Introduction

Ventilation strategy represents one of the most consequential decisions in retrofit design. The choice between Mechanical Ventilation with Heat Recovery (MVHR) and Mechanical Extract Ventilation (MEV) fundamentally shapes energy performance, installation complexity, occupant comfort and whole-life costs. Both technologies have legitimate applications within the retrofit landscape, but their suitability depends on building characteristics, retrofit scope and performance objectives.

Understanding MVHR

MVHR systems operate on a balanced principle: air is simultaneously extracted from wet rooms and living spaces whilst fresh air is supplied to habitable areas. A heat recovery core transfers thermal energy from outgoing warm air to incoming fresh air, typically recovering 75-90% of heat that would otherwise be lost.

Key characteristics of MVHR:

MVHR performs optimally when buildings achieve good airtightness. In retrofit contexts, this creates a chicken-and-egg scenario: MVHR benefits compound when carried out alongside fabric improvements, but the investment in both measures simultaneously increases project costs substantially.

Understanding MEV

MEV systems extract moisture and pollutants from specific locations—typically kitchens, bathrooms and utility rooms—and exhaust them outside. Supply air enters through background ventilators or trickle vents in windows and external walls. No mechanical supply system exists; air movement relies on pressure differentials and natural infiltration routes.

Key characteristics of MEV:

MEV functions effectively in buildings with natural infiltration characteristics. Retrofit projects focusing on specific improvements—upgraded windows, insulation, heating—can integrate MEV without requiring comprehensive airtightness work.

Comparative Performance Analysis

Energy efficiency

MVHR delivers superior thermal performance where airtightness standards are met. Modelling suggests MVHR can save 1.5-2.5 kWh/m²/year compared to MEV in well-sealed dwellings. However, this advantage diminishes significantly in leaky buildings. MEV may actually deliver better overall energy outcomes in retrofit scenarios where airtightness improvements remain incomplete or infeasible.

Indoor air quality

MVHR's filtered supply delivers superior air quality, particularly beneficial in urban or polluted locations. MEV provides moisture extraction and pollutant removal but relies on unfiltered ambient air entry, which may carry dust, pollen and external contaminants.

Installation and disruption

MEV demands far less invasive installation work. MVHR requires ceiling voids, wall chases and extensive coordination with other services. In occupied retrofit projects, this disruption creates significant logistical challenges and cost implications.

Selecting the Right Strategy

The decision framework should consider:

Conclusion

Neither MVHR nor MEV represents a universally optimal solution. MVHR delivers superior performance in buildings achieving high airtightness standards and justifies its complexity and cost within comprehensive retrofit programmes. MEV provides pragmatic performance benefits with lower capital and disruption costs, particularly suited to staged retrofits and buildings where airtightness improvement remains limited. Effective retrofit strategy demands matching ventilation technology to the specific building context, occupancy needs and programme objectives rather than defaulting to assumed best practice.