PIV (Positive Input Ventilation) Explained

PIV explained — positive input ventilation, and where it fits.

PIV is the simplest whole-house ventilation strategy and a common, low-cost answer to condensation in leakier homes. It works by gently pushing filtered air in, rather than pulling stale air out. Understanding how it differs from MVHR and extract systems — and its real limitations — tells you when it's the right tool and when it isn't.

Positive Input Ventilation (PIV) works by the opposite logic to most ventilation. A small fan — usually mounted in the loft, sometimes in a hallway cupboard — draws air from the roof space (or filtered from outside) and gently pushes it into the home, typically through a single central diffuser on the landing. This slight positive pressure displaces stale, moist indoor air, forcing it out through the building's background leakage, trickle vents and gaps.

How PIV controls condensation

PIV's main job is dilution. By continuously introducing drier air and pushing out humid air, it lowers the average indoor relative humidity, which reduces surface condensation and the conditions mould needs. For a leaky home with a condensation problem and no other ventilation, a PIV unit is often a quick, affordable improvement — which is why it's a popular retrofit fix.

PIV vs the alternatives

PIV compared to other strategies

Strategy	Mechanism	Heat recovery	Best suited to
PIV	Push filtered air in; displace stale air out	None	Leakier homes; budget condensation control
Extract (dMEV/MEV)	Continuously extract from wet rooms	None	Moisture control at source
MVHR	Balanced supply + extract with heat exchanger	75–95%	Airtight, insulated homes
Trickle vents + intermittent extract	Background + on-demand extract	None	Minimum Part F compliance

The limitations

No heat recovery — PIV pushes in cooler air, so there's a heating penalty (mitigated slightly if drawing tempered loft air, but still real). It dilutes humidity at the cost of some warmth.
Can feel cool — occupants sometimes notice the cooler incoming air, especially in winter; some units add a small heater, which adds running cost.
Loft-air quality — drawing from the loft assumes the loft air is clean and dry; in a damp or dusty loft that assumption fails (filtered-from-outside units avoid this).
Unsuitable for airtight homes — as above, it needs leakage to work.
Dilution, not extraction at source — it lowers average humidity but doesn't remove moisture at the point it's generated as effectively as extract or MVHR.

When PIV is the right choice

Common questions from clients.

Will a PIV unit cure my condensation and mould?

In a leakier home with a humidity-driven condensation problem, PIV often helps significantly by lowering average indoor humidity. But it's diluting the symptom, not addressing cold surfaces — so if thermal bridges or poor insulation are the main driver, PIV alone may not be enough. We diagnose the cause first; PIV is one possible part of the solution, not a guaranteed cure.

PIV or MVHR — which should I get?

It depends mainly on how airtight your home is. For an older, leakier house wanting affordable condensation control, PIV can be a sensible choice. For an airtight, well-insulated home — or one being deeply retrofitted — MVHR is the right strategy, because it ventilates a tight envelope and recovers 75–95% of the heat. PIV needs leakage to work and offers no heat recovery.

Does PIV waste heat?

Yes, to a degree — it pushes cooler air into the home with no heat recovery, so there's a heating penalty, partly offset if it draws tempered loft air. That's an acceptable trade-off in a leaky home where the alternative is uncontrolled draughts, but it's exactly why airtight homes use MVHR instead, recovering most of the ventilation heat.