Five steps to success with ASHRAE 90.1

ASHRAE Standard 90.1: Energy Standard for Buildings Except Low-Rise Residential Buildings is one of the main drivers used in any building design. Read the top five best practices for engineers to understand the key elements of this standard.
By Cory Duggin, PE, LEED AP BD+C, BEMP, TLC Engineering for Architecture, Tenn. January 25, 2017

This article is peer-reviewed.Learning Objectives

  • Define ASHRAE Standard 90.1: Energy Standard for Buildings Except Low-Rise Residential Buildings.
  • Demonstrate how ASHRAE 90.1 and other codes and standards influence the design of building systems.
  • Compare the prescriptive and performance paths to meeting code and standard requirements.

Energy codes are adopted and enforced differently in every state. Some have statewide energy codes, while others have exempt jurisdictions with their own local code departments. ASHRAE Standard 90.1: Energy Standard for Buildings Except Low-Rise Residential Buildings does a lot to influence the design of buildings, but it is not always a direct influence.

Most jurisdictions adopt a version of the International Energy Conservation Code (IECC), which is often viewed as a codified version of ASHRAE Standard 90.1 even though they are developed independently. A few jurisdictions do adopt Standard 90.1 as their energy code, but the application of 90.1 to projects is largely through above code programs, such as U.S. Green Building Council‘s LEED program, and as an alternate compliance path to the parallel edition of the IECC.

The 2015 IECC and ASHRAE 90.1-2013 are considered to be parallel standards because Standard 90.1-2013 is an alternate compliance path. If the 2015 IECC has been adopted, either standard may be used for compliance (unless a local amendment doesn’t allow it). Generally, ASHRAE 90.1-2013 is seen as more stringent than the 2015 IECC (see Figure 2). They have some requirements that are very similar, such as prescriptive window-to-wall ratio (WWR) limits, and others that are different, like the prescriptive "additional efficiency" packages. They even have different mandatory requirements. For instance, automatic receptacle controls have been a mandatory requirement for certain space types because ASHRAE 90.1-2010, but they still haven’t appeared as a requirement in any of the currently published versions of the IECC.

For this reason, design teams should weigh which would be the most appropriate standard on a project-by-project basis. Requirements between the two standards cannot by mixed and matched.

Figure 1: The University of Florida Joseph Hernandez Hall is a chemistry and chemical biology lab designed to achieve U.S. Green Building Council LEED NC v2009 Gold. The design incorporates exhaust system heat recovery, daylight harvesting, occupancy sensASHRAE 90.1-2013 should be considered more often as a path to compliance. If a project can incorporate the more stringent mandatory requirements of ASHRAE 90.1-2013, then a project can take advantage of the lack of additional efficiency packages in the prescriptive path or the lower compliance threshold in the performance path. The 2015 IECC requires the design energy cost to be less than or equal to 85% of the standard reference design (baseline) rather than just being less than or equal to as in ASHRAE 90.1-2013. Here are five steps to move a project to compliance with ASHRAE 90.1-2013:

  1. Review mandatory requirements
  2. Assess the prescriptive path
  3. Evaluate the performance path
  4. Decide on the compliance path
  5. Perform compliance calculations and documentation.

Standard 90.1-2013 has two paths to compliance: the prescriptive path and the performance path. Both paths require that all mandatory requirements be met. The prescriptive path is a straightforward recipe for compliance but is restrictive in that all items must be met. The performance path is more flexible in that nearly any component’s performance can be traded off for something else, but it requires energy modeling-and more effort on the part of the design team. That effort typically involves collaboration between the architect and engineer on an integrated design and can result in a much higher quality end product. For example, the energy model could be used to show how glass with a lower solar-heat-gain coefficient (SHGC) reduces peak cooling loads.

Review the mandatory requirements

Mandatory requirements are exactly what the term implies: There is no alternate path around them. The entire project team needs to review the mandatory requirements to ensure every discipline includes them in their design. Not including a mandatory requirement causes the entire project to be noncompliant.

Minimum mechanical equipment efficiencies have been mandatory since the first edition of Standard 90 was published in 1975. For the first time, minimum efficiency requirements have been added for commercial refrigeration equipment and commercial refrigerators and freezers. The space types that are required to have automatic receptacle control have been expanded to include conference rooms, print/copy rooms, break rooms, and classrooms. Previously, they were only required in private offices, open offices, and computer classrooms.

A requirement also was added in the power section for separate electrical-energy monitoring for total electrical energy, HVAC systems, interior lighting, exterior lighting, and receptacle circuits. Significant changes have been made to the interior lighting controls. The space-by-space lighting-power density (LPD) table has been revised to include the minimum lighting control requirements per space type using the nine types of lighting controls introduced in ASHRAE 90.1-2013, section 9.4.1.1. The possible lighting control functions from section 9.4.1.1 are:

  • Local control
  • Restricted to manual on
  • Restricted to partial automatic on
  • Bi-level lighting control
  • Automatic daylight responsive controls for sidelighting
  • Automatic daylight responsive controls for toplighting
  • Automatic partial off
  • Automatic full off
  • Scheduled shutoff.

Table 9.6.1 divides the lighting control functions into columns and the space types into rows. All lighting control functions with REQ in their column for a space type are required. When ADD 1 or ADD 2 appears in the row for a space type, at least one of the ADD1 functions and one of the ADD2 functions must be used for that space type.

There are exceptions to the lighting control function requirements, though. For example, enclosed offices less than 250 sq ft are required to have automatic daylight responsive controls for sidelighting, but 9.4.1.1(e) exception 2 says it is not required if the total glazing area in that space is less than 20 sq ft.This is why it’s important to start with the lighting control functions in the table and cross-check the specifics of their requirement and exceptions in the text of section 9.4.1.1.

Another important distinction is that if ASHRAE 90.1-2013 is chosen as a compliance path to the IECC, only the mandatory requirements of Standard 90.1 must be met. This distinction is especially important if a project is also seeking certification with an above code program in addition to meeting minimum code compliance, which does not provide a certification. If an IECC compliance path was chosen, then the mandatory requirements of both the IECC and the above code program standard must be met.

For example, if a project is pursuing certification under LEED v4 and the energy code is the 2015 IECC, the project would have to meet the mandatory requirements of ASHRAE 90.1-2010 and the 2015 IECC.

To further complicate things, if Standard 90.1-2013 is chosen as the compliance path for the 2015 IECC, then the mandatory requirements of both standards 90.1-2010 and 90.1-2013 would have to be met.

No requirements from the 2015 IECC would need to be included even though it is the adopted code. It is important to talk to the authority having jurisdiction (AHJ) in advance to ensure they understand how you intend to comply with the adopted code when using a compliance path with which they may not be as familiar.

Assess the prescriptive path

Figure 2: This graph tracks the relative energy performance of ASHRAE Standard 90.1 versus the International Energy Conservation Code over several years. Courtesy: American Council for an Energy-Efficient EconomyThe prescriptive path is the most commonly used path to compliance. It has become synonymous with ComCheck, the Department of Energy’s program for documenting prescriptive compliance. Because many professionals are comfortable with this approach, it is often the first path investigated. The best way to plan for prescriptive compliance is to have all team members include the prescriptive requirements in their base design. Electrical engineers need to check whether any lighting requirements would cause them to exceed the lighting-power allowance. Mechanical engineers need to check whether components like economizers and energy-recovery ventilators are required. Architects need to look at the minimum envelope performance. If there is a problem with any prescriptive item, it should be brought up at this time.

Generally, if any one prescriptive item is not met, then the project cannot comply with the prescriptive path; when it comes to the envelope, however, there is another option. ASHRAE 90.1-2013, Section 5.6 describes the building envelope trade-off option, which allows just the performance of the envelope components to be traded off with one another within the prescriptive path.

This is how many buildings with high WWR have complied with the standard even though the prescriptive maximum is 40%. The easiest way to use the Building Envelope Trade-Off Option is through the ComCheck software.

With each version of Standard 90.1, the minimum performance of the envelope is raised. The Building Envelope Trade-Off Option only works if some other part of the envelope has significantly higher performance than the prescriptive minimum to trade off. Higher-performance minimums allow for less opportunity to trade off, so it is no longer the "get out a jail free" card it used to be.

For instance, a project with 45% WWR ratio in climate-zone 4A would not meet the prescriptive requirements because it exceeds the max WWR of 40%. However, if the SHGC of the glazing is 0.3 rather than the prescriptive 0.4, then the trade-off option could show that the reduction in SHGC makes up for the increased WWR, which would allow for prescriptive path compliance.

The vertical fenestration orientation requirement has been heavily revised. Instead of just having one option purely based on the areas of glazing facing a particular direction, now a second option uses the fenestration areas weighted by the SHGC of the glass compared to the total fenestration area times the prescriptive max SHGC divided by four.

The angle thresholds for what is considered north-, south-, east-, and west-facing fenestration have been revised as well. The change in angle thresholds eliminates ranges from the ASHRAE 90.1-2010 definition that were unclassified, making the east and west ranges 7.5 deg wider and the north range 45 deg larger. The increase in the east and west ranges will make it more difficult to comply with this requirement; but overall, the ranges better align with the solar path.

From a mechanical standpoint, economizers are prescriptively required in more climate zones, and energy-recovery ventilators (ERVs) are required on smaller systems with less outside air. Most of the electrical performance increases are mandatory. The maximum LPDs are getting incrementally lower, though. If the team doesn’t think it is appropriate for an ERV or economizer to be included in the design when required, the max LPD is exceeded, or any other prescriptive reqirement is not met, then the only option for compliance is the performance path.

Evaluate the performance path

Figure 3: This graph indicates the energy-performance impact of varying wall R-values. Courtesy: TLC Engineering for ArchitectureUnless every single prescriptive element is met, then a project must comply via the performance path. The design team’s energy modeler will need to determine whether the building complies with the performance path. This will give the design team its first real check for how the design components work together. The energy model may then be used to see what energy-conservation measures (ECMs) need to be incorporated into the project to comply. Because the energy model gives an estimate of annual energy cost, the return on investment for each ECM can be calculated to determine the most cost-effective way to achieve compliance. With energy-performance goals becoming more common in projects, it may be a required deliverable to evaluate different ECMs.

On the performance path, the performance of any prescriptive component may be traded off with another. An energy-cost budget (ECB) model must be performed using the instructions in Chapter 11 of ASHRAE 90.1-2013 to show that the design energy cost does not exceed the ECB.

The performance path is often seen as a conciliation prize for failing to comply with the prescriptive path, but it really offers the most flexibility to the design. An energy model can be used to compare the performance of the walls, roof, and windows to determine the optimum values for the particular building-rather than using the code minimum. Using code-minimum values is a great starting point, but minimum values aren’t optimal for every building. High internally loaded buildings can actually use more energy when the insulation is increased.

For example, increasing the roof insulation of a hospital in climate-zone 4 to R-60 can trap heat in the building, which in turn increases the cooling energy because the heat now has to be removed using the HVAC. An energy model quantifies the impact of these decisions for the design team. Mechanical systems and control sequences also can be compared rather than using rules of thumb. This way, the payback for a higher-efficiency mechanical system can be weighed with increased envelope performance to find the greatest cost benefit.

Decide on the compliance path

Figure 4: This graph is an example of the energy-performance impact of varying glazing U-values. TLC Engineering for ArchitectureOnce both paths have been evaluated, the team has to decide on a direction. It’s important to make this decision at the schematic design phase. If a significant change, like reducing the WWR, has to be made at design development (DD) for code compliance, there are major downstream effects due to the cost of redesign for all disciplines of the design team.

There are pros and cons to each. Code officials are used to seeing the standardized ComCheck reports, so it is often seen as a more streamlined path to approval. There are standard forms for documenting the performance path, but there can still be confusion when code officials aren’t familiar with reviewing them. ComCheck can easily be filled out by each discipline, while the performance path requires an energy model. If a budget for energy modeling wasn’t part of the owner’s vision, it can be an additional expense that has to be proven to benefit the project.

There is a lot of simplicity with the prescriptive path, but it doesn’t offer the tailored solution of the performance path. The performance path allows for the best solution for each particular project to be used. A building may even be able to comply via the performance path without certain components required in the prescriptive. For example, air-side economizers don’t have the opportunity to operate often in climate-zone 2A, but they are still required prescriptively. The performance path allows them to be omitted as long as the performance is compensated for somewhere else.

After deciding the prescriptive path was too limiting, architecture/engineering design teams have found that they still couldn’t comply with the performance path. A building with code-minimum glass performance and code-minimum HVAC equipment still won’t comply if it has an 80% WWR. At that point, if the HVAC and lighting performance can’t be improved to compensate for the increased WWR, teams often resort to reducing the WWR, sacrificing elements of the architect’s vision. In many cases, this is dictated by the project budget and/or HVAC system restrictions.

This is why it’s important to evaluate both compliance paths before the project gets to the DD phase. Significant design changes after DD can be expensive, delay the schedule, and irritate the client. Simple energy studies using shoebox models can be done during conceptual design to ensure noncompliant design scenarios don’t make it past schematic design (SD). The results of earlystudies also facilitate cross-discipline discussion of design elements.

Perform compliance calculations and documentation

If the prescriptive path is chosen, then each discipline should complete its portion of the ComCheck. Because the prescriptive path is a recipe for the code-minimum building, ComCheck requires each prescriptive requirement to be input. There is also a section to confirm that each mandatory requirement has been met. Each section’s compliance report can be generated separately to be easily combined into a submission package. The ComCheck reports are typically accepted in two ways, either by placing the reports on sheets in the drawings or by submitting them separately with the permit documents. Some states, however, have their own software tools for documenting compliance. Check the DOE energy codes website for state-by-state compliance options.

Documenting the performance path is not quite as straightforward. Some energy-modeling software can generate an ECB report for submittal. Certain states, such as Florida and California, only allow specific software to be used for generating the acceptable report format. The best way to submit performance-path documentation is to use the ECB compliance forms in the ASHRAE 90.1-2013 User’s Manual and attach the energy-model output report as supporting documentation. ASHRAE also has interactive versions of the forms on its website. Most important, meet with the AHJ early to learn about expectations for performance-path-compliance documentation so there aren’t any surprises when applying for a building permit.


Cory Duggin is an energy-modeling wizard at TLC Engineering for Architecture, providing building-performance simulation efforts across the more than 375- person firm through both direct project involvement and by supporting project teams on specific and unique modeling issues. He has experience working on projects of all sizes in multiple states under various versions of ASHRAE 90.1 and the IECC. His project experience includes comparing energy-code-compliance paths for projects under the 2015 IECC. He is a member of the Consulting-Specifying Engineer editorial advisory board.

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