Monday, December 18, 2017

Part III: Time Value of Carbon

when you save matters
what you build matters

what you don't build matters more
by Larry Strain, FAIA

(continued from Part II)
Given the amount of embodied emissions from new construction and the amount of operating emissions from existing buildings, a combined strategy of reusing and upgrading existing buildings, building fewer new buildings and reducing embodied carbon emissions from construction is the most effective way to reduce emissions quickly. When the renovations include deep energy upgrades—even making existing buildings net zero energy and emissions, we address two sources of GHG missions at the same time. We reduce embodied emissions compared to new construction, and we reduce operating emissions from existing buildings by making them more efficient.

Reducing operating emissions is not technically difficult; we already know how to do it.

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

We recently co-authored a study with the Integral Group of a two-story office building renovation and upgrade for DPR Construction. This remodeled office building is currently generating more energy that it consumes, making it a net positive building.

The interior remodel upgraded equipment and lighting, added skylights and photovoltaics (PVs), with only minimal upgrades to the envelope (roof insulation). The remodel generated about 1/3 of the embodied emissions that building a new building would have, and because it is producing more power than it uses it is paying off that embodied carbon debt.

The building wasn’t even a particularly ideal candidate for a net-zero retrofit. It is partly shaded by taller buildings, and the single glazed aluminum storefront windows couldn’t be replaced because they were historic. The most compelling part of this story is that even without ideal conditions, it still made sense to retrofit; the project came in on budget and on time.

The priorities for deep energy upgrades for existing buildings have changed over the last decade. In the past, you started by upgrading the building envelope—adding insulation and high performance windows, and maybe upgrading the lighting. With the use of blower-door tests and highly efficient and relatively inexpensive heat pump technology, air sealing and equipment upgrades are now also among the first upgrades we might undertake.

Efficiency strategies may vary depending on whether the building is residential or commercial. Commercial buildings generally have higher internal loads, which means that for commercial buildings, lighting and equipment upgrades may have a larger impact on reducing energy and emissions than envelope upgrades. Residential buildings tend to be dominated by external heating and cooling loads, and envelope upgrades may have a bigger impact, although appliances and equipment upgrades are also important.

Another thing that has changed is our understanding of the urgency of addressing climate change. As previously stated, we need to drastically reduce total carbon emissions—operating and embodied—over the next 10-30 years, and to do that we need to evaluate energy efficiency strategies based on the initial embodied carbon investment to achieve the strategy against the future operating savings generated from the efficiency upgrade. How much carbon did we spend to reduce operating emissions, and how long will it take the savings from increased efficiency and clean energy production to off-set that initial investment? When you do this analysis it may change your approach to efficiency upgrades. Blowing in insulation, re-commissioning or even replacing inefficient HVAC and lighting systems are likely to have a good return on carbon invested; re-skinning a building with a high-performance aluminum / glass curtain wall or wrapping the building in foam insulation may not be a good investment from a carbon standpoint. It may even make sense to add PVs before achieving that last few % of efficiency. The point is, we need carbon reduction strategies that have a positive payback within a 30-year time frame and ideally within 10 years.

Because reusing the foundation, structure and envelope saves a lot of embodied carbon compared to building new, some buildings are more important to reuse than others. Large, heavy commercial buildings offer a greater potential for reducing embodied emissions, because replacing this type of building will have a higher carbon footprint than replacing small residential structures. The good news is we have a lot of these buildings. There are almost six million commercial buildings in the U.S. and the majority of them are one- to three-story, flat-roofed buildings.


What are the best candidates for energy upgrades? We start with the buildings that use a lot of energy compared to similar buildings with similar uses. Poor performing buildings have a higher potential to save energy and reduce carbon emissions than more efficient buildings. These buildings usually have:
  • Poor thermal envelopes, little or no insulation, single glazed windows, unshaded windows, leaky, drafty buildings.
  • Old, inefficient HVAC and lighting systems and controls, and equipment and appliances.


Bringing existing buildings up to ASHRAE 90.1 2013— efficient but not necessarily to super-efficient—passive-house standards, would allow most of them to be converted to net zero energy. We have an abundance of one- to two-story strip malls, warehouses, schools and office buildings with large expanses of flat roofs, not to mention millions of single family homes. These are all prime candidates for efficiency and net zero upgrades.  The best candidates for zero net energy upgrades are: 
  • Buildings with unshaded flat roofs or south (in the northern hemisphere) and west facing sloped roofs.
  • One- to three-story story buildings: 76% of commercial square footage and 96% of residential square footage are one- to three-stories.
  • The majority of existing buildings in most climate zones—offices, retail, schools, warehouses, apartments and single family homes—could be converted to net zero energy.
  • Buildings with adjacent unshaded land. Parking lots with PV canopies produce power and have the added benefit of shading the cars and pavement and reducing the heat island effect around the building.
  • There are a number of building types and configurations that can’t be made NZE using only the roof and walls of the building—buildings over four stories, high-energy use buildings such as restaurants, hospitals and data centers. These will require off-site district or community based solutions.
  • To also achieve net-zero emissions, we also need to eliminate the use of on-site fossil fuel combustion and convert them to all electric.


  • Identifying the best buildings to retrofit and upgrade to zero.
  • There are limited incentives and regulations that require existing buildings be upgraded.
  • It can be expensive to make an existing building more efficient and power it with renewable, clean energy.
  • Addressing potential moisture and condensation issues when we upgrade existing buildings.
  • All upgrades require an investment of eCO2. We need simple ways to calculate the carbon invested and how long will it take the savings from increased efficiency to offset that investment.


  • We still will need new buildings. Buildings wear out, priorities change, populations shift and grow, but we need to make reusing and upgrading existing building a much higher priority.
  • Every building won’t get to net zero, but we can make all existing building more efficient. We need to identify and target the best candidates and focus on them first, high energy use buildings and low-rise commercial and residential buildings. We could be retrofitting a lot more buildings to very low energy or ZNE.
  • Reusing and upgrading existing buildings makes more sense in places that are mostly developed, such as the US 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 them ZNE).
  • Although this paper does not address transportation directly, locating buildings to minimize transportation impacts associated with building use is another a critical strategy for reducing emissions associated with buildings.  


Architecture 2030, 2030 Challenge for Products.

Carnegie Mellon, EIO LCA.

Carbon Leadership Forum, “Embodied Carbon Benchmark Study”

Cole, R. Kernan, P. “Life Cycle Energy Use in Office Buildings” Building and Environment Vol. 31, No.4, 1996.

Department of Energy, United States Energy and Information Administration, “Commercial Buildings Energy Consumption Survey” 2012.

Department of Energy, United States Energy and Information Administration, “Residential Energy Consumption Survey,” 2009.

Eley, C. “Design Professionals Guide to Zero Net Energy Buildings.”

Environmental Protection Agency, “U.S. Greenhouse Gas Inventory.”

Fernandez, N.P. “ The Influence of Construction materials on life-cycle energy use and carbon dioxide emissions of medium size commercial buildings” Thesis, School of Architecture, Victoria University of Wellington, NZ, 2008.

McGraw Hill Construction/Dodge, “U.S. Construction Outlook,” 2015.

Stein, R.G, “Architecture and Energy,” 1977.

Stein, R.G; Hannon, B.M.; Segal, B.Z.; Serber, D. “Energy Use for Building Construction,” 1977.

United States Census, “Characteristics of New Housing.”

United States Climate Action Report 2014 to the Intergovernmental Panel on Climate Change. No longer available on the State Department’s website, but still available as a PDF from:

For the complete report, please click here. 

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