Trial / Demo
During our recent webinar, Navigate Title 24 (2025) with Confidence, we received a lot of technical questions from engineers, energy modelers, and building performance specialists navigating the latest updates to California’s Title 24 performance compliance pathway.
With such a wide range of questions submitted during the session, it wasn’t possible to answer them all live. However, clear themes emerged around the updated performance compliance requirements, modeling assumptions in the proposed vs standard workflow, simulation engine behavior within IESVE, and interpretation of key metrics such as Longterm System Cost (LSC) and source energy.
Below, our experts answer the most common questions, offering practical guidance on how IESVE supports Title 24 compliance modeling through the Apache simulation engine and streamlined compliance workflows. To learn more about the latest updates included in Feature Pack 2 of VE 2025, visit our release overview, or explore how IESVE supports Title 24 compliance more broadly. For more opportunities to see the IESVE Title 24 2025 workflows live, register for our upcoming, free, virtual training with PG&E.
1.1 What types of buildings can be modeled for Title 24 within IESVE?
IESVE is approved for Title 24 compliance modeling for new construction non-residential buildings (Title 24 versions 2025, 2022, and 2019).
1.2 Do I need to build all three Actual Design, Proposed and Standard geometry for Title 24 compliance
A single actual design geometry is sufficient, as IESVE automatically applies the necessary restrictions associated with the proposed model. As for the Standard model, the VE will automatically generate the standard model based on the Alternative Calculation Method (ACM).
1.3 Can I use IESVE for multi-family building compliance with Title 24?
IESVE can be used to develop the building geometry for multifamily buildings, which can then be exported into CBECC for compliance modeling. Modelers may use the geometry in the VE for above code analysis.
2.1 Why does Title 24 use a “partially locked” proposed model, and does that reduce accuracy?
The partially locked proposed model is a requirement of Title 24. It standardizes key inputs—such as HVAC systems and performance curves—to ensure consistent comparisons across projects. Refer to 2.2 for examples of differences between models.
2.2 Can you explain the different models that are possible in IESVE’s Title 24 workflow?
Three models are possible within IESVE's Title 24 workflow – proposed, standard, and actual design. Title 24 requires both a proposed and standard model. IESVE automatically generates the standard model based on your proposed model. The “proposed model” represents the building design, but with Title 24’s required constraints applied (such as predefined HVAC systems and performance assumptions). The auto-generated “standard model” is an equivalent building using the prescriptive option. The restriction/requirement of the standard and proposed model is detailed in the ACM. The third, optional model is the “actual design” model, which has no restriction and can be used for above code studies like parametric studies, design exploration and more. Note that the third model is an option model in the VE.
2.3 What kinds of assumptions are built into the standard model, and can they be overridden?
The standard model is defined by Title 24 rules as stated in the Title 24 ACM and includes fixed assumptions for parameters such as internal gains, schedules, HVAC systems, and envelope performance and more. The intention is for the “Standard model” to be equivalent to the proposed model with all the prescriptive rules applied. Therefore the user cannot edit/override the standard model.
2.4 Do you have to use the Navigator workflow in IESVE for Title 24 modeling?
Yes. Using the navigator facilitates Title 24 compliance via automatically importing all relevant HVAC systems, envelope constructions, and other model elements and settings. The navigator also automatically generates the standard model and contains the documentation needed for compliance.
3.1 What simulation engine does IESVE use?
Simulations in IESVE run on the Apache engine. Apache is IES’s in-house dynamic thermal simulation engine, developed and maintained specifically within the IESVE platform. Furthermore, as per the requirement of Title 24 ACM, IESVE is tested in accordance with ASHRAE Standard 140-2023.
3.2 What are the benefits of using IESVE for Title 24 modeling?
IESVE empowers project teams pursuing Title 24 by having three distinct models – proposed, standard, and actual design. See question 2.2 for a thorough description of the different model types. Additionally, IESVE is approved for Solar Access Roof Area (SARA) as well.
4.1 Can I create a custom construction for my proposed model?
Yes, users can create a custom construction, however we are limited to used only Title 24 approved materials, i.e. materials with [T24_####].
4.2 How does air barrier selection impact infiltration rates in IESVE?
Air barrier selections are applied at the space level and influence infiltration using a calculation defined in the Title 24 ACM methodology (see ACM 2025’s section 5.4.2 on Infiltration). Improved air barrier performance reduces the modeled design infiltration rate, which in turn lowers overall building heating and cooling loads and improves energy performance within the compliance model. Users can select the air barrier in the ‘Building Data’ section of the navigator and at the template level.
5.1 Can PV systems be simplified during early design or compliance modeling?
Yes. PV arrays can be modeled as simple rectangular surfaces, especially during early design stages or for compliance-only purposes.
5.2 Is it acceptable to model the entire roof as a single PV surface early in design?
Yes. This is an acceptable approach when detailed panel layouts are not yet designed.
5.3 How is PV modeled for Title 24 compliance in IESVE?
For proposed model, the PV systems must be physically modeled in the geometry in the ModelIT application, as they are used for both SARA assessment and renewable energy calculations.
5.4 Can parametric PV panels be used in Title 24 proposed models?
While parametric panels can be used in general modeling, they cannot be used for Title 24 proposed model.
5.5 How can SARA be used to justify a PV exception due to limited roof space?
If SARA analysis shows that less than 70% of the roof is viable for solar (e.g., due to shading) - the analysis can help demonstrate eligibility for a prescriptive exception. Please review NRCM Chapter 9 for detail.
6.1 Are there limitations on HVAC systems when modeling for Title 24?
Only HVAC systems defined in the Title 24 ACM Sections 5.6-5.8 can be used. If the HVAC system is not Title 24 compliant, it cannot be modeled for Title 24 code compliance.
6.2 Can I edit the HVAC system in the standard model?
No, HVAC systems in the standard model are not editable. But the user can review them in ApacheHVAC.
6.3 Can I create a custom HVAC system for proposed model?
No, systems must be selected from the Title 24 compliant HVAC systems listed in HVAC wizard.
6.4 Can I use a custom performance curve for my HVAC equipment?
Users can use custom performance curves in the actual design model only. Standard and Proposed models are required to use pre-defined Title 24 compliant curves.
6.5 How do I know which of the performance curves are Title 24 approved?
The Title 24 approved performance curves generally have a suffix or a prefix called “Title 24.”
6.6 Do I need to turn off my autosizing for Title 24 compliance?
As stated in the “HVAC Autosizing On/Off for the Proposed model” step in the navigator, the user is not allowed to use the autosized value as the proposed HVAC equipment capacities must match the construction documents.
6.7 How do i model a system that is not listed in the Title 24 complaint system?
Our recommendation is to use a system similar to your model and add the assumptions you made in the “Narrative” section of the “Building Data.” This will be copied over to the PRF1 report that is submitted to AHJs.
6.8 Can I edit DHW capacity in IESVE?
Users can edit the DHW capacity of new or existing DHW system through the HVAC wizard for the proposed model. DHW capacity cannot be edited in the ApacheHVAC application. Please note the ACM (Section 5.9.2) says the rated capacity needs to be manually entered, therefore this value is not auto sized. The steps to edit the DHW capacity is given in our userguide.
