Passive homes are designed to be ultra-energy-efficient, with airtight construction and high-performance insulation that drastically reduces heating and cooling demands. However, a surprising number of passive homes experience lower-than-expected Energy Performance Certificate (EPC) ratings due to issues surrounding ventilation.

In EPC assessments, inadequate ventilation strategies in airtight homes can trigger penalties that offset the benefits of their energy-saving designs.

Airtightness: The Double-Edged Sword in EPC Assessments

Passive homes excel at minimizing unwanted air leakage, which significantly improves thermal efficiency. However, airtightness without adequate ventilation poses risks of poor indoor air quality, condensation, and dampness. EPC software accounts for this by penalizing properties where mechanical or passive ventilation strategies are either absent or poorly specified.

Key Reasons Ventilation Issues Downgrade EPC Ratings in Passive Homes:

1. Assumed Poor Air Quality in Absence of Documented Ventilation

If a passive home lacks documented ventilation systems (such as Mechanical Ventilation with Heat Recovery – MVHR), EPC assessors may apply default assumptions of poor indoor air quality, resulting in a lower rating.

2. No Credit for Uncontrolled Natural Ventilation

While many passive homes use strategic window placement and passive stack ventilation, EPC methodologies prioritize mechanical solutions. Uncontrolled natural ventilation may not be credited as an effective system.

3. Heat Recovery Efficiency Overlooked

If MVHR systems are installed but lack documented efficiency ratings (e.g., SFP – Specific Fan Power, and heat recovery percentages), default conservative values are used, which can downplay their true impact.

4. Moisture & Humidity Penalties

EPC software models risk scenarios where airtight properties trap moisture, leading to condensation. Without a proven ventilation solution, the software reduces the dwelling’s assessed energy performance.

5. Over-Insulation Without Ventilation Mitigation

An airtight, over-insulated envelope without corresponding ventilation strategies is flagged as a potential overheating risk, further impacting EPC scores.

Real-World Efficiency vs EPC Calculations

Passive homes are designed to outperform conventional builds in terms of energy efficiency. However, EPC software’s limitations in recognizing advanced natural ventilation strategies or undocumented MVHR performance can result in ratings that don’t reflect real-world energy savings.

How to Prevent Ventilation-Related EPC Downgrades

  • Document Ventilation Systems Thoroughly: Provide specifications, performance data, and installation details for MVHR or passive ventilation systems.

  • Airtightness Testing is Essential: Ensure that formal air permeability test results are provided to avoid default leakage assumptions.

  • Include Passive Ventilation Strategies in Assessment Notes: Highlight passive stack ventilation, trickle vents, and other features that aid air circulation.

  • Use Experienced EPC Assessors: Choose assessors familiar with passive house standards who can input nuanced details correctly.

EPCrate: Accurate EPC Assessments for Passive Homes

At EPCrate, we specialize in EPC assessments for airtight, energy-efficient homes. Our EPC assessment services ensure that ventilation systems are properly accounted for, giving your passive home the rating it deserves.

Explore our transparent pricing and schedule an appointment via our online booking platform.

Book Your EPC Assessment Today

Ensure your passive home’s cutting-edge ventilation and energy-saving features are recognized. Contact EPCrate to arrange a professional EPC assessment tailored to passive house standards.