Building Materials and the Time Value of Carbon

When you save matters. What you build matters.
Here’s why we need to build well and rebuild better.

 

By Larry Strain

The Paris climate accord set a new target for global temperature rise: to keep global temperatures from rising above 2°C and thus avoid catastrophic, irreversible climate change. The countries of the world came together and set a more aggressive goal of 1.5°C temperature rise. To meet that goal, emissions need to peak by 2020, and fossil fuels need to be phased out by 2055.

Given those goals, there are two critical things we need to consider when we evaluate carbon reduction strategies:

• The first is the amount of potential savings a strategy offers.
• The second is the time frame of those savings.

 

We need strategies that produce large savings quickly, and because some reduction strategies result in an initial increase in carbon emissions, we need to pursue strategies that can produce a net reduction within that critical 10- to 20-year time frame.

Over the last year, a group of us from Siegel & Strain Architects, the Carbon Leadership Forum, and Architecture 2030 have been working to build connections between individuals and organizations working on different strategies for reducing GHG emissions in the built environment. Recognizing that we don’t have much time left to address climate change, we are focusing on strategies that can deliver the largest reductions in the shortest time.

These strategies include:

• reducing embodied greenhouse gas emissions from materials and construction
• reusing and retrofitting existing buildings
• building new net-zero-energy buildings

 See below for how you can get involved.

The scale of the problem

As an end user of fossil fuels, the built environment accounts for more emissions than any other single sector—somewhere between 40% and 50% of global greenhouse gas (GHG) emissions.

The current gold standard for reducing emissions from buildings is to build new, net-zero-energy (NZE) buildings—super-efficient buildings powered by renewable energy sources. This is an important piece of getting to a carbon-neutral built environment, but there is a problem with this strategy: building those new NZE buildings will generate a lot of emissions.

Two other sources of emissions may be even more important to address in the short term:

• embodied emissions from building materials, products, and construction processes
• operating emissions from the buildings we already have.

 

Embodied emissions (eCO2) in new construction

Embodied emissions are the first emissions a building generates.

In the U.S., we are currently building about 5.7 billion ft2 of new buildings a year, and the embodied emissions from building those new buildings is about 300 million metric tons per year.

Over the next 20 years, the embodied emissions from those new buildings will outweigh the operating emissions from those buildings. And if we manage to make them NZE, then the only emissions will be the embodied emissions. So while new buildings need be NZE, we also need to reduce embodied emissions.

Based on Siegel & Strain’s own experience with building low-carbon buildings, we know we can reduce embodied emissions by around 30% by selecting existing materials and technology, by using lower-carbon materials, and by employing more-efficient design and construction processes. But an even more effective way to reduce embodied emissions is to reuse existing buildings. Building renovation generates significantly less emissions than new construction does and creates an opportunity to reduce operating emissions from existing buildings.

Operating Emissions  (oCO2) in existing buildings

Operating emissions from existing buildings are an even bigger source of emissions. There are about 310 billion ft2 of buildings in the United States, and operating them generates about 2.2 billion metric tons of GHG emissions every year—about one-third of total U.S. GHG emissions. The majority of the buildings in use today will still be in use in 2030, so existing buildings are the buildings we need to improve.

Which brings us back to the strategy of renovating more existing buildings and building fewer new buildings.

When the renovations include deep energy upgrades—even making them NZE buildings—we address two sources of GHG missions at the same time. We reduce embodied emissions compared to new buildings, and we reduce operating emissions from existing buildings.  And the good news is, we already know how to do this:

• Improve efficiency: upgrade the lighting, HVAC systems, equipment, controls, etc.
• Improve the building envelope: insulation, windows, shading, air sealing, daylighting.
• Power them with renewable energy.

NZE remodels are not as hard as we might think. We just co-authored the Total Carbon Study, a detailed case study of a two-story office remodel and upgrade that is now generating more energy that it consumes—a net-positive building. This interior remodel upgraded equipment and lighting, and added skylights and photovoltaics (PV), with only minimal upgrades to the envelope (roof insulation). The remodel generated about one-third of the embodied emissions that rebuilding the building would have.

For NZE retrofits, we need to evaluate the initial eCO2 investment against the savings from the upgrade. How much carbon was invested to get to zero, and how long will it take the savings from increased efficiency to offset that investment?

When you do this analysis, the answers may surprise you. Blowing in insulation or re-commissioning existing HVAC and lighting systems are likely good investments of eCO2; re-skinning a building with a high-performance aluminum and glass curtain wall may not be worth it. We need to start evaluating all of our reduction strategies by how well they perform within a 10- to 20-year time frame.

Shifting Priorities

It’s time to rethink our goals for reducing emissions from the built environment:

• Make all new buildings net zero by 2030.
• Reduce embodied emissions from new buildings to zero by 2050.
• Upgrade all existing buildings to net zero by 2050.
• Prioritize early savings over long-term savings.

 

I want to clarify a few things:

1. Yes, we still need new buildings. Buildings wear out, priorities change, and populations shift and grow. That said, we could be reusing a lot more buildings than we currently do.
2. Every existing building won’t get to net zero. We need to identify and target the best candidates and focus on them first. Low-rise commercial and residential buildings are high on my list. We could be retrofitting a lot more buildings to very low energy or NZE.
3. Reusing and upgrading existing buildings makes more sense in places that are mostly developed, like the U.S. and the EU. For countries that are still building a lot of new buildings, like China and India, the focus will need to be more on reducing the embodied carbon in new construction (as well as making these new buildings NZE).

Larry Strain, FAIA, is principal at Siegel & Strain Architects. Erin McDade, program manager at Architecture 2030, and Kate Simonen, AIA, S.E., founding director of the Carbon Leadership Forum, contributed to this article.


If you are working on these issues and would like to be on the contact list this group is developing, please contact Erin McDade at Architecture 2030.