A Simple Equation for Conducting a Whole-Building LCA
As embodied carbon gains more attention, more project teams are pursuing the Building Life-Cycle Impact Reduction credit in LEED v4.0 and v4.1 and choosing option 4, Whole-Building Life-Cycle Assessment. If you are in that boat but wondering where to start, we would like to offer a simple equation.
To conduct a whole-building life-cycle assessment (WBLCA), you need two pieces of information: the quantity of each material and the embodied environmental factor for that material.
Quantity of material x Environmental factor = Embodied environmental impact
Let’s break down each component.
Finding Material Quantities
To identify the material quantity for a specific building, you can use a variety of sources. Ultimately, you need a total weight or volume for each material, depending on the units used to measure the material you are working with.
Tip: Always check your units! In LCA terms, this is referred to as the “functional unit.”
Typical material units are kilograms (kg), cubic feet (cu ft), cubic yards (cu yd), cubic meters (cu m), tons (imperial), or tonnes (metric).
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The great thing about BIM models (like Revit) for projects is that you can easily pull material quantities from the model information. Other sources of quantity information include:
- a bill of materials used for cost estimating
- area take-offs for programming design
- ordering invoices
- design schedules
Tip: Verify that the BIM or other project documents on which the bill of materials is based include the entire scope of the WBLCA.
For large, complicated projects, sometimes the structural models are in two parts. To meet the requirements of the LEED WBLCA option, the entire structure needs to be included in the study.
Tip: Don’t get bogged down by the little stuff.
The ISO 14044 standard states that a 5% cutoff can be applied. This means that materials that make up less than 5% of the total mass of a project can be excluded from the analysis.
Examples of materials typically excluded from a WBLCA are nails, screws, wallboard edges, etc.
If you can, find a way to double-check that all the materials you intended to include in the study are in the quantity list.
Finding the Right Environmental Factors
Once you have the quantity of materials in your compiled bill of materials, it’s time to match the material to the environmental factor. Environmental factors are the numbers that represent the impact that materials have on air, water, and soil. Different factors are relevant to different impacts. For example, carbon (CO2) is the greenhouse gas factor used to represent global warming potential.
These environmental factors are gathered in life-cycle-inventory databases and are further compiled into environmental product declarations (EPDs). WBLCA tools for design teams embed this information by material to make this exercise as fast and as intuitive as possible. Different tools might make this process easier by:
- Grouping materials typically installed together into one selection. For example, a typical wall includes framing, gypsum board, and paint. This can make initial selection easy but will limit the modeler’s ability to customize.
- Limiting the materials in the database to those relevant to buildings, such as excluding ethanol, or only including typical concrete strengths.
- Limiting the selection of data by region our country of origin. The U.S. has a growing database of life-cycle-inventory data and EPDs, but it is not as comprehensive as those of other countries. Sometimes you can’t find the product you are looking for in a U.S. database.
Tip: Check that your life-cycle environmental factor data complies with ISO standards.
Environmental factors must comply with ISO 14044 and 14020 to meet the LEED requirements for a WBLCA. Interestingly, a published EPD is required to use ISO-compliant data, but an LCA study or report is not. Sometimes it depends on the software program being used for the LCA. Look for third-party verification, software documentation of life-cycle-inventory databases being used, and report bibliography.
Tip: Prepare to do some quality control.
If using Revit as the basis for the tool or mapping, you will still need the drawings and specifications to fill in gaps about the material information. Many materials are not labeled accurately in the BIM model.
In addition, the modeler will make a series of decisions and assumptions about the most appropriate data to use. It can be a frustrating and long process, depending on the level of detail needed for the assessment and the quantity of materials that need to be mapped. There is a famous saying in the LCA world: “garbage in, garbage out.” This saying forewarns that the less accurate the model inputs are, the less accurate the results will be.
There is another saying: “All models are wrong, but some are useful.” You should interrogate your results to understand what they can tell you and what they can’t tell you. WBLCA contributes information on the relative “goodness” or “badness” of choosing one building material or product over another, and that leads to more informed decision making. In addition, creating databases that quantify environmental impacts will allow us to benchmark where we are and drive reductions in the building industry.