Positive Input Ventilation: How PIV Works, Costs and Limits

Positive input ventilation (PIV) is a whole-house system that runs a quiet fan continuously, usually in the loft, to push filtered air into the home through a central ceiling diffuser. The fresh air dilutes and displaces the humid indoor air, which leaves through gaps and vents. Done properly, it lowers the moisture that causes condensation and mould. Done in the wrong building, it can make damp worse. This guide explains how a unit works, what it costs, how it compares with MVHR, extractor fans and trickle vents, and how to tell whether it suits a particular property.

TL;DR

• PIV is one fan unit, normally in the loft, that supplies filtered air to the hall or landing and continuously dilutes humid indoor air. Installed with the unit and diffuser, it typically costs £700 to £1,000 for a typical home.
• It is a strong retrofit option for condensation in houses with a suitable loft, but it is not a cure for rising damp, leaks or a badly ventilated loft.
• MVHR recovers heat and filters air, but needs ducting and an airtight building. UK systems start at around £3,000.
• PIV outcomes depend on the roof, the diffuser position, door undercuts, occupancy and maintenance, so a survey should come before a purchase.
• Filters need occasional cleaning or replacement, and trickle vents and internal doors should stay open to let air move.

What positive input ventilation is

The Property Care Association defines PIV as the continuous supply of filtered, low-moisture external air to a central point in the home, so that indoor air is diluted, displaced and replaced. Its 2020 guidance on PIV systems describes a fan that draws air from a ventilated loft or from outside and delivers it through a central diffuser.

In practice that means a small box in the loft above the landing, a fan inside it, a filter on the intake and a diffuser fitted into the ceiling below. The fan runs all day at a low speed. It does not heat the house and it does not extract air from the kitchen or bathroom.

Where the stale air goes

The incoming air creates a slight positive pressure inside the building. The moist air is pushed out through natural leakage paths: gaps around windows, door undercuts, trickle vents and, where fitted, extract fans. The PCA guidance describes the same route for displaced air, through leakage paths, background ventilation and, where needed, extract fans.

That is the key difference from an extract system. BEAMA’s guide to energy efficient ventilation notes that PIV does not remove moisture directly at the kitchen or bathroom. The moisture has to find its own way out, so the building must be able to let it go.

Loft-mounted and wall-mounted units

There are two common layouts.

The PCA guidance describes both arrangements. Homebuilding says PIV is less suitable for flats, vaulted ceilings and flat roofs unless a wall-mounted alternative is used.

Some loft units include a heater. A heated unit warms the incoming air slightly, which takes the chill off the supply in colder months, at the price of a higher running cost. An unheated unit costs less to run but delivers colder air in winter. Which is right depends on the property and who lives in it, which is a reason to have the layout assessed first.

Can PIV stop condensation and mould?

Condensation forms when warm, humid air meets a cold surface. Cooking, showering, drying laundry indoors and simply breathing all add moisture. If it cannot leave, it settles on walls, window reveals and behind furniture, and mould follows.

The PCA, RVA and BEAMA joint position statement from February 2023 says that correctly applied, designed, installed and commissioned PIV can reduce indoor moisture. It also says PIV is not a universal solution. Depending on the layout, enhancement extract fans may be needed, and an existing MVHR system should be repaired, adjusted or replaced rather than supplemented with PIV.

A 2023 peer-reviewed study of a UK dwelling reported that PIV combined with an enhancement extract fan reduced ambient moisture and stopped active mould growth. The authors stressed correct location, installation, operation and maintenance, along with the building’s characteristics and how many people live there. One home is not a guarantee for every home, but it shows why the pairing of PIV and extract fans comes up so often.

For the difference between condensation and other moisture problems, see how to tell damp and condensation apart.

The disadvantages of PIV

The limits matter as much as the benefits.

• Air that cannot escape. Homebuilding reported in April 2025 that PIV can worsen damp where air has nowhere to go or is driven into gaps in the walls.
• A poorly ventilated loft. The same article says PIV cannot solve condensation in a loft that is itself poorly ventilated. BEAMA adds that loft units need a well-ventilated loft and an adequately airtight upstairs ceiling to limit recirculation.
• Building-dependent results. The 2023 research perspective is that results depend on roof and ceiling construction, diffuser location, door undercuts, occupancy, climate, settings and continuing maintenance.
• Other moisture sources. PIV does nothing for rising damp, penetrating damp from defective masonry, or a leaking pipe. Those need to be diagnosed and treated at source.
• Limited research. BEAMA’s GPG268 guide notes limited research and says additional measures may be required depending on building shape and layout. It also says PIV is not best practice for new build.

PIV vs MVHR

MVHR stands for mechanical ventilation with heat recovery. BEAMA’s guide explains that it extracts warm, moist air from kitchens, bathrooms and other wet rooms through ductwork and passes it across a heat exchanger. Filtered outdoor air, preheated by the recovered heat, is then supplied to living rooms and bedrooms.

The installation is far larger than a PIV. It involves a central unit, designed and insulated duct routes, terminals, condensate management, controls, balanced airflow in each room and commissioning.

The PIV figures are indicative prices from Homebuilding, with higher prices for more advanced models and controls. The MVHR figures come from the Centre for Sustainable Energy, which says systems start at around £3,000 and can exceed £10,000 depending on size and type. Decentralised MVHR starts at about £600 per fan.

CSE says MVHR is most effective in relatively airtight new-build or comprehensively refurbished homes. Retrofitting is difficult because of the space needed for ducts and the unit, and the need for an airtightness strategy and testing. BEAMA lists the advantages as controlled preheated fresh air, lower heating demand, filtration and reduced condensation risk, and the disadvantages as high initial cost, installation complexity and the need for good airtightness.

So the choice often comes down to the building. A draughty Victorian terrace or a 1960s semi with a standard loft is a natural PIV candidate. A new or deeply retrofitted, airtight home is where MVHR earns its cost. Homebuilding says PIV is probably the better option if airtightness cannot be guaranteed or low-resistance duct runs cannot be designed and installed.

PIV vs extractor fans and trickle vents

These are not rivals so much as parts of one ventilation strategy.

Extractor fans remove moisture at the source, in the kitchen and bathroom, in the minutes after it is produced. PIV works across the whole home and does not do that job. That is why PIV and enhancement extract fans are often paired. If a bathroom fan is weak or ducted badly, the first fix is the fan. Our guides to bathroom extractor fan upgrades and kitchen ventilation options under Part F cover what to fit and what goes wrong with ducting.

Trickle vents are small openings in window frames that allow background airflow. They are passive: the air only moves when wind or temperature differences drive it. GOV.UK’s existing-home ventilation guide tells occupants to leave trickle vents open and unblocked, including in winter, and to keep internal doors open where possible so air can move between rooms. With PIV, those vents and gaps become the exit routes for the displaced air, so closing or sealing them undermines the system.

A simple way to see it:

1. Extractor fans deal with spikes of moisture at the source.
2. Trickle vents give the building a way to breathe.
3. PIV provides a steady, whole-house supply of drier air that pushes moisture out.
4. MVHR does all three jobs in one ducted system, and recovers heat as well.

Is PIV worth it?

For the right property, usually yes. The cost is modest next to the cost of repeated mould treatment, redecoration and the wider damage that persistent condensation causes. The conditions that make it worthwhile:

• Condensation and mould on walls and ceilings, especially in bedrooms and upstairs rooms
• A two-storey house or bungalow with a ventilated, accessible loft
• A building that is not airtight, so air has routes out
• Existing extract fans in the kitchen and bathroom that work properly

The conditions where it is the wrong tool:

• Damp caused by a failed damp-proof course, leaking gutters, defective render or plumbing leaks
• A loft with its own condensation problem
• Flats and vaulted ceilings with no suitable loft, unless a wall-mounted unit is chosen
• Homes with an existing MVHR system

The National Home Improvement Council says the right ventilation solution depends on the Approved Documents, layout, occupancy, pets, heating schedules, pollution, acoustics and overheating concerns, and recommends a competent installer. It treats ventilation as a question of indoor air quality and building fabric together: moisture control, heating, airtightness and pollutant removal all need to be considered.

What installation involves

For a typical loft installation:

1. Assessment. The surveyor checks the moisture readings, the layout, the loft ventilation and the existing fans and vents.
2. Unit placement. The unit is fixed in the loft above the hall or landing.
3. Diffuser. A diffuser is fitted into the ceiling below and connected to the unit.
4. Power and controls. The unit is wired in and set up, with heater and speed settings where fitted.
5. Commissioning. Airflow is checked and the settings explained to the occupants.

BEAMA describes PIV as easy to install and suitable for both major and minor refurbishment. Even so, mechanical ventilation is regulated work. In England, the 2026 edition of Approved Document F says mechanical ventilation must be installed by a suitably competent person, certified through a registered competent-person scheme or building control, and inspected and tested. That edition takes effect on 24 March 2027 for ordinary building work, subject to transitional arrangements. Rules differ in Scotland, Wales and Northern Ireland.

Running costs and maintenance

Maintenance is light. BEAMA says the work centres on inspecting and cleaning or replacing the intake filter. MVHR also needs regular filter and grille cleaning, fan and heat exchanger cleaning and airflow checks. Follow the manufacturer’s instructions for both.

To get the most from a PIV:

• Check the filter on the schedule the manufacturer gives and replace it when it is clogged.
• Keep trickle vents open and internal doors able to move.
• Keep the loft ventilated and do not block eaves vents with insulation.
• Do not switch the unit off in winter to save a few pounds. Heated models cost more to run, so ask about settings at commissioning.

Perspective: fix the cause before the symptom

We see PIV recommended as a universal answer to damp, and it is not one. Our surveys start from the cause of the moisture, not the product. A wall that stays wet because a damp-proof course has failed will not dry out because a fan is running in the loft. A bedroom with black mould on the external corner may have a ventilation problem, a cold-bridge problem, or both.

That is why we look at the full picture: moisture readings, ventilation routes, loft condition, heating pattern and how many people live in the property. If PIV is the right fix, we say so. If extract fans, insulation or repair to the building fabric are the right fix, we say that instead. Our in-house specialists carry out the work themselves, we are PCA aligned and accredited, and treatment work carries a 25-year insurance-backed guarantee.

Our PIV and air quality solutions page sets out how we approach it, and our condensation and moisture control service covers the wider picture. A related case is our mould and condensation solution for a Bournemouth bedroom, where insulation and ventilation were considered together.

— Proofterior

Get the diagnosis before you buy a unit

Mould on the walls does not tell you why it is there. A complimentary survey from our specialists across Dorset, Hampshire and Wiltshire identifies whether the problem is condensation, penetrating damp, rising damp or a mix, and whether PIV, extract fans, insulation or repair is the right answer. You receive a detailed written report that explains the findings.

Book a complimentary property survey, or see the full list of damp, mould and ventilation services. Landlords, agents and property managers can use the same route. Our guide to what a damp survey involves explains what to expect.

FAQ

What is PIV?

PIV is a whole-house ventilation system that continuously supplies filtered air from a central fan unit, usually in the loft, through a ceiling diffuser. The air dilutes and displaces humid indoor air, which leaves through leakage paths, vents and extract fans. It is mainly used to reduce condensation and mould.

Is PIV better than an extractor fan?

They do different jobs. An extractor fan removes moisture where it is produced, such as a shower or hob. PIV lowers moisture across the whole house but does not extract at source. In many homes the best result comes from working extract fans combined with PIV, which matches the findings of the 2023 UK case study.

Can PIV make damp worse?

Yes, in the wrong conditions. If the air cannot escape, or the loft itself is poorly ventilated, moisture can be pushed into wall cavities and gaps. A survey that checks the loft, the layout and the other moisture sources reduces that risk.

How much does PIV cost?

Indicative UK figures are about £250 to £500 for the unit and £700 to £1,000 installed, with higher prices for advanced models. MVHR costs considerably more, starting at around £3,000. Actual prices depend on the property, so ask for a written quote after a survey.

Should I choose PIV or MVHR?

PIV suits most existing homes that are not airtight and have a usable loft. MVHR suits airtight new builds or homes in full refurbishment where ducting can be designed in. If your home already has MVHR, repair or adjust it rather than adding PIV.

Sources

• Guidance Document: Positive Input Ventilation (PIV) Systems, Property Care Association, May 2020
• PIV Joint Position Statement, February 2023
• Energy efficient ventilation in dwellings, BEAMA
• Positive Input Ventilation explained, Homebuilding
• How to fix a PIV unit causing damp, Homebuilding
• Can positive input ventilation tackle surface condensation, 2023 study
• Mechanical ventilation with heat recovery, Centre for Sustainable Energy
• Approved Document F: Ventilation, 2026 edition
• Existing home ventilation guide, GOV.UK
• Ventilation and air quality in buildings, NHIC

Recommended

• PIV & Air Quality Solutions Dorset
• Kitchen Ventilation Options: UK Part F, 30 L/s, and Ducting Pitfalls
• Bathroom Extractor Fan Upgrade: What to Fit and Why
• Interstitial Condensation: Causes, Risks and Fixes