Considerations for carbon investment decisions, passive design strategies, more
Tips for making decisions about carbon and energy reduction strategies.
Though much discussion lately has been around embodied carbon, CO2 emissions from building operations (72%) are a much larger challenge than those from construction material embodied carbon (28%), according to Architecture 2030. By 2050, projections anticipate a close to 50/50 split between operational and embodied carbon, but this hinges on building designs and renovations between now and 2050 making progress on improving building operations.
To reduce the carbon impact of buildings, it is best to start with the largest piece of today’s pie, operations in the use phase. Within the range of choices that can impact operational carbon impacts, not all selections are equally important. Choices that impact the building’s passive design features, like orientation, massing, and enclosure systems, have the largest and longest impact related to the lifespan of the overall building.
The examples in this article will use the Department of Energy (DOE) Standalone Retail Prototype Building located in Climate Zone 4A. This building is modeled to comply with the ASHRAE 90.1-2019 energy requirements for the entire building. Then the model is rerun using EnergyPlus with incremental levels of polyiso roof insulation above the deck to determine the overall building energy usage. Similar to the DOE metric of the Energy Use Index (EUI), the ASHRAE 90.1-2004 insulation requirements are used as a baseline for comparison. The carbon dioxide emission equivalent (CO2e) is determined by using the DOE 2011 Buildings Energy Data Book to get an average CO2 emission value for operations of a standard 1,000 square feet of a commercial building space across the U.S. The insulation product’s embodied energy and carbon data is gathered from the PolyIsocyanurate Insulation Manufacturers Association’s industry-wide environmental product declaration.
When making decisions about carbon and energy reduction strategies, it is helpful to approach the problem as an “investment” rather than a “discretionary expense.” Discretionary expenses are easier to eliminate by simply consuming less. But that concept doesn’t apply well to building operations. Building occupants expect to have the building heated and cooled, so “off” is not a very good option. As an investment, however, it is more appropriate to look at improving the building enclosure and understanding what the return on the investment is from an energy and/or carbon perspective.
Not only is roof insulation a good carbon investment at R-30 for this example building, but increasing levels of insulation can also result in additional carbon and energy savings over the operational life of the roof. For the same building and location, the increased roof insulation can provide measurable whole building energy and CO2e annual savings at levels well beyond the current minimum code.
Passive design strategies
To impact operational carbon, the largest carbon challenge facing buildings today, the returns on the investment need to be consistent over time. This is where passive design strategies like building enclosure improvements really shine. They have much longer service lives than mechanical equipment and finish materials, leading to sustained returns.
Below demonstrates the cumulative carbon savings for a 75-year building life, as outlined in the polyiso EPD. In the example, the roof insulation begins at R-30 and increases every 20 years by R-10, corresponding with periodic roof membrane replacement. In this analysis, the initial insulation product’s embodied carbon, replacement embodied carbon, and end-of-life disposal are considered. What is clear is that shortly after the building becomes operational, the embodied carbon of the materials is dwarfed in comparison to the savings provided by the use of the insulation product. The increased roof insulation ultimately saves a net of 705 metric tons of carbon over the 75-year life of the building.
Using the same example as above, the direct energy savings for the building is significant. The embodied energy in the improved roof system from R-15 to R-30 for the entire building is 38.8 GJ. Compared with the annual savings the initial improvement provides, this investment in energy, rather than carbon, to produce the insulation pays back in less than five months. The cumulative energy savings over the life of the building for the same example over 75 years as shown is also very notable at 9,030 GJ of energy.
Here are some general strategies to address both operational and embodied carbon when making “carbon investment” decisions: First, ensure that the alternatives and substitutes you consider have equivalent performance attributes. If it has lower initial embodied carbon but performs worse or doesn’t last as long as the specified product, it may not be a good carbon investment. After considering performance, determine if the product has the potential to provide operational carbon savings. If the product has operational carbon impacts over a long period of time, don’t sacrifice operational carbon savings for initial embodied product carbon alternatives. Lastly, if a product has high operational carbon savings and relatively low embodied carbon (a short payback period) include MORE in your designs and utilize those additional carbon returns to supplement the carbon use of other areas of the design, like aesthetic finishes, where the decisions may be discretional but desired.
To learn more about this topic, view GAF’s A’21 session Solving for Carbon, Divided by Roofing.