Whole-building LCA Frequently Asked Questions

1. I saw two different studies of the same building, with different results – why are they different?
LCA studies have so many variables that it’s difficult to compare results from one study to another, even if for the same product or building. Some key variables include:

• The LCA software tool. There are different methodological approaches to LCA, and each tool may use a different method, a different interpretation of standards, and different underlying data. Also, be mindful that software tools may update their methods and data periodically, which means two identical studies with different versions of the same software may be different.
• The underlying data. Whole-building LCA results rely on environmental data on materials and construction processes, and these data can vary depending on the source. Each whole-building LCA software tool has different data built in, with a variation in original source, amount of detail in the data, level of data regionalization (if any), and other factors. Software tools also differ in how they approach product substitution when a material is missing, life cycle assumptions like product replacement schedules, and so forth.
• Scope of the assessment. A common source of big differences in results is variations in life cycle scope – not all studies include all the life phases. Before comparing, check which of the EN 15804 life cycle modules are included (modules A1-C4 and optionally module D).
• Scope of the model. Another factor that will greatly affect comparability is differences in which components of the building were included. For example, results from a study on a core-and-shell model can’t be compared to a study that additionally includes interior partitions and finishes.
• Accuracy of the bill of materials. Different models may have a different degree of diligence in accuracy (e.g., actual material types and quantities versus an estimate) and comprehensiveness (e.g., fasteners included versus excluded).
• Building life for the model. Check how many years were used for each study. This affects product replacements and the impacts related to maintenance.

2. What are the implications when doing LCA studies at different times during project design?
Doing an LCA in early design can be helpful for broad-brush guidance at a time when there is still opportunity for adjusting the decisions that have big consequences for environmental impact. An early LCA provides baseline results – this can be a starting point when seeking improvements in environmental performance. As design progresses and decisions become more locked-in or specific, an LCA at this point will deliver results that have more precision and can help the design team fine-tune. (However, be mindful of all the comparability rules discussed in the first FAQ.) An LCA when design is finished, or even post-occupancy, reports the final results and can be used for transparency purposes like an environmental building declaration (EBD).

3. What’s the difference between LCA for LEED points and LCA for an EBD?
Every LCA study starts with a decision on goal and scope: identifying the purpose of the study, what will be included and ultimately its intended audience. LCA studies can vary widely in purpose and approach. For example, LCA used to earn points in LEED (and in similar LCA provisions in other programs) has a very specific purpose and a very limited scope. Design teams perform a simplified comparative LCA study for a self-defined reference building versus a proposed building, following USGBC-defined guidelines. There is no formal verification, peer review, or reporting per LCA industry-standard practices.

At the other end of the spectrum are Environmental Building Declarations (EBDs). LCA performed in this context is a comprehensive study and reporting process conforming to international standards. The purpose is to produce a transparency report for a single building describing its LCA performance, without comparing it to anything.

4. How do I know if whole-building LCA results are “good”?
LCA data is most useful if it can be viewed in context – for example, in comparison to some kind of benchmark, which enables a judgment. This is currently a challenge.

When LCA is used internally in industry for product development, the benchmark may be an existing product that the manufacturer is looking to improve. Similarly, during building design, the benchmark could be an early version of the design. For subsequent design iterations, a “good” result is simply an improvement over the self-defined benchmark. In this context, how “good” the result is depends on how high or low the baseline is. If the baseline is an “F” and the final result is a “D”, the final result is an improvement but might not earn the value judgment of “good.”

To truly judge the relative LCA performance of a building requires comparison to a defensible benchmark that reflects industry average practice. Putting in place a system to enable that kind of comparison is harder than it sounds. The Athena Institute is working on this.

5. Can the information in a whole-building LCA study or an EBD be useful for other projects?
First, an explanation of EBDs: an environmental building declaration (EBD) is a transparency report, conforming to standards for LCA practice and reporting, that provides LCA results for a building. It is similar in principle to an environmental product declaration (EPD).

No matter what reporting mechanism is used, whether it’s a formal EBD, an informal and internal report, or something in between, the question may arise as to the utility of the results. Beyond the usefulness of LCA to the design team during design of that particular building, are there lessons for other projects down the road? Maybe.

Currently, EBDs (and EPDs) are primarily most useful for awareness-building. These are advocacy vehicles for the value of measurement, transparency and accountability achievable through the use of LCA. They help educate on the need for performance data when making environmentally conscious decisions and sustainability claims. And they demonstrate the application of standards in reporting.

In addition, an EBD is useful internally to the design team and the owner, in the same way that an EPD is most useful to the product manufacturer (rather than the consumer), because creating these documents requires the incorporation of LCA into design or manufacturing practice. In other words, the real value is in the LCA process itself, and not so much in the reporting of the LCA results – at least for now. A team using LCA during building design will almost certainly end up with a building that has a lighter footprint than otherwise.

The data in an EBD does contain some lessons of potential value to future projects by any design team. For example, architects and engineers can look at the detailed results tables to better understand what is driving the building impacts and can use those lessons in guiding their own material and design decisions on other projects. They could also use the EBD results as a benchmark for their own next project (as a target to beat – with caution, being mindful of comparability issues), and perhaps will be inspired to produce an EBD themselves.

6. How much time does it take to do a whole-building LCA or create an EBD?
Assuming the use of a simplified software tool specifically intended for whole-building LCA, like the Athena Impact Estimator, it can take anywhere from a couple of hours to a couple of weeks to create a model. An experienced software user modeling a simple building and using the Impact Estimator’s built-in features for calculating material quantities could probably get LCA results in as little as one hour. At the other extreme is a highly-detailed model using actual material quantities – it can take a lot of time to gather and validate accurate data about material types and quantities.

Once the information about material quantities is in hand, the actual LCA calculations in the software are very fast. But there is additional user time needed to review results (do they make sense, where are the hot spots, etc) and perhaps some time to gather information into whatever format users wish to report their study.

Creating an environmental building declaration (EBD) is another story. The Athena Institute applies a high degree of accuracy in gathering the bill of materials, which can slow down the LCA process. In addition, an EBD is a very in-depth, comprehensive, custom report, which takes time to produce.

7. How are all the materials accounted for in a whole-building LCA?
The key input for whole-building LCA is an appropriate bill of materials. The degree of accuracy and comprehensiveness required of the bill of materials is a function of scope and objectives of the study, and a good deal of user judgment.

In general, users need to create a building model that includes all the major materials for the building elements included in the scope – each type of material, and its quantity.

The accuracy in identification of materials and their quantities is dictated by the purpose of the study. For example, in early design (before those details are known), material estimates are fine – Athena Impact Estimator users familiar with the “assembly dialogues” are working in this manner, with material quantities estimated by the software based on simplified quantity take-off algorithms.

As design progresses and actual material types and quantities become known, Impact Estimator users can either manually adjust the automated generic quantities calculated by the software, or they can import a complete bill of materials from another source. Actual material quantities can come from drawing take-offs, BIM models, and cost estimates.

What to include: all materials within the study scope that are present in a significant quantity or are likely to have a significant impact on the results. Users need to apply their judgment regarding which materials, if any, are deemed so insignificant that they can be excluded.

Validating the materials: users need to be diligent in verifying the accuracy and completeness of an imported bill of materials. This is particularly an issue with materials extracted from BIM models, which can be incomplete (many important elements for LCA are typically missing, like rebar) and inexact.

When results are reported, it’s important to clearly state what elements of the building were included and to provide the associated bill of materials. In addition, it’s a good idea to provide a qualitative statement on the source for the bill of materials; for example, identifying it as an estimate calculated by the Impact Estimator, as an actual take-off from construction documents, etc. It is also common practice to identify any major elements or materials that were excluded, for whatever reason (for example, HVAC equipment, due to a lack of available life cycle inventory data).

8. If I want to compare two whole-building LCA studies, what should I look for to determine if they are comparable?
First, check for functional equivalency. Do both buildings serve the same function, more or less? If not – for example, if one is a high-rise office building and the other is a single-family home – then LCA results would not be comparable. Next, check for alignment in scope. Do both studies include the same life cycle stages? If not, you cannot compare total results (but you may be able to compare results for individual life cycle stages). Do both studies include the same building elements, to the same degree of detail in the bills of materials? This can be hard to check, but it’s crucial. For example, perhaps both studies include concrete structure, but one doesn’t include the rebar – you cannot compare these results.

For other factors to watch for, take a look at the first FAQ.

9. Why is biogenic carbon sequestration considered “beyond the building life cycle”; i.e., in Module D, in the Impact Estimator?
First, an explanation of life cycle modules. We use the terminology per the EN 15804 standard, where Modules A through C cover the cradle-to-grave life cycle phases, and Module D considers benefits and loads beyond the building life.

It’s important to recognize the temporal aspect of environmental impacts for long-lived infrastructure like buildings. In LCA, we compress the impacts over time into a single snapshot taken at the point in time set for the study (e.g., 75 years down the road, a reasonable assumption for the lifespan of a building).

Additionally, we look beyond the life of the building to determine the likely disposition of the materials once the building is decommissioned. Here, we typically account for material reuse, metals recycling and possibly energy recovery potential from materials. Obviously, these are assumptions looking far ahead in the future. While these judgements are often informed by the current state of the art, they are nonetheless forecasts and should be reported separately due to the high degree of uncertainty in trying to predict the future.

In fact, everything beyond the product manufacturing stage is really an assumption about the building. But we still have a good degree of confidence in assumptions (i.e., “scenarios” in LCA lingo) through Module C. With the scenarios beyond building life, we prefer to apply a very high level of caution and transparency.

Giving a building credit for biogenic “carbon sequestration” (i.e., reducing the building’s global warming potential impact due to the offsetting effects of carbon storage in wood) is an LCA methodological judgment for which standards are still evolving, which means different LCA practitioners and tools may take different approaches.

We have chosen a conservative approach to this: we only give credit for “permanently sequestered carbon,” and we report this carbon in Module D. Biogenic sequestered carbon is the carbon dioxide removed from the atmosphere by a living tree and converted into carbon, which makes up about half the mass of wood. When wood is transferred from the forest to a building, it continues to store carbon, but we consider much of this storage to be temporary and therefore we do not give the building a carbon credit for the carbon dioxide that will eventually be released from this wood some time down the road, through decay or incineration.

However, we do give credit for carbon in wood that will never decay or burn – this is the proportion of wood in landfill that will remain in stasis indefinitely. To make this prediction, we use the US EPA model WARM to forecast wood’s decomposition in landfill 100 years in the future. We apply the model to determine the fate of wood at the end of the building’s life: how much is going to landfill, the rate of decomposition in landfill, the resultant emissions of carbon dioxide and methane, the rate of methane recovery, and the disposition of the methane (flared or burnt for energy recovery).

International standards for biogenic carbon accounting are still evolving. While the newest version of ISO 21930 now addresses biogenic carbon and requires reporting it within Module C, we have chosen to continue reporting this result in Module D for transparency. We feel this is important, due to the current uncertainty and methodological questions in biogenic carbon accounting. For example, there is a very high degree of uncertainty in assumptions about waste management so far in the future, e.g., proportion of wood landfilled, rate of decomposition in aerobic and anaerobic landfills, rate of landfill gas recovery, etc. By reporting biogenic carbon sequestration in Module D, we are acknowledging this uncertainty and enabling Impact Estimator users to decide for themselves whether or not they wish to include it. We may change this down the road, depending on improvements in consensus and understanding about biogenic carbon accounting methodologies and any further development in international standards.

10. What is the difference between an EPD and an EBD?
An environmental building declaration (EBD) is a transparency report produced by the Athena Institute, conforming to standards for LCA practice and reporting, that provides LCA results for a building. It is similar in principle to an environmental product declaration (EPD), in that it summarizes LCA results for a product according to standards. But it does not follow the same lengthy and complex administrative path of an EPD, which works for mass-produced and commodity products but makes little sense for buildings, where each one is a unique product.