As sustainable design trends push for passive ventilation systems—like stack ventilation, cross-ventilation, and night-purge strategies—property owners are discovering a frustrating truth:
Energy Performance Certificate (EPC) assessments may not fully credit natural airflow solutions, and in some cases, might even penalize them.

At EPCrate, we specialise in assessing eco-conscious buildings where traditional EPC methodologies fall short in recognising passive design efficiencies.

What is Passive Ventilation?

Passive ventilation involves using natural airflows (without mechanical fans or systems) to regulate indoor air quality and temperature. Common strategies include:

  • Cross-Ventilation: Opening windows or vents on opposite sides to allow airflow.

  • Stack Ventilation: Utilizing differences in air density (warm air rises) to drive airflow vertically.

  • Thermal Chimneys, Atriums, and Wind Towers enhancing airflow through architectural features.

These systems are integral to low-energy building design, reducing reliance on mechanical HVAC systems.

How EPC Software Views Ventilation Systems

EPC assessments, especially those using RdSAP (for existing dwellings), rely on simplified inputs:

  • Ventilation is typically categorised as:

    • Natural Ventilation (with or without chimneys)

    • Intermittent Extract Fans (kitchen/bathroom)

    • Mechanical Ventilation (with or without Heat Recovery)

EPC algorithms favour Mechanical Ventilation with Heat Recovery (MVHR) because it quantifiably reduces heat loss through ventilation.
Passive systems, despite their real-world efficiency, are often lumped into generic “natural ventilation” categories with default air permeability and infiltration penalties.

Are Passive Ventilation Strategies Penalized in EPCs?

Not directly—but indirectly, yes:

  1. Default Assumptions on Air Tightness
    If passive ventilation relies on user-operated openings (windows, vents), EPC software may assume higher air leakage rates unless airtightness tests are provided.

  2. Lack of Heat Recovery Credit
    Passive systems don’t mechanically recover heat from outgoing air. The EPC model therefore assumes higher ventilation heat losses compared to MVHR systems.

  3. No Behavioural Efficiency Recognition
    EPC software doesn’t account for dynamic usage patterns where passive ventilation reduces the need for active cooling, resulting in an efficiency underestimation.

How to Ensure Passive Ventilation is Properly Represented in EPCs

  1. Conduct an Air Tightness Test (Blower Door Test)
    Prove that the building envelope is airtight even while using passive ventilation strategies. This can override default air leakage assumptions.

  2. Provide Detailed Design Documentation
    Architectural drawings, CFD airflow simulations, and ventilation strategy reports can support non-default inputs during SAP assessments (SAP 10).

  3. Consider a Full SAP Assessment Instead of RdSAP
    For bespoke-designed passive buildings, RdSAP is too simplistic. A full SAP calculation allows for more detailed airflow modelling and recognition of passive design elements.

  4. Utilise High-Performance Glazing & Thermal Mass Documentation
    Demonstrating how the building fabric supports passive temperature regulation can positively influence EPC outcomes.

  5. Select Assessors Experienced in Sustainable Architecture
    EPCrate’s assessors understand how to navigate the limitations of EPC software for passive buildings, ensuring every possible credit is achieved.

Passive Design vs EPC Ratings: The Methodology Gap

While passive ventilation aligns with real-world energy efficiency and occupant comfort, current EPC methodologies still favour mechanically-driven systems due to their quantifiable data inputs.

This gap is being addressed in future EPC reforms, but until then, documentation and assessor expertise are your best tools to avoid under-scoring.

Book a Passive Design-Aware EPC Assessment with EPCrate

Own a property relying on passive ventilation? Ensure your EPC reflects its design strengths accurately.
Book an assessment with EPCrate for detailed, architecture-sensitive EPC calculations.
Explore our pricing plans or contact us for tailored advice on passive building EPCs.