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
| 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
Running costs and the heater question
PIV's appeal is its low cost — both to install and to run. The fan itself is small, typically drawing only a few watts continuously, so the electricity to run it is modest. The real cost is indirect: the cooler air it introduces has to be heated by the home's heating system, an energy penalty PIV cannot avoid because it has no heat recovery. Some loft-mounted units take the edge off by drawing air that the loft has slightly warmed on a sunny day, but in the depths of winter the incoming air is cold. Many units offer an integral electric heater to temper the supply on the coldest days; this removes the cool-draught complaint but adds meaningful running cost, since resistance heating is expensive. The honest position is that PIV trades a small fan cost and a modest heating penalty for effective humidity dilution — a good deal in a leaky home, a poor one in a tight house where MVHR would recover most of that heat instead.
Where the air comes from matters
A loft-mounted PIV unit assumes the loft air it draws on is clean and dry, and that assumption is not always safe. A damp loft (poorly ventilated, with its own condensation), a dusty one full of old fibreglass, or one harbouring contaminants will have that air pushed into the living space. In areas with elevated ground radon, drawing from the loft is generally preferable to drawing from low level, but the loft's own condition still matters. Better units draw filtered air directly from outside rather than from the roof void, which gives a known, filtered supply at the cost of a slightly more involved installation. When specifying PIV, the source of the air — and its filtration — deserves as much thought as the unit itself; pushing poor-quality loft air around a home solves a humidity problem while creating an air-quality one.
