Introduction: Ventilation in Retrofit Design

As retrofit professionals implement PAS 2035 compliant upgrades, ventilation strategy has become central to project success. When improving airtightness through fabric upgrades, the need for effective mechanical ventilation becomes critical. Two primary systems dominate UK retrofit specifications: Mechanical Ventilation with Heat Recovery (MVHR) and Mechanical Extract Ventilation (MEV). Understanding their differences is essential for specifying appropriate solutions.

What is MVHR?

Mechanical Ventilation with Heat Recovery systems actively manage both supply and extract air throughout a property. Fresh air is drawn in through a central unit, filtered, and distributed via ductwork to habitable rooms. Simultaneously, stale air is extracted from wet rooms and kitchens. Critically, a heat exchanger recovers warmth from outgoing air and transfers it to incoming fresh air, typically recovering 70-90% of heat energy.

MVHR advantages

• High heat recovery efficiency reduces heating demand and operating costs
• Filtered supply air improves indoor air quality
• Controlled, predictable ventilation rates ensure compliance with Building Regulations
• Particularly effective in well-sealed properties where passive ventilation is insufficient
• Supports achievement of lower EPC ratings and fabric energy efficiency targets

MVHR considerations

• Higher capital cost (typically £2,500-£5,000+ installed)
• Requires extensive ductwork, presenting design challenges in retrofit contexts
• Needs regular filter maintenance (quarterly to six-monthly intervals)
• Requires dedicated space for plant room or loft-mounted unit
• More complex commissioning and ongoing performance verification

What is MEV?

Mechanical Extract Ventilation removes stale air from wet rooms and kitchens through ducted extract fans, creating negative pressure that draws fresh air through background ventilators or infiltration. Unlike MVHR, MEV does not condition incoming air or recover heat. It is a simpler, extract-only approach to mechanical ventilation.

MEV advantages

• Significantly lower capital cost (typically £800-£1,500 installed)
• Minimal ductwork required; often uses existing building cavities
• Straightforward installation, particularly in retrofit scenarios
• Low maintenance requirements
• Simpler commissioning and commissioning verification

MEV considerations

• No heat recovery; heating energy is lost with extracted air
• Relies on uncontrolled background ventilation or infiltration for fresh air supply
• Performance dependent on adequate background ventilation provisions
• May not achieve same indoor air quality standards as MVHR
• Less effective in properties with poor natural ventilation characteristics

Regulatory and Standards Context

Building Regulations Part F (Ventilation) requires adequate fresh air supply and moisture removal in all properties. For retrofit work, compliance depends on the scope of works and energy performance requirements under Part L.

PAS 2035 emphasises fabric-first approaches, which typically involve sealing air leaks and upgrading insulation. These interventions significantly reduce uncontrolled infiltration, creating a dependency on mechanical ventilation to meet Part F requirements. The standard expects retrofit coordinators to specify ventilation systems that maintain indoor air quality whilst minimising energy loss.

Where fabric improvements achieve airtightness levels below 5 air changes per hour at 50Pa, MVHR becomes increasingly justified on both performance and regulatory grounds. In less aggressive retrofit scenarios, MEV may satisfy requirements at lower cost.

Selecting the Right System

Effective ventilation strategy requires considering several factors:

Property characteristics

• Existing airtightness and planned fabric improvements
• Available space for plant and ductwork
• Structural constraints affecting installation

Performance targets

• Required improvement to EPC rating
• Fabric energy efficiency objectives
• Occupant comfort and indoor air quality expectations

Financial considerations

• Available budget and lifecycle cost analysis
• Potential grants or incentive schemes
• Operating cost implications over system lifetime

Maintenance capacity

• Occupant capability and willingness to maintain systems
• Access arrangements for filter replacement

Conclusion

Neither MVHR nor MEV represents a universal solution. MVHR delivers superior performance in highly retrofitted properties where heat recovery justifies higher investment. MEV provides cost-effective compliance where fabric improvements are moderate and existing ventilation characteristics remain suitable. Retrofit coordinators should base system selection on detailed site assessment, performance modelling, and alignment with overall retrofit objectives, ensuring specifications serve both regulatory compliance and client outcomes.