7 min read
What is a PIV unit?
A PIV unit is the single most effective fix we fit for whole-house condensation and the mould that comes with it. It is also one of the most misunderstood. Here is what it actually does, what it costs, and the situations where it will not work — so you can decide if it is right for your home before spending a penny.
How a PIV unit works
PIV stands for positive input ventilation. A small, quiet fan unit — usually fitted in the loft above the landing — draws fresh air from the loft space, filters it, and gently pushes it down into the house through a ceiling diffuser.
That constant trickle of fresh air slightly pressurises the house. Moisture-laden air is continuously diluted and pushed out through natural leakage points, trickle vents and gaps, rather than settling on cold walls and windows as condensation.
The result is that the whole house's humidity drops to a level where condensation — and therefore mould — struggles to form, without you having to change how you live.
What it costs to buy and run
A quality unit supplied and professionally installed typically costs a few hundred pounds depending on the model and the layout of the house. Running costs are genuinely tiny — most units draw less power than a low-energy light bulb, usually costing around £1 to £2 a month on electricity.
Many units include a low-wattage heater option that takes the chill off the incoming air in winter. It is worth having in a cold loft, and it adds only a small amount to the running cost.
Which homes it suits
PIV works best in houses and bungalows with a loft where the problem is general condensation: streaming windows most mornings, black mould appearing in several rooms, musty smells, and damp patches on cold external walls.
It is particularly effective for families — drying clothes indoors, cooking and showers produce litres of moisture a day that no amount of window-wiping will keep up with.
Flats without a loft can use a wall-mounted PIV version that draws air from outside instead. The principle is the same.
When a PIV unit will not fix the problem
PIV deals with moisture in the air. It will not fix water coming through the fabric of the building. If your damp is caused by rising damp, a leaking gutter, cracked render or a plumbing leak, that has to be repaired first — no ventilation unit can dry out a wall that is being constantly wetted.
It also will not overcome a house that is never heated. If rooms sit at ten degrees all winter, surfaces will condense whatever the airflow. PIV works alongside reasonable, steady heating, not instead of it.
This is exactly why we survey before recommending anything. A quick set of readings tells us whether your problem is condensation (PIV territory) or a structural damp issue that needs different treatment.
PIV versus extractor fans and dehumidifiers
Extractor fans are the right tool for wet rooms — bathrooms and kitchens — where moisture is produced in bursts. But they only work while they run, and most households underuse them.
Dehumidifiers treat one room at a time, need emptying, and cost noticeably more to run than a PIV unit. They are a decent short-term measure, not a whole-house fix.
The setup we fit most often is both: decent extraction in the bathroom and kitchen to catch moisture at source, plus a PIV unit keeping the rest of the house dry. Between them they cover every room, every day, automatically.
What happens on an installation
A typical loft installation takes two to three hours. We fit the unit, run the ducting to a diffuser over the landing, wire it in, and set the airflow to suit the size of the house. There is no mess in the living space and nothing to maintain beyond an occasional filter check.
If mould is already established, we clean and treat the affected areas at the same visit, so the house is dry and clean from day one rather than waiting months for the mould to die back on its own.
Frequently asked
Is a PIV unit noisy?
No. A correctly installed unit is quieter than a fridge hum and sits in the loft, so most households never hear it. Rattling or droning usually means a cheap unit or poor installation.
Will a PIV unit make my house cold in winter?
The airflow is a gentle trickle, not a draught. Units with a thermostatic heater take the chill off incoming air, and because drier air is cheaper to heat, most households find heating bills stay the same or improve.
How long before the condensation stops?
Most homes see windows clear within days. Existing mould stops spreading once humidity drops, but the staining itself still needs proper cleaning and treatment.
Do I need a PIV unit if I already have trickle vents?
Trickle vents help, but they are passive — they rely on wind and temperature difference. A PIV unit actively pushes fresh air through the house every hour of the day, which is why it succeeds where vents alone have failed.
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