, to pre-heat/pre-cool incoming fresh air and reduce heating and cooling demands.
- Use the highest efficiency heating and air-conditioning systems available. Ground- or air-source heat pumps, which use electricity to either expel heat from the indoors during the cooling season or capture heat from the outdoors during the heating season, are three to five times more efficient than combustion-based furnaces and boilers. About 40% of a building’s total operational carbon emissions are from its HVAC system, so maximizing the efficiency of this equipment throughout the building’s lifetime—with smart operations, scheduled maintenance, and ongoing, monitoring-based commissioning—will have a significant impact.
- Install on-site renewables and/or procure electricity from off-site carbon-free sources.
What about the embodied carbon emitted in the construction of the building? With new construction, reducing this embodied carbon impact, which often makes up between 30-60% of a building’s whole life carbon, will be the greater challenge. Fortunately, the embodied carbon of a building project can be reduced 10-20% without adding to construction costs.
Typically, the structure accounts for 70-80% of a new building’s total embodied emissions. But once structural materials are in place, they generally stay in place and have the lifetime of the building to amortize this carbon debt.
If using concrete:
- Reduce use of Portland cement as much as possible. It’s the component most responsible for concrete’s carbon emissions.
- Instead of traditional cast-in-place concrete, work with the structural engineer to optimize the design to lighten the weight.
- Use hollow core or voided slabs made with high-recycled content concrete and high-recycled content rebar.
- Replace cement with non-fossil fuel-based supplementary cementing materials (SCMs).
If using steel:
- Make sure the structural design is right-sized and uses the lightest shapes and members possible.
- Use higher-grade, high-recycled content steel from mills that use electric arc-furnace (EAF) production, which produces less than half the emissions of basic oxygen furnace (BOF) production and can use power from renewable energy sources.
If using wood:
- Using mass timber for some or all of the building’s structure can significantly reduce its embodied emissions. Timber sourced from climate-smart, sustainably managed forests has significantly less embodied carbon than concrete or steel.
Here things get tricky. The enclosure system is critical to achieving a net-zero energy building as it has a huge impact on the building’s loads and the efficiency of its systems. But what are the potential embodied carbon implications of constructing a high-performance envelope? If decarbonization is the goal, when making design decisions it is crucial to evaluate the implications for both operational carbon and embodied carbon alongside each other. This means looking at the building as a whole and accounting for all the ways in which its different components interrelate. The only really effective way to make informed design decisions to reduce carbon is with whole-building lifecycle assessment (WBLCA).
For example, envelope insulation has a huge impact on the building’s operation and serves several critical purposes beyond thermal performance. But some types of insulation (EPS, XPS, Polyiso, spray foam) have high embodied carbon. Is it as simple as avoiding these products? No. To learn why, read the upcoming post, Five Principles for Low Embodied Carbon Buildings.
That said, there are some general guidelines that all projects can follow to reduce the embodied carbon impact of the enclosure:
- Reduce the weight
Heavy cladding materials, such as concrete and masonry, have high embodied carbon in and of themselves. Moreover, a heavier façade can also require additional structure, resulting in an even larger embodied carbon cost.
- Minimize glazing
Glazing units are heavy and they are high in embodied carbon and difficult to recycle. The frequency at which IGUs generally need to be replaced is a primary reason why the embodied carbon in the enclosure does not tend to amortize over the building’s lifetime. On top of all that, the glazing is usually the weakest point of the enclosure’s operational energy performance. So minimizing glazing can result in a significant reduction of lifecycle carbon emissions.
- Keep it simple
Using fewer cladding materials in a simplified assembly system will not only contribute toward reducing embodied emissions, but could also have implications for operational emissions. The fewer connections required between the cladding and the structure, the less thermal bridging there will be to decrease the building’s thermal efficiency.
- Design for durability
Frequent replacement of enclosure components adds significant embodied emissions, so design the envelope to be as durable as possible by using reliable and repairable materials. Use envelope commissioning to ensure these materials are correctly assembled and protected from damage.
Without the need to manufacture and construct entirely new structural and enclosure systems, building reuse can dramatically decrease a project’s embodied carbon emissions. In fact, building reuse will almost always beat out new construction for lower whole life carbon emissions. Remember that it can take up to 80 years for a new building that’s even 30% more efficient than an average-performing existing building to offset due to its higher operational performance and the embodied carbon emissions related to the construction process.
But what about the operational emissions of the existing building selected for reuse? What will be needed to retrofit an existing building to achieve net-zero energy?
Depending on the conditions of an existing building, the task of cost-effectively retrofitting for net-zero energy performance is often well within reach and represents the path to the greatest overall avoided emissions.
- Reduce loads
The strategies for reducing the internal and envelope loads will be the same as for new construction, but with the added constraint of the existing building itself. Options for retrofitting and upgrading the enclosure components that are critical for operational performance may be limited by the existing envelope design. For example, the windows may need to be replaced and insulation may need to be added. As discussed above, these can have significant embodied carbon costs. The design challenge is to find the lowest embodied carbon strategies for achieving net-zero energy performance.
- Improve efficiency
Again, the strategies for improving the building’s efficiency will be the same as for new construction, but with reuse there may be specific challenges posed by the conditions of the existing building. For example, it’s possible that there may be space constraints that make replacing the HVAC system challenging. And, replacing this equipment carries an embodied carbon cost that should be taken into account.
- Electrification + Renewables
In some cases, the easiest way to eliminate operational emissions of an existing building may be to convert the building to all-electric and source the electricity from 100% renewable sources. Options for on-site renewables may be more limited for existing buildings. For example, the existing roof may not be able to accommodate the added weight of a new PV system without additional structural support, which could carry significant embodied carbon cost.
Assuming the existing structure and enclosure will satisfy the building’s new use, a greater focus for reducing embodied emissions will shift to the interior materials.
A “hot spot” study of the proposed renovation design can be employed to identify the materials that will have the largest impact on embodied carbon. Common high-impact interior materials include:
- Specify lighter-weight and thinner products.
- Use EPDs to find lower embodied carbon options.
- Design to minimize waste and enable deconstruction at end-of-life.
- Specify durable, high-recycled content carpet tile products with solution-dyed nylon yarn.
- Eliminate carpet and other floor finishes where possible, using exposed structural materials as finish materials instead.
Whether the community decides to build new or reuse an existing building, a net-zero carbon outcome depends on a holistic, whole life carbon approach. Keep an eye out for a follow-up post titled Five Principles for Low Embodied Carbon Buildings on what this means in practice.
Written by James Wilson, Sustainability Consultant
Written by Catherine Paplin, Senior Building Enclosure Consultant