PIV
Two of them are almost right in a way that quietly rules PIV out of houses where it would have worked. The third is the reason installers keep fitting it in houses where it does not.
"PIV blows cold outside air into the house."
What actually happens
It draws air from the loft, which is warmer and drier than outside air for most of the UK winter because the loft is inside the insulation envelope. The heating then warms the incoming stream by a couple of degrees; the marginal heat cost is small and the dehumidifier savings usually more than offset it.
"All damp is a ventilation problem, so PIV will fix it."
What actually happens
PIV solves condensation mould, which is one specific pattern: moist indoor air meeting cold surfaces. Rising damp, penetrating damp and plumbing leaks look similar to the untrained eye and PIV solves none of them; fit PIV to those and the underlying fault gets worse under a false sense of progress.
"Heated PIV is worth the upgrade for winter comfort."
What actually happens
In a British loft, the incoming air is rarely cold enough to justify a 400 to 800 watt pre-heater running through the winter. The £30 to £60 annual running cost is small in isolation but rarely earns its keep against a diffuser sited sensibly in a hallway; cold PIV is the default answer.
The honest question
Is Positive Input Ventilation the right fix for condensation and mould in your home?
The short verdict
For a pre-1990 UK house with condensation mould in the corners, a ventilated loft above and no easy path to a whole-house retrofit, PIV is the highest-leverage air-quality pound you can spend; for airtight retrofits, warm-roof conversions, flats without a loft, or damp problems that turn out to be rising or penetrating damp, PIV is the wrong tool and will fail in a way that damages both the fabric and your patience.
Condensation on the windows every winter morning, or black mould creeping back into corners no matter how often it's painted over.
Rising damp, penetrating damp from a failed roof or render, and condensation running off cold-bridged surfaces all look like a PIV problem and are not. A damp survey before spending anything makes sure the tool matches the fault.
Kitchen and bathroom extractors, humidistat-controlled and actually run long enough after cooking and showering, remove most of the moisture a UK home generates. Confirming they work is a free first step.
Lids on pans, laundry not dried on radiators, trickle vents open, and a right-sized dehumidifier in the worst room can knock indoor humidity below the mould threshold without any install. If it does, PIV isn't needed.
If you've considered these and PIV is still the right answer for your home, here's how to choose well.
Where the fabric is genuinely leaky and the mould is condensation mould, a £600 to £900 cold PIV install solves the problem in a day and pays back the running cost of daily dehumidifier use within two winters.
The decision, in six questions
Six questions decide whether PIV is genuinely the right answer for your home, worth serious consideration, or the wrong tool for the fault you actually have.
Is the visible mould around windows, in corners or on external-wall surfaces, and worst on north-facing walls?
This is the pattern PIV is designed for; condensation mould from moist indoor air meeting cold surfaces.
Strong candidate
Mould at floor level, on internal walls, or spreading downward from a ceiling suggests rising, penetrating or plumbing damp; get a survey before fitting anything.
Probably not for your home
Do you have a ventilated cold roof loft above the living space (not a warm-roof conversion or flat roof)?
The loft provides the drier air source PIV depends on.
Strong candidate
Without a ventilated loft, PIV cannot draw the air it is designed to; a different ventilation strategy is required.
Probably not for your home
Is the house genuinely leaky by modern standards (roughly above 5 m³/h/m² at 50 Pa, or simply not retrofitted for airtightness)?
The house has enough background leakage to shed the moist indoor air PIV displaces; the physics works.
Strong candidate
In an airtight retrofit, PIV forces moisture into the fabric instead of out through leaks; MVHR is the correct tool.
Probably not for your home
Is a fungicidal wash of the existing colonies planned before the PIV goes in?
Killing the mould gives the PIV a clean slate; without this, the visible improvement is slower and the household loses confidence.
Strong candidate
PIV doesn't clean existing mould; it prevents new growth. Sequence the wash before the install.
Worth considering
Can the diffuser be sited in the central hallway or landing, away from a bed or seating position?
The mild draught is unnoticed in circulation space; cold PIV works comfortably.
Strong candidate
If the only viable diffuser position is directly above a bed or sofa, either accept a heated unit and its running cost, or reconsider the layout.
Worth considering
Is the local air quality tolerable for a G3 or G4 filter (not right beside a heavily trafficked road or industrial source)?
Basic filtration is adequate for pollen and gross dust; the ventilation benefit is real without contaminating the supply.
Strong candidate
On a busy urban main road, PIV's basic filter is not appropriate as an air-quality strategy; look at MVHR with F7 filtration instead.
Probably not for your home
What usually changes the answer
Change one of these and our recommendation often changes with it.
The damp survey identifies rising or penetrating damp.
What changes
The house has been (or is being) retrofitted below 5 m³/h/m² at 50 Pa.
What changes
The problem is confined to one bedroom or bathroom, not the whole house.
What changes
The house sits on a heavily trafficked urban road with real PM2.5 concerns. G3 or G4 filtration is not adequate as an air-quality strategy; MVHR with F7 filters is the honest answer even at higher cost.
What changes
The loft is a warm-roof conversion, an unventilated flat roof or a shallow-pitched cold roof with no accessible space.
What changes
The existing extract fans are broken, disconnected or never used.
What changes
If this were our house
For a 1930s semi with corner mould, a ventilated loft, and daily dehumidifier running
We would kill and clean every mould colony with a fungicidal wash on a Saturday morning, book a cold PIV install for the following weekend, and site the diffuser in the central landing rather than any bedroom. The £700 fitted cost lands well inside the range where PIV pays back the running cost of the dehumidifier the household is currently using every day.
We would live with the cold unit for one full winter before deciding on anything else. If the January diffuser draught became genuinely uncomfortable, the heated version is a straightforward upgrade later; but nine houses in ten find the cold version imperceptible in a hallway. Only after that winter, and with a fabric plan in mind, would we consider whether MVHR belongs in a future deep retrofit conversation.
When we'd say no
We would rather lose the sale than take you somewhere the numbers don't stand up.
The mould pattern is at floor level, spreading downward from a ceiling, or on internal walls. This is rising, penetrating or plumbing damp; PIV will not solve it and the delay lets the underlying fault get worse.
The house has been retrofitted airtight (below 5 m³/h/m² at 50 Pa). Pressurising it drives moisture into the wall build-up and can rot the fabric; MVHR is the correct tool.
The loft is a warm-roof conversion, an unventilated flat roof or otherwise sealed. Without the ventilated loft as an air source, PIV loses the physics it depends on.
The house sits on a heavily trafficked urban road with elevated PM2.5. A basic G3 or G4 filter is not an air-quality strategy in that environment; MVHR with proper filtration is the honest answer.
The problem is one damp bedroom or bathroom, not the whole house. A £600 to £1,200 single-room heat-recovery unit or a humidistat extract fan is the proportionate answer; PIV is overreach.
The quote lands materially above £900 fitted without a specific reason (long cable run, structural work, ducted diffusers). PIV is a £600 to £900 job in almost every UK house that suits it; margin above that needs to be earned.
What most people get wrong
The trade at a glance
A five-step editorial scale, not a rating. Green weight is a gain; grey weight is a cost you carry.
Up-front cost
£600 to £900 fitted for a cold PIV in a straightforward install; roughly a tenth of a whole-house MVHR retrofit.
Effectiveness against condensation mould (right house)
Corner mould, weeping windows and stuffy morning air resolve within one to two winters in a leaky house with a diagnosis that matches.
Effectiveness in an airtight retrofit
Physics reverses; pressurising a tight envelope drives moisture into the fabric rather than out. Wrong tool.
Running cost per year
A cold PIV draws under 20 watts continuously; £30 to £45 in electricity a year. Heated versions add £30 to £60 more.
Install disruption
Half a day, single trade, one diffuser cut in a hallway ceiling; no ductwork through habitable rooms.
Filtration quality
G3 or G4 filter is adequate for pollen and gross dust; not appropriate as a PM2.5 strategy on a busy urban road.
Ongoing maintenance
Filter change every 12 to 24 months; a few pounds and a minute in the loft. Fans are typically five-year replaceable.
Applicability across the UK stock
Most pre-1990 semis, terraces and detached houses with a ventilated loft; a very large slice of the housing stock.
What it costs
The unit itself; add install labour to reach fitted cost.
Includes a 400 to 800 watt pre-heater; rarely earns its keep in a British loft with a sensibly-sited diffuser.
One diffuser in the central landing, short cable run to the consumer unit, existing loft access.
The site-wide standard range; anything materially above needs a specific reason.
The premium is rarely worth it; cold PIV in a hallway is imperceptible to almost all households.
Legitimate premiums where the install genuinely needs more time.
Household job if you're comfortable with a mask and gloves; professional decontamination for widespread colonies.
Under 20 watts continuous at 2026 UK tariffs; typically less than a fortnight of dehumidifier running.
Adds the pre-heater's winter draw; rarely justified against a well-sited cold unit.
A minute in the loft; often the maintenance homeowners forget.
Ranges reflect UK installer quotes from Nuaire, EnviroVent and Vent-Axia approved fitters across five UK regions in 2025 to 2026, plus manufacturer specifications for the standard three-bed dwelling flow rate of 50 to 60 litres per second (BS 5250). Airtightness thresholds draw on Approved Document F guidance and BRE indoor-air research. Refreshed each spring.
You've got the answer
If the mould is around windows, in corners or on north-facing walls, the loft above is ventilated, and the house has not been retrofitted airtight, PIV at £600 to £900 fitted solves the problem within one to two winters and pays back the running cost of daily dehumidifier use inside two years. Get a damp survey if the pattern is anywhere else; fit MVHR instead if the envelope is tight.
You can stop here if you're still deciding. The rest of this page is for readers who have already committed and want to spend the money well.
If you're going ahead
This half of the page is written for the reader who has decided. It reads in the order the decisions come at you.
Which version
A loft-mounted unit that draws loft air through a filter and pushes it into the central hallway at 50 to 60 litres a second. No heater; the incoming stream sits at loft temperature and is warmed by the house's heating. The default answer for most UK homes.
When we'd choose it
The vast majority of pre-1990 UK homes with an accessible ventilated loft and a diffuser position in a hallway or landing rather than directly above a bed.
The same unit with a 400 to 800 watt pre-heater that tempers the incoming stream when the loft drops below roughly 14°C. Slightly higher capital, £30 to £60 more running cost per winter, and a small comfort improvement where the diffuser sits in a lived-in room.
When we'd choose it
The narrow case where the only viable diffuser position is directly above a bed or seating area and cold PIV would be noticeably uncomfortable in January.
A compact through-wall unit for one damp bedroom or bathroom; supplies and extracts on the same wall with a small heat exchanger between the airstreams. Not PIV; a different tool with a different job at £600 to £1,200 fitted per room.
When we'd choose it
When the problem is confined to one room rather than the whole house, and pressurising the whole envelope would be overreach.
Low-wattage humidistat extract fans running continuously in wet rooms; supply air comes in through trickle vents and background leakage. Different physics from PIV but similar cost bracket and similar disruption.
When we'd choose it
Houses where the moisture load is concentrated in the kitchen and bathrooms and the rest of the home is not showing mould; an honest alternative to PIV in a leaky house.
Whole-house Mechanical Ventilation with Heat Recovery. Filtered, tempered fresh air to every habitable room and heat recovery from the extract. Requires airtightness below 3 m³/h/m² at 50 Pa and a specialist designer; £8,000 to £14,000 fitted retrofit.
When we'd choose it
Airtight retrofits and new builds where PIV would compromise the fabric; the correct tool once the envelope is tight enough to support it.
Your house
PIV depends on the house being leaky enough for the moist indoor air to shed through the background gaps. Above roughly 5 m³/h/m² at 50 Pa (typical for an untreated pre-1990 UK home) the physics works cleanly.
Below 5 the recovery of moisture-laden air slows; below 3 the pressurisation starts driving moisture into the wall build-up rather than out through leaks. If airtightness works are planned, PIV is a short-window tool, not a long-term one.
A ventilated cold roof is the assumption; loft air breathes with the outside and stays drier than internal air for most of the winter. Diffuser sits in the ceiling of a central hallway with the unit above.
Warm-roof conversions, sealed flat roofs and unventilated slate roofs with felt underlays that dew heavily are ruled out; the air source is either sealed off or has its own moisture problem.
The central landing or hallway is the natural position; a mild continuous draught is unnoticed in circulation space. Bedrooms and sitting rooms where a diffuser would sit above a bed or sofa are the awkward cases; either accept a heated unit or reconsider the layout.
Diffuser position is a design decision, not a homeowner one; a good installer will walk the loft with you before quoting rather than deciding on the day.
The standard G3 or G4 filter is adequate for pollen and gross dust in most UK settings; it is not an air-quality strategy against PM2.5 on a busy urban road or near an industrial source.
Where filtration quality matters, MVHR with F7 or better filters is the honest answer even at higher capital cost; retrofitting better filters to a PIV unit is possible but limited by the fan's static-pressure capability.
In use
PIV runs at 50 to 60 litres a second, sized for a typical three-bed dwelling under BS 5250. That is a small enough flow to be inaudible and unnoticed at the diffuser; large enough to lift indoor humidity from the mould-supporting mid-60s per cent range down toward 50 per cent within a few hours of continuous operation.
The pressure differential is a few pascals; nothing structural. Existing windows, doors, trickle vents and skirting-board gaps carry the moist air out; the unit doesn't need any dedicated extract path.
The standard G3 or G4 filter catches pollen and gross dust; it is not an F7 HEPA-grade filter and PIV is not a filtration strategy. Its job is moisture management, not particulate control; where particulates are the concern, MVHR is the correct tool.
In a British loft, the incoming stream sits between 5 and 15°C for most of the winter; the house's heating warms it by the time it has crossed the hallway. A cold unit in a sensibly-sited diffuser is imperceptible to almost all households.
The heater adds a small comfort benefit only where the diffuser sits directly above a bed or seating area; in those cases, £30 to £60 a year of running cost buys a few degrees of tempering. Cold is the default; heated is a specific answer to a specific placement problem.
Dust mite populations collapse when indoor humidity stays below 50 per cent for extended periods (BRE indoor-air research). PIV in a leaky house typically brings winter humidity from the mid-60s to the high-40s and stays there; the dust mite reduction is a secondary but genuine health benefit for allergy sufferers.
Installation
Half-day install, single trade.
Loft access at first light; the unit is bracketed to a joist or a proprietary frame, the diffuser is cut into the ceiling of the central hallway or landing, a short flexible duct connects the two, and a fused spur off the nearest lighting circuit powers the unit. Commissioning is a humidistat setpoint and a boost speed check.
Same day, afternoon.
The installer runs the unit on boost for an hour to prove airflow at the diffuser, sets the humidistat threshold (typically 65 per cent for existing mould, dropped to 55 per cent after a winter of running), and hands over the filter access instructions. Household is left with a running unit and a quiet house by teatime.
Common mistakes
PIV fitted to a house where the damp turns out to be rising or penetrating; the visible improvement never comes and the underlying fault gets worse.
Diffuser sited directly above a bed or seating area with a cold unit; the mild draught becomes noticeable in January and the household switches the unit off.
PIV fitted to a house that is subsequently retrofitted for airtightness; the physics reverses and moisture starts driving into the fabric. Decommission the PIV as part of the airtightness works, not after.
Filter never changed; airflow drops, the unit works harder and louder, and after two or three years the household switches it off. Filter access is the single biggest maintenance failure.
Existing mould left untreated before the install; the visible improvement is slow, the household loses confidence, and the unit gets blamed. Fungicidal wash first, PIV second.
Unit fitted in a warm-roof conversion or sealed flat roof where the air source is compromised; PIV either draws its own moisture problem into the house or fails to move any useful volume.
Choosing an installer
PIV is a simple product installed well or badly by the same trades; the questions below distinguish the ones who diagnose from the ones who just fit.
Maintenance
Change the filter every 12 to 24 months depending on local air quality; a rural site can stretch the interval, an urban main road cannot. Cost is a few pounds a filter and a minute in the loft. Check the fan runs quietly at annual intervals; if the pitch changes, the bearings are on their way and a five-year replacement is due. Adjust the humidistat setpoint down from 65 per cent to 55 per cent after the first winter of running once the household is used to the drier indoor air.
Real questions
PIV is the honest £600 to £900 fix for condensation mould in a leaky pre-1990 UK house with a ventilated loft; it works precisely because the house leaks, and pays back the running cost of daily dehumidifier use inside two winters. It is the wrong tool for rising or penetrating damp, for airtight retrofits, for flats without a loft, and for houses on busy roads where PM2.5 filtration matters. Diagnose before spending; the diagnosis is the smallest lever with the largest effect on the outcome.
The short-term tool for damp; often the running cost PIV pays back.
The correct tool once the envelope is tight enough to make PIV wrong.
How PIV fits alongside extract fans, MVHR and passive ventilation.
Fabric performance is the other half of the condensation story; cold surfaces are half the fault.
The health case and the cleaning sequence that has to precede any ventilation fix.