The shift toward energy efficiency will require engineers and designers to become proficient in the updated methods and requirements contained within ASHRAE Standard 90.1-2022.
Learning Objectives
- Be able to identify the various compliance paths and subpaths within ASHRAE Standard 90.1.
- Learn what is required for the performance-based compliance options.
- Develop a decision-making algorithm to choose which path to follow.
ASHRAE 90.1 insights
- The increasing demand for energy efficiency is driving a shift toward performance-based code compliance and ASHRAE 90.1 rises to the forefront.
- Because of the ever-increasing complexity, many senior engineers are not far ahead of their junior counterparts and in the absence of their guidance, a structured decision-making approach is essential.
While the 2022 version of ASHRAE Standard 90.1: Energy Standard for Buildings Except Low-Rise Residential Buildings has been available for three years, no states have adopted or referenced it as a statewide energy code, although that is changing in 2025. As a result, few within the architecture, engineering and construction industry have experience applying the latest compliance methods, such as the additional energy requirements required by Section 11 or the new mechanical system performance rating (MSPR) method outlined in Appendix L.
Many engineers, designers, architects, contractors and clients struggle to keep up with evolving energy codes and standards. Even senior engineers with decades of experience applying prescriptive requirements can find themselves behind. Because energy efficiency requirements have changed so significantly in the last few code cycles, engineers often struggle to find colleagues with tacit knowledge of the nuances found in these standards.
The need for a decision-making algorithm
Because ASHRAE Standard 90 was first published in 1975, the complexity of the standard has increased significantly. Figure 1 attempts to illustrate this by showing the number of pages per edition of ASHRAE 90.1, as well as Standards 90.2 and 90.4, which were originally under the umbrella of Standards 90 and later 90.1.
Recent code cycles have seen the inclusion of the Performance Rating Method — often known by its location within the standard, Appendix G — as a compliance path, the introduction of energy credit requirements and a new performance-based alternative for heating, ventilation and air conditioning (HVAC) system compliance within the overall prescriptive path.
As energy standards become more complex, building owners and design teams will increasingly lean on the mechanical and electrical disciplines for advice on how to comply with the requirements. In the absence of intuition developed after many years of experience, a roadmap or algorithm is needed to help new designers identify the best path forward.
ASHRAE 90.1’s performance-based methods within the prescriptive path
The prescriptive compliance path is typically the most familiar, allowing each discipline to operate relatively independently of one another. Building designs following the prescriptive path must comply with Sections 5 to 11, which cover the building envelope, HVAC, service water heating, power, lighting, other equipment and additional efficiency requirements. However, even within the prescriptive path, performance-based options exist for certain disciplines.
For building envelope, Section 5.5 offers a fully prescriptive path, while Section 5.6 enables a performance-based approach. This approach allows for trade-offs between different envelope materials or subassemblies if the overall envelope performance meets the specified criteria. The procedure for the envelope trade-off approach is outlined in Appendix C. Energy modeling is required for this path, which often falls outside the architect’s scope of expertise, meaning that the engineering team needs to be included even if energy modeling is not otherwise required.
HVAC systems have three compliance paths within the prescriptive path: the simplified approach (Section 6.3), the prescriptive path (Section 6.5) and the mechanical system performance rating method (Section 6.6), which was added in the 2022 edition. The MSPR allows for trade-offs within HVAC systems but not with other disciplines like the envelope trade-off approach mentioned above.
As seen in Equation 1, the new approach compares energy efficiency, known as the total system performance ratio (TSPR), of the proposed design with that of a baseline HVAC system, normalized by a given mechanical performance factor. A new appendix, Appendix L, covers the calculation procedures.
One benefit to the MSPR is that, while it does require energy modeling, it allows for simplifications, such as reducing the building geometry to simple shapes. This may be very useful on small renovation projects, although the utility of this simplified approach is likely reduced on new buildings or complete renovations/alterations, where the building geometry must be modeled to a reasonable degree of accuracy for building loads.
Equation 1:
TSPRp > TSPRr / MPF
Where:
TSPRp = proposed TSPR
TSPRr = reference TSPR
MPF = mechanical performance factor
New for ASHRAE 90.1-2022 are the additional efficiency requirements in Section 11, which cover all disciplines in Sections 5 to 10. Having experience with U.S. Green Building Council LEED or other points-based building energy performance rating systems is helpful for understanding this section. There are 33 individual energy efficiency measures to choose from and while most of the measures do not require the creation of an energy model, some of the most substantial, such as E01: Improved Envelope Performance, do require energy modeling. Future measures are likely to continue moving in this direction.
Performance-based compliance within ASHRAE 90.1
The energy cost budget (ECB) method has been around in some form or another since ASHRAE Standard 90-1975 was first published and computer-based, 8,760-hour energy simulations were first mentioned in ASHRAE 90.1-1989.
However, it’s not uncommon for the prescriptive method to be preferred, as it was often simpler to understand, required less interdisciplinary coordination and computing power and was usually easily achievable. The ECB was only used as a last resort if some unique building property made the prescriptive path not feasible.
In the years since, the prescriptive requirements have become increasingly stringent, while at the same time architectural designs have grown more complex. Fortunately, computing power is no longer the concern it once was, making the ECB more attractive. The ECB essentially compares the estimated annual cost of energy for the proposed building against that for a minimally compliant building, using the requirements given in the prescriptive path.
Equation 2:
DEC < ECB x (1 – ECreq / 1,000 x Aadj)
Where:
DEC = design energy cost
ECB = energy cost budget
ECreq = energy credits required per Section 11
Aadj = an adjustment factor to normalize additions or alterations by the total area in the model
In terms of compliance, the performance rating method (PRM) is a relative newcomer to ASHRAE 90.1. First appearing as an informative appendix in 2004, it was not officially considered part of the standard.
However, it has subsequently been promoted first to a normative appendix (2010) and finally to a third compliance path (2016). Like the ECB, it compares the proposed building against a baseline and allows for tradeoffs between disciplines, while allowing for even more flexibility than the ECB.
The energy model for the proposed design is compared to a more normative baseline building than that used in the ECB. This baseline model uses standardized HVAC systems and the same percentage of glazing on each wall, among other characteristics as required by Appendix G. Furthermore, the baseline building energy performance is simulated after it is rotated 90, 180 and 270 degrees in addition to the actual orientation, with the results of all four orientations being averaged.
The result of the simulations is the calculation of a performance cost index, as shown in Equation 3. Because the proposed building is rated against a baseline building of the same footprint, the methods for calculating the baseline performance do not need to change much from one edition of the standard to another; the target performance cost index (PCIt) can simply be decreased.
Equation 3:
PCI = PBP / BBP
Where:
PCI = Performance cost ndex
PBP = Proposed building performance
BBP = Baseline building performance
Who determines the compliance path?
As energy standards continue to become more stringent and complex and push for performance-based compliance, mechanical and electrical designers will often be asked to advise and assist in achieving a code-compliant project. Even if the prescriptive path is the compliance method chosen, securing the additional energy credits required by Section 11 will require increased collaboration between the various design team disciplines.
While an integrated design approach is strongly recommended, at the very least a meeting solely focused on energy code compliance should be held at the beginning of the project. While the mechanical and electrical disciplines may not own the project, it is their responsibility to push, if need be, for inclusion as early as possible in the design process.
They will also be called on to educate the design team on new energy code requirements, including the expanded commissioning requirements, which are often forgotten. Scheduling an energy code coordination meeting early in the design process will be beneficial to all stakeholders. During this meeting, each discipline should identify the subpath of the overall prescriptive path they intend to pursue and whether they have identified any challenges with meeting code compliance.
Achieving ASHRAE 90.1 compliance
The robust nature of ASHRAE Standard 90.1 can make it difficult to determine which path to pursue. Choosing a compliance option will allow designers a reference point as energy questions arise. While there is no one-size-fits-all approach for selecting the best path, following a flow chart can help identify the best course of action (see Figure 2). The idea is to ask a series of questions at the beginning of a project that will lead to achieving energy compliance with fewer difficulties. The algorithm is presented as a flow chart and consists of the following series of yes or no questions:
- Is there a desire for beyond-code building performance? In this context, beyond code refers to anything above and beyond the base requirements of ASHRAE 90.1, such as a formal certification program like LEED, Green Globes or Energy Star, a state or federally required stretch code or a client’s desire for sustainable building performance, often phrased as “LEED-lite” or “as close to net-zero as possible.” If the answer to this question is yes, then the PRM should be used. This method has long been required for many of the formal programs mentioned above and has been an acceptable method of proving code compliance since the 2016 edition of the standard.
- Is the project a new building or an addition? New buildings and additions depend heavily on the characteristics of the building envelope for energy performance. At the energy code compliance coordination meeting, the design team should review the prescriptive requirements of the building envelope section and identify potential challenges. If it is a simple building and the architects do not anticipate any significant challenges with the prescriptive path, the team should continue with questions the next questions. Challenges with prescriptive compliance often come in the form of fenestration exceeding the allowable limits found in Section 5.5.
- If such challenges are identified, there are generally two options: the envelope trade-off or a performance-based path. Since the envelope trade-off option under Section 5.6 already requires energy modeling and because Section 12 is more flexible by allowing for tradeoffs between all disciplines, it likely makes sense to pursue the energy cost budget method.
- Does the design team perceive any challenges in complying with Section 11 of the standard? These issues could range from technical challenges to obtaining certain credits to simply not wanting to spend time evaluating each point and completing subsequent documentation. As compliance with Section 11 is not required for the performance-based paths, the energy cost budget method is likely to save overall time and effort.
- Will energy credits that require energy modeling, such as improved envelope performance (E01), be pursued? If so, the creation of an energy model lends itself to performance-based compliance.
- Is energy modeling required for the mechanical, electrical and plumbing design? Certain design choices, such as geothermal systems or energy storage, are likely to require an energy model as a matter of best practice. Again, the energy model lends itself to a performance-based path.
- Does the mechanical team anticipate any challenges meeting the prescriptive requirements of Section 6.5? Certain design constraints can sometimes cause challenges in meeting the minimum efficiency requirements in Section 6.5. If there are no issues with Section 6.5, then the prescriptive path can be used. If challenges are identified, then one final question should be asked.
- Is simplified building geometry acceptable for a loads/energy model? If the project is a relatively simple building or a renovation, an energy model with simplified building geometry may be acceptable. In this case, the mechanical team may decide to use the mechanical system performance rating method, which is otherwise a part of the overall prescriptive path but allows for tradeoffs between mechanical equipment. If simplified building geometry is not acceptable, which is likely to be the case with a new building for which a loads model has been created, then the energy cost budget should be used. If the mechanical team does not foresee any challenges to prescriptive compliance, then the overall prescriptive path can be used.
The ongoing evolution of ASHRAE 90.1
As energy codes continue to evolve, understanding and effectively navigating the compliance paths of Standard 90.1 is crucial for designing code-compliant buildings and delivering successful projects. A collaborative approach among architects, engineers and contractors is essential for successful building performance. Open communication, early coordination and a shared understanding of energy performance goals are vital.
While a framework is helpful for decision-making, ASHRAE 90.1 will continue to change and it is important to stay updated on the latest requirements. By following a structured decision-making approach to choosing an ASHRAE Standard 90.1 compliance path, engineers can navigate the complexities of energy efficiency requirements and lead their project teams to achieve energy-efficient building designs.
