Lesson 9 – Comparison Between Ventilation Rate Procedure (VRP) and IAQP Approaches
Ventilation design decisions are often framed as a choice between following prescribed airflow rates or pursuing a performance-based approach.
In practice, the Ventilation Rate Procedure and the Indoor Air Quality Procedure are not competing methods, but tools designed for different levels of understanding, responsibility, and project complexity.
This lesson compares VRP and IAQP conceptually, highlighting their differences in philosophy, application, and implications for system sizing and energy performance.
Two Different Questions, Two Different Approaches
The most important difference between VRP and IAQP lies in the question each method is trying to answer.
The Ventilation Rate Procedure asks how much outdoor air should be supplied based on space type and occupancy assumptions.
The Indoor Air Quality Procedure asks what level of indoor air quality must be achieved and how it can be maintained over time.
Although both aim to ensure acceptable indoor air quality, they start from fundamentally different premises.
Prescriptive Versus Performance-Based Thinking
VRP is a prescriptive approach.
It defines minimum outdoor air rates using predefined tables and equations, assuming typical contaminant sources and average conditions.
IAQP is a performance-based approach.
It allows the designer to define acceptable contaminant concentration limits and demonstrate, using analysis and justification, that these limits are met.
Prescriptive methods prioritize simplicity and consistency.
Performance-based methods prioritize flexibility and accountability.
Data Requirements and Design Effort
One reason VRP is widely used is its relatively low data requirement.
VRP relies on:
- Occupancy counts
- Floor area
- Standard ventilation rates
- Simplifying assumptions about contaminant sources
IAQP requires more detailed information, including:
- Identification of relevant contaminants
- Emission characteristics
- Acceptable concentration limits
- Effectiveness of removal mechanisms
This difference in required effort explains why IAQP is less commonly applied in routine projects.
Implications for System Sizing
Under VRP, conservative assumptions often accumulate, leading to higher outdoor air flow rates and larger HVAC systems.
IAQP has the potential to reduce unnecessary ventilation when contaminant sources are controlled or well understood.
However, it can also result in similar or higher ventilation requirements if contaminant generation is significant.
The key distinction is that IAQP ties system sizing decisions directly to air quality outcomes rather than default assumptions.
Accountability and Risk Allocation
VRP offers a clear compliance pathway.
If prescribed ventilation rates are met, the designer can demonstrate conformity with the standard.
IAQP shifts responsibility toward the designer.
Compliance depends on demonstrating that indoor air quality targets are achieved, which requires sound assumptions and technical justification.
This shift in accountability is a major reason why VRP remains the preferred option in many projects.
When VRP Is the Appropriate Choice
VRP is well suited for:
- Typical occupancy patterns
- Standard building types
- Projects requiring straightforward compliance
- Situations where detailed contaminant data is unavailable
In such cases, VRP provides a practical and defensible design approach.
When IAQP Becomes Valuable
IAQP becomes attractive when:
- Ventilation rates from VRP appear excessively conservative
- Contaminant sources are identifiable and controllable
- Energy performance is a critical design driver
- Advanced filtration or air-cleaning strategies are employed
In these scenarios, IAQP allows ventilation decisions to reflect actual air quality behavior rather than generalized assumptions.
Complementary, Not Competing, Methods
VRP and IAQP should not be viewed as mutually exclusive.
Understanding IAQP improves the way VRP is applied by encouraging engineers to think critically about assumptions, contaminant sources, and system configuration.
Even when VRP is ultimately used for compliance, IAQP thinking can inform better engineering judgment.
Key Takeaway
The Ventilation Rate Procedure and the Indoor Air Quality Procedure represent two different philosophies for achieving acceptable indoor air quality.
VRP emphasizes simplicity and prescriptive compliance, while IAQP emphasizes performance, flexibility, and accountability.
Choosing between them is not about right or wrong, but about selecting the approach that best matches the project’s complexity, risk tolerance, and design intent.
Reflection Question
In your projects, is the choice between VRP and IAQP driven mainly by technical suitability, or by comfort with compliance and approval processes?
Pause here and reflect before continuing.
Consider how understanding both approaches could influence future ventilation decisions.