Positive Input Ventilation (PIV) Explained

A positive input ventilation (PIV) unit is typically fitted in the loft (or sometimes in a hallway cupboard in flats without loft access) and continuously introduces filtered fresh air into the home at a low level, gently pushing stale, humid air out through natural gaps, trickle vents, and extractor fans rather than relying on those alone to clear moisture.

How it differs from extractor fans

An extractor fan removes moist air from a specific room, on demand or on a timer. A PIV system works continuously and whole-house, diluting humidity throughout the property rather than targeting one room at a time. The two aren’t mutually exclusive — a PIV unit doesn’t replace bathroom or kitchen extraction, it supplements it for the wider condensation problem that affects hallways, bedrooms, and other rooms without their own extraction.

When it’s worth considering

• Condensation and mould affecting multiple rooms, not just the usual bathroom/kitchen hotspots.
• You’ve already addressed the basics (habits, existing extraction) from our prevention checklist without enough improvement.
• A property with limited natural ventilation — for example, a well-sealed, recently double-glazed home where draughts that used to clear moisture naturally have been eliminated.

What it costs and what to check

A PIV unit is a bigger investment than an extractor fan but meaningfully less than damp-proofing treatments like DPC injection or tanking — get quotes from more than one installer, and check whether the property’s loft has enough clear space and suitable access for the unit and its ducting before assuming it’s a straightforward fit.