Phius Building Scientist & Certification Manager Al Mitchell outlines modeling processes included in the ASHRAE 209 Building Performance Simulation Process standard and how they can be optimized for use on Phius projects.
Recently, ASHRAE released an update to standard 209, which was first released in 2018 under the name Energy Simulation Aided Design for Buildings Except Low-Rise Residential Buildings. Renamed Building Performance Simulation Process, this standard aims “to establish minimum requirements for the process of using simulation to evaluate building performance and inform decision making.” I served on the committee for this release, and will continue to work on maintenance of the standard for at least the next two years.
The Phius standard has long been based on building performance simulation (BPS or energy modeling), and I think many of us are good about using WUFI for compliance modeling. However, WUFI and BPS can also be useful tools for making design decisions about a project.
This blog details a few of the 11 modeling cycles from Standard 209 that are most useful for the Phius crowd, and outline how to implement them in WUFI. I also recommend this slide deck from the Phius Pro Forum this past October that Haley Kalvin-Gold, James Ortega and I put together on conducting a feasibility study. These slides contain useful information and solid baseline assumptions to use in a model before the detailed information is available from the design team.
Note that chapters 4 and 5 of the Standard 209-2024 deal predominantly with compliance for the standard as well as other administrative aspects. Some of the informative notes about what to include in reporting and quality assurance in the simulation process are useful, but Phius practitioners can ignore aspects that do not apply. For example, 5.1 requires the use of whole-building simulation software that meets the requirements of ASHRAE 90.1, section G2.2. WUFI does not meet this requirement, but as we are using the standard for guidance rather than compliance, we can ignore this requirement. I would argue that the modeling cycles in chapters 6 and 7 are most useful to Phius Certified Consultants (CPHCs®).
If you have not yet worked in the climate location of the project or on the building type or size, this simple box modeling is useful. Even if you are familiar with the climate and building scope, this can still prove useful to test out ideas, assemblies or building orientation.
For this example, I am going to work through the design and modeling of a mid rise apartment building in Atlanta, Georgia. This is a typology I am fairly familiar with, but a climate in which I am not well versed.
For my simple box model, I want exactly that – to keep it simple. Looking at the informative appendix, I am going to draw up a ~34,000 sf building that is four stories in height with an aspect ratio of 2.7 and a window to wall ratio of 0.15. Taking the size of the building, and an estimated 600 sf per 1 bed unit, I am treating this as a 48-unit, 48-bedroom building. This allows for some quick estimation of MELs (miscellaneous electric loads) and lighting using the Phius Multifamily Calculator using the floor method and an estimated ICFA. I can then calculate the space conditioning targets and start experimenting.
The standard advocates for experimenting with geometry, window-to-wall ratio, window orientation and shading, building orientation, and enveloped specifications. Currently, the building is located long ways north and south, and I have R-30 in the attic, R-5 on the slab, R-17 in the walls, and U-0.19 windows. I have an 80 percent sensible efficient ERV serving the spaces.
About one third of the way down the PH Verification reports are my favorite graphs in WUFI. These four graphs show the energy balance of each of the space conditioning targets, and when I am trying to optimize a design manually I will refer to the graphs to see what the biggest losses are and modify those aspects. Currently, we are well under our targets for the space conditioning loads, so I can reduce envelope performance to save cost and embodied carbon. I will also test spinning the building 90 degrees to see how sensitive the building is to orientation. It looks like I can back off the insulation in the roof as it does not show up on the graph. The heat loss to the ground is also minimal, so potentially I can reduce or eliminate the insulation under the slab. Again, I am not well versed in 3A climates, so this is part of the exploration.
I was able to reduce the insulation in the walls to R-13, eliminate the insulation under the slab, and reduce the window performance to U-0.29. The rotation reduced the loads as well. For a 1 percent increase in source energy usage, I can significantly reduce the cost and embodied energy in the building.
I tested a few different solar heat gain coefficients (SHGC), and kept the lower value to reduce excessive heat gain. The final window had a solar heat gain of 0.31, with some room to reduce if the cooling load changes. Using this simple box model, I was able to explore what building performance values are needed for the 3A climate in Atlanta, and am ready to move forward with the design.
Modeling cycle two involves more exploration of the building architecture, and exploring conceptual designs. At this point, I would ask my architect to provide me with some of the massing models, and I would get more of the context modeled in (the previous model had some shading factors to sample surrounding buildings). For each of these cases I would copy the previous cases from the simple box model and import the conceptual geometry. Then, I can apply the assemblies determined from the previous cycle, update the internal gains, ventilation rates, etc. and calculate the new targets for the building.
Maintaining stable assemblies allows you to test the impacts of the architecture, and show reductions or increases over the previous cases targets. Because the targets are going to change, I would calculate the percentage of the target rather than absolute. For example, if my first case has a heating demand limit of 4.0 kBtu/sf yr, and the design is performing at 2.0 kBtu/sf yr, I am “beating” the target by 50 percent. If my conceptual model geometry has a target of 5.0 kBtu/sf yr but the heating demand in my model is 3.0 kBtu/sf yr, I am beating the targets by 40 percent, at 10 percent reduction in performance from the previous case. Source energy can be compared in absolute terms.
Welcome to where passive buildings shine!
The key element in passive building is load reduction, which reduces demand and therefore source energy. In this case, I know we are going to be applying heat pumps to the project, so I want to reduce whichever load is higher to better balance the HVAC system. WUFI Passive is not a load sizing tool. WUFI calculates the peak loads using a differing protocol, and the cooling load does not include the latent load. This round of modeling should be done in conjunction with the design engineer, who should run load calculations in parallel to ensure accurate sizing information. If looking at WUFI, it is fair to assume that the calculated load has a sensible heat ratio of around 0.6, so I would divide the target by 0.6 to get a decent estimate of the total load.
From our previous optimized case (pretend we are now modeling more detailed geometry with separated windows, etc.), we have a cooling peak load of 2.15, which – including the latent load estimation – is around 3.58, so my cooling peak load is driving the system sizing. The standard recommends evaluating building envelope elements such as insulation, shading, thermal mass, window-to-wall ratio, and other building elements including lighting and daylighting controls, internal equipment selection, and ventilation strategies. For this case, I will change the window performance, shading, and thermal mass of the building.
I was able to reduce the SHGC from 0.31 to 0.25, add a little more reveal depth, and increased the thermal mass of the building (say double-layer gypsum and concrete topping slabs) to reduce the peak cooling load to 2.0 Btu/hr sf (3.33 estimated). My heating load increased to 3.03 Btu/hr sf, but this more closely aligns with my peak cooling load. Now is the time to check with the design and engineer to see if we can reduce equipment sizing to save money and refrigerant charge. The next few modeling cycles guide the reader through more detailed optimization of the building, and are applicable later in design. I will now skip ahead to the other two cycles that intertwine strongly with the Phius process.
Modeling Cycle nine focuses on the impacts of change orders on the project's energy performance goals. Whether it is a new roof assembly or window manufacturer, make a copy of the design-certified case and make the changes. Be sure to note the changes in the feedback form, and ensure the model passes all the targets. Please also be cognizant of any hygrothermal impacts, such as a change from closed-cell to open-cell spray foam, or a previously concrete and foam floor that got changed to an air permeable insulation cavity assembly that is now at bulk water risk.
Modeling Cycle 10 revolves around the as-built performance that is confirmed during final certification. The final certification team at Phius will review the Phius Certified Rater or Verifier’s documentation to confirm that the building was built as designed and review testing performance, inputting the final blower door numbers into the WUFI model. With this, the model reflects the building in its final form, and achieves full certification with the Phius certification program sought.
It is worthwhile to review this model compared to earlier ones, identifying the things that have changed over design and construction and their relative impacts to the performance of the project. It is prudent to note these down for the next project to improve your modeling and analysis skills, as well as to be aware of common changes that happen and advise the design architect on the constructability of the project based on your previous experience.
Studies have shown the experience of the team improved the project's cost efficiency, and making changes earlier on in design saves time and money.
Being a CPHC is not just a credential, it is a craft you can hone over time, and I think ASHRAE 209-2024 is a valuable framework to work within for this continual improvement. I would recommend you purchase and read it from the ASHRAE Standards Store.
