Yes, a PIV unit works, for the problem it is designed for. Positive input ventilation cures condensation and the black mould that follows it, across a whole house or flat, by gently pushing filtered air in so that moist air is pushed out. It costs £600 to £1,000 installed and pennies a day to run. It does nothing for penetrating damp, rising damp or a leak, and it is a poor answer to one cold room in an otherwise dry house.
Key takeaways
- A PIV unit sits in the loft (or a cupboard in a flat) and pushes filtered air into the hallway, so stale moist air leaves through vents and gaps.
- It works for whole house condensation: mould in several rooms, streaming windows every morning, a musty smell in winter.
- It does not work for penetrating damp, rising damp, a leak or failed cavity fill, because those walls are wet from outside, not from the air.
- Indicative cost £600 to £1,000 installed; the fan draws only a few watts, so running cost is small. 20% VAT.
- A PIV needs the house to leak a little. In a very airtight home, or where every trickle vent is sealed, it has nowhere to push the air.
- Alternatives: humidistat extract fans (£200 each from our quote book), air bricks (£85), insulation to warm a cold wall, or MVHR in a whole house retrofit.
What does a PIV unit actually do?
A positive input ventilation unit is a quiet fan in a box, usually mounted in the loft with a diffuser in the landing ceiling. It draws air from the loft, filters it, and pushes it into the hallway continuously at a low rate. That puts the house at a very slight positive pressure, so the moist air from cooking, showers and breathing is pushed out through trickle vents, extract fans and the small gaps every older house has.
The effect is to replace the moist air in the house several times a day without opening a window. Humidity drops, surfaces dry out, and mould stops growing because there is no longer enough moisture on the wall to feed it. In a flat with no loft, a wall mounted PIV draws air from outside through the wall and does the same job.
When does a PIV unit work?
A PIV is the right answer when the problem is condensation across the whole home rather than in one place, and the house has enough background ventilation for the pushed air to leave. Typical signs:
- Black mould in more than one room, worst in the bedrooms and worst in winter.
- Windows streaming with condensation every morning and puddles on the sills.
- A musty smell that clears when the windows are open and returns when they are shut.
- New windows fitted without trickle vents, after which the mould started.
- Rented homes, where a system that does not depend on the tenant is the practical choice.
Our guide to condensation or damp shows how to tell which kind you have before you buy anything.
When is a PIV unit the wrong fix?
- Penetrating damp. A patch that grows after rain is water coming through the wall from a gutter, downpipe, cracked render or failed pointing. Drier air inside changes nothing. Fix the leak.
- Rising damp. A tide mark with salts at the base of a ground floor wall is ground water. It needs a damp proof course, not ventilation.
- Failed cavity wall insulation. Wet fill carries rain across the cavity. The fill has to come out; see cavity wall extraction.
- One cold room in a dry house. If a single north-facing wall or window reveal has mould and the rest of the house is fine, the problem is a cold surface. Insulate the wall or the reveal.
- A very airtight home. A modern build or a fully retrofitted house with no trickle vents gives the PIV nowhere to push air. It needs a balanced system with extract as well as supply.
How much does a PIV unit cost to buy and run?
| Item | Indicative cost | Notes |
|---|---|---|
| PIV unit, loft mounted, supplied and fitted | £600 to £1,000 | Includes diffuser, wiring to a fused spur and commissioning |
| Humidistat extractor fan, fitted | £200 each | From our quote book; kitchen or bathroom, ducted to outside |
| Air brick, fitted | £85 each | From our quote book; background ventilation |
| Ventilation improvements package | From £500 | From our quote book; fans, air bricks and trickle vents |
Running cost is small. The fan draws only a few watts, similar to a small LED bulb left on. The built in heater, where fitted, runs only in cold weather to take the chill off the incoming air. Because cool air is being pushed in, the boiler does slightly more work in winter; the trade-off is that dry air and dry walls are cheaper to heat than damp ones.
Ventilation is 20% VAT. It is not an energy-saving material under HMRC VAT Notice 708/6. Where a PIV is fitted as part of a wall, loft or floor insulation job, the insulation element is 0% until 31 March 2027 and the quote shows the split.
PIV, extract fans, insulation or MVHR: which one?
| Option | Best for | Indicative cost | Limits |
|---|---|---|---|
| Humidistat extract fans | Moisture at source in kitchens and bathrooms; fit these first | £200 each fitted | Do not treat the rest of the house on their own |
| PIV unit | Whole house or flat condensation in a home with some air leakage | £600 to £1,000 | Needs vents or gaps for the air to leave; no help for damp from outside |
| Wall insulation | Mould on one cold wall or reveal; cold solid walls | IWI from £1,800 a room; EWI £110 to £140 per m² | Must be fitted with working ventilation; 0% VAT |
| MVHR (heat recovery) | Airtight homes and whole house retrofits | Priced on the retrofit design | Needs ducting to every room, so a major job in an occupied house |
Most London homes with condensation need two of these together: humidistat fans in the wet rooms, plus either a PIV for the rest of the house or insulation for the cold walls that carry the mould. PAS 2035 requires the ventilation to be assessed whenever insulation is added, so we do both in one visit; see ventilation after insulation. MVHR belongs in a whole house retrofit where the house is being made airtight on purpose.
How we decide and fit a PIV
- Free survey. We measure wall moisture and surface temperature, check the outside of the house for leaks, and look at the fans and vents, so we know it is condensation and not damp from outside.
- Written diagnosis and fixed quote, stating whether a PIV, fans, insulation or a combination is the right fix, and why.
- Installation by our own team, normally half a day for a loft unit, wired to a fused spur and set for the size of the house. We check the extract fans and vents so the pushed air has a way out, and show you the controls and the filter.
The damp, condensation and ventilation service page lists everything we fit and arrange. If the real cause is a cold wall, internal wall insulation is the room by room option.
Frequently asked questions
Do PIV units really work for condensation and mould?
Yes, for condensation across a whole house or flat. A PIV lowers the humidity indoors so surfaces dry and mould stops growing. It needs working extract fans in wet rooms and some vents or gaps for the air to leave. It does not fix damp coming through the wall from outside.
How much does a PIV unit cost to run?
Very little. The fan draws only a few watts on its normal setting. The optional heater runs only in cold weather to take the chill off the incoming air. In winter the boiler does slightly more work because cool air is being pushed in, which is the cost of moving the moisture out.
Is a PIV better than an extractor fan?
They do different jobs. A humidistat extractor fan removes moisture where it is made, in the kitchen and bathroom, and every home should have those first. A PIV treats the whole house. Where mould is in several rooms, the usual answer is both.
Will a PIV unit fix rising damp or penetrating damp?
No. Those walls are wet because water is coming in from the ground or through the wall, and a PIV only changes the air inside. Rising damp needs a damp proof course; penetrating damp needs the gutter, pointing, render or cavity fault repaired.