Can Positive Input Ventilation (PIV) systems force moisture into building fabric, causing interstitial condensation, and what are the practical challenges in social housing?

Yes, authoritative sources highlight significant risks when PIV systems interact with building fabrics—especially in retrofitted homes with Internal Wall Insulation (IWI). Below is a detailed breakdown:

1. The Science of Interstitial Condensation

Interstitial condensation occurs when warm, moisture-laden air penetrates wall cavities and cools, depositing water within the structure. PIV systems can exacerbate this by actively pressurizing indoor air, forcing it into cracks, joints, or imperfectly sealed insulation layers. Over time, trapped moisture compromises structural integrity and promotes mould growth.

2. PIV + IWI: A High-Risk Combination

When combined with IWI (common in energy retrofits), PIV’s positive pressure can drive moisture into insulated walls. Official guidance warns that improperly managed ventilation may trap dampness behind insulation, preventing drying and accelerating decay. Moisture risk assessments are critical before installation.

3. Occupant Burden in Social Housing

PIV systems demand high occupant compliance:

• Understanding mechanics: Residents must grasp how moisture movement impacts walls.
• Operational diligence: Keeping trickle vents open, avoiding obstructions, and reporting faults.
  In social housing, tenant turnover, literacy barriers, or limited maintenance support make consistent compliance unrealistic. Landlords often lack resources for ongoing education or monitoring.

4. Mitigation Strategies

Authoritative frameworks recommend:

• Pre-installation risk assessments: Using moisture modelling tools.
• Alternative ventilation: Decentralized extractors or humidity-controlled systems to localize moisture expulsion.
• Design upgrades: Airtight insulation detailing to block air pathways into walls.

Key Takeaway

PIV systems require rigorous moisture management, especially in IWI-retrofitted social housing. Without occupant education and proactive maintenance, these systems risk hidden structural damage. Official guidance prioritizes holistic design over reliance on user behaviour.