Ventilation Strategy in PAS2035 Retrofit

Ventilation design represents one of the most challenging aspects of retrofit work under PAS2035. As buildings are progressively sealed to reduce heat loss, managing indoor air quality becomes increasingly important. A poorly designed ventilation strategy can undermine energy savings, create condensation problems, and compromise occupant health.

Understanding the Challenge

Traditional buildings typically achieve air changes through infiltration—uncontrolled leakage through cracks, gaps and poor seals. During retrofit, when you improve the building fabric, these unintended air changes are reduced or eliminated. This improvement in airtightness means you must now deliberately provide ventilation where it previously occurred accidentally.

PAS2035 requires that ventilation strategy forms part of your retrofit specification. The approach must be evidence-based, proportionate to the building type, and documented clearly for the homeowner or building manager.

Assessment and Design Process

Step 1: Establish Current Ventilation Performance

• Conduct air permeability testing (blower door test) to establish baseline infiltration rates
• Assess existing mechanical systems—extract fans, cooker hoods, trickle vents
• Review moisture patterns and condensation history
• Identify moisture generation points—kitchens, bathrooms, laundry areas

Step 2: Determine Ventilation Requirements

PAS2035 guidance aligns with building regulations and standards. Consider:

• Continuous background ventilation: Typically 0.3–0.5 air changes per hour (ACH) to maintain air quality
• Moisture extraction: Kitchens require rapid extraction during cooking; bathrooms need extraction during and after use
• Occupancy patterns: Family homes, care facilities and offices have different ventilation needs
• Building characteristics: Solid walls, suspended timber floors and pitched roofs each influence strategy

Ventilation Strategy Options

Passive Approaches

For buildings with good natural ventilation potential:

• Trickle vents and background ventilators: Low-cost, passive, but limited control and may increase heating demand
• Stack effect ventilation: Relies on temperature differences to drive air movement; effective in tall buildings with vertical shafts
• Cross-ventilation: Opening windows on opposite sides of a room; user-dependent and unreliable in retrofit scenarios

Mechanical Approaches

For sealed or thermally improved buildings:

• Mechanical Extract Ventilation (MEV): Simple extract-only fans in kitchens and bathrooms; requires passive background ventilation
• Mechanical Ventilation with Heat Recovery (MVHR): Extracts stale air while recovering 75–90% of heat energy; most effective for highly insulated buildings
• Demand-controlled ventilation: Uses sensors (humidity, CO2) to adjust extraction rates, improving energy efficiency

Hybrid Approaches

Many retrofits benefit from combined strategies—passive background ventilation with mechanical extract in high-moisture areas, or MVHR with supplementary trickle vents as backup.

PAS2035 Compliance Requirements

Your ventilation strategy must include:

1. Written specification: Document the chosen approach, installation standard and rationale
2. Commissioning plan: Outline how the system will be tested and balanced
3. User guidance: Clear instructions on operating any mechanical systems and managing passive vents
4. Maintenance schedule: Filter changes, ductwork cleaning and system checks
5. Performance targets: Quantified air change rates or CO2 targets to verify adequacy

Common Pitfalls and Solutions

• Over-sealing without ventilation provision: Creates risk of condensation and poor air quality. Always implement ventilation before or concurrent with air-tightening work.
• Undersizing extract capacity: Leads to inadequate moisture removal. Size kitchen and bathroom fans according to room volume and usage patterns.
• Poor ductwork design: Rigid ducts with minimal bends perform better than long flexible runs. Terminate outside with appropriate backdraughts prevention.
• Inadequate commissioning: Mechanical systems must be balanced and tested to verify they deliver designed air changes.
• User confusion: Provide simple, visual guidance on manual controls and filter replacement intervals.

Energy Interaction

Ventilation represents a heat loss pathway even with recovery systems. When modelling energy performance under PAS2035:

• Account for continuous ventilation losses in calculation tools (SAP, RdSAP)
• MVHR systems typically recover 75–90% of sensible heat; model conservatively
• Demand-controlled extraction can reduce unnecessary heat loss by 10–20% compared to continuous running

Summary

Effective ventilation strategy is integral to successful retrofit. Start with assessment of current conditions, match ventilation approach to building type and occupancy, and document decisions transparently. Avoid the false economy of saving on ventilation specification—poor air quality and condensation problems quickly undermine retrofit benefits and homeowner satisfaction.

Engage specialist ventilation designers where MVHR or complex hybrid systems are proposed, and ensure thorough commissioning and user education form part of project delivery.