Ventilation
Positive Input Ventilation (PIV) Explained
Positive Input Ventilation is a popular whole-house approach to condensation. Here is a neutral look at how it works, where it shines and where it falls short.
In short
Positive Input Ventilation (PIV) is a whole-house system that gently introduces filtered air from a central unit — usually in the loft — to dilute humid, stale indoor air and encourage it out through natural leakage. It can be effective against condensation in the right home, but it does nothing for penetrating or rising damp and relies on the house being able to let air out.
Key takeaways
- PIV works by gently pressurising the home with filtered air, diluting and displacing humid air.
- It targets condensation across the whole house, not localised building faults.
- It helps most in homes with a genuine condensation problem and somewhere for air to escape.
- It does nothing for penetrating or rising damp, which need physical repairs.
- It is one option among several — compare it with extract and heat-recovery systems.
Positive Input Ventilation, almost always shortened to PIV, is one of the most heavily marketed solutions to condensation and mould. It can genuinely help in the right circumstances — but it is not magic, and it is the wrong tool for some problems. Here is a straight, neutral explanation.
How PIV works
A PIV unit, typically mounted in the loft, draws in air, filters it, and gently pushes it into the home through a central diffuser, usually on the landing ceiling. This creates a slight positive pressure indoors.
That gentle pressure does two things:
- It dilutes the humid, stale indoor air with drier air.
- It gently encourages the existing moist air out through natural gaps — around windows, doors and trickle vents.
The result, in a suitable home, is a steady lowering of indoor humidity across the whole property, which reduces condensation on cold surfaces and the mould that follows. Some units draw their air from the loft space, while others draw filtered air from outside; some include a small heater to temper incoming air in cold weather.
When PIV helps
PIV tends to work well when:
- The core problem is genuinely condensation, spread across the house rather than confined to one spot.
- The home has a reasonable amount of natural leakage so the displaced air has somewhere to go.
- Moisture levels are moderate and the occupants want a continuous, low-maintenance background solution they do not have to think about.
In those conditions it can quietly transform a home that was prone to streaming windows and mould in cold corners.
When PIV is the wrong solution
Important
PIV does nothing for penetrating or rising damp. If moisture is entering the building through a defect, pushing air around the house will not stop it — the physical cause must be repaired.
It is also a poor fit when:
- The house is very airtight with nowhere for displaced air to escape — the system cannot push air out through walls that do not leak.
- The real issue is a single wet room best handled by a targeted extractor fan.
- Humidity is being driven by a fixable source — an unvented tumble dryer, constant indoor washing — that would be cheaper to address directly.
- The incoming air is cold and untempered, making rooms feel chilly, unless the unit heats it or you keep the home warm.
PIV compared with other systems
| System | Approach | Best suited to |
|---|---|---|
| PIV | Supplies air, pushes stale air out | Whole-house condensation with natural leakage |
| Extract fans | Remove moisture at the source | Wet rooms — bathrooms, kitchens |
| MEV | Continuous central extraction | Consistent whole-house extraction |
| MVHR | Supplies and extracts, recovers heat | Airtight homes, often new builds |
See how home ventilation works for how these fit together.
Honest expectations
PIV is a legitimate, widely used approach to condensation, not a cure-all. Before investing, confirm the problem really is condensation rather than a leak or rising damp, check that your home can let the displaced air out, and weigh it against simpler measures like extractor fans and better habits. Where the diagnosis fits, many households find it a convenient, effective background solution; where it does not, the money is better spent elsewhere.
Frequently asked questions
In a home where the problem is genuinely whole-house condensation and air can escape naturally, PIV can substantially reduce it by diluting humid air and lowering indoor humidity. It will not help with condensation driven by a specific fixable source or with leaks.
Sources
- Energy Saving Trust: Energy Saving Trust · UK · accessed 27 October 2026
- GOV.UK: Understanding and addressing the health risks of damp and mould in the home · England · accessed 27 October 2026
DampSense Editorial Team
Research & editorial
Our editorial team researches UK building-moisture topics using government guidance, public-health sources and established building-science references. We write to help homeowners and tenants make informed decisions, and we flag where professional assessment is needed.
Credentials & limitations: Research-led editorial team. We are not chartered surveyors; our guides are general information, not a professional building survey.