Three different jobs, not three competing products
People shopping for a ventilation fix tend to compare an extractor fan, trickle vents and a PIV unit as though one of them is the right answer. They are not alternatives. They do three separate jobs — removing moist air where it is produced, letting replacement air in without opening windows, and diluting the moisture that escapes into the rest of the house — and a home with a condensation problem usually needs at least two of them working together. Get clear on which job you are failing at and the choice becomes much simpler.
Extract ventilation: dealing with moisture at source
This is the kitchen and bathroom fan, and it is the highest-value ventilation in most homes because it removes water vapour at the point of generation, before it spreads to cold bedrooms and condenses there.
Approved Document F, the ventilation part of the Building Regulations, sets minimum intermittent extract rates for work covered by the regulations:
- Kitchen with a cooker hood extracting to outside: 30 litres per second
- Kitchen without an extracting hood: 60 litres per second
- Utility room: 30 litres per second
- Bathroom: 15 litres per second
- WC or sanitary accommodation: 6 litres per second
These are the standards for new installations and work under the regulations rather than a retrofit obligation on every existing home, but they are a useful benchmark. Many older fans, and most fans with a clogged grille, a crushed flexible duct or a long run to the outside, deliver a fraction of their rated figure.
Two practical points. A fan that vents into a loft rather than to outside is not extracting — it is relocating your bathroom's moisture into the coldest part of the building, where it condenses on the underside of the roof. This is a common and damaging installation error. And running time matters as much as rate: most of the moisture from a shower leaves in the fifteen to twenty minutes after you finish, so a fan wired to the light switch is doing half the job.
Trickle vents: the unglamorous one that makes the rest work
Trickle vents are the slotted background ventilators in window frames, and they are the component people most often block deliberately — taped over, screwed shut, or specified out when new windows go in.
Their job is to let replacement air in. Air that leaves through an extractor fan has to be made up from somewhere. In a well-sealed home with no background ventilation, the fan fights to pull air through whatever gaps remain, its throughput drops well below its rated figure, and the room stays humid despite the fan running.
Approved Document F sizes these by equivalent area in square millimetres. For a natural ventilation system it gives minimum equivalent areas of 8,000 mm² for habitable rooms and kitchens in multi-storey dwellings, 10,000 mm² for the same rooms in single-storey dwellings, and 4,000 mm² for bathrooms.
Closing them in winter feels sensible and is usually a false economy: the heat lost through a trickle vent is small, and the condensation that follows from sealing the room is not.
PIV: what it actually does
A positive input ventilation unit is normally mounted in the loft, with a diffuser in the upstairs landing ceiling. It draws air from the loft space, filters it, and pushes it gently into the house continuously. The house sits at very slightly positive pressure, so moist internal air is displaced outwards through gaps, vents and flues.
The mechanism is dilution. It does not remove moisture at source; it continually replaces humid indoor air with drier air, lowering average humidity across the whole dwelling.
Where it genuinely helps
PIV suits homes with widespread, low-level condensation across multiple rooms — several bedrooms with mould in the corners and window condensation every morning, rather than one specific wet patch. It is unobtrusive and needs little maintenance. Running costs for an unheated unit are typically a few pounds a year, though heated models in larger homes can reach around £100 annually. Units generally cost between £300 and £1,000 plus installation, and need a competent electrician to connect.
Where it disappoints people
The honest limitations are worth knowing before you spend the money:
- It is not a substitute for extract at source. Diluting a kitchen's worth of cooking moisture across the whole house is less effective than removing it from the kitchen. The Building Regulations approach — extract where moisture is generated — exists for a reason.
- The air is only as good as your loft. The unit draws from the roof void. If that space has its own condensation problem, a leak, or heavy dust, filtration quality becomes important, and lofts are rarely inspected to the standard you would apply to a bedroom.
- Cold draughts are a real complaint. Unheated units introduce loft-temperature air in winter. Where insulation is poor or the diffuser sits above a landing people use constantly, this is noticeable, and some households end up turning the unit down until it does very little.
- Flats without a loft need a different product. Wall-mounted units drawing directly from outside exist for that situation; a standard loft unit is not an option.
- It does nothing about liquid water. A leaking gutter, a failed damp-proof course or penetrating damp through a wall will continue exactly as before.
Which do you actually need?
A reasonable order of priority for most UK homes with condensation problems:
- Check the fans you already have. Confirm they vent to outside, clean the grille and duct, and add or use an overrun timer. This is cheap and frequently solves the problem on its own.
- Open the background ventilation. Unblock trickle vents, or have them fitted if the windows have none.
- Address the moisture you are adding. Indoor laundry drying, lids on pans, doors closed while showering.
- Then consider PIV, if condensation is genuinely spread across the house and the first three steps have not been enough.
Buying a PIV unit while a bathroom fan vents into the loft is spending several hundred pounds to dilute a problem you could have removed.
What none of them will fix
Ventilation manages water vapour in air. It does not remove mould that has already grown, and it does not clear the musty smell that lingers after a damp episode — that comes from spores and microbial compounds on surfaces and in the air, not from humidity itself. Where a room still smells stale after the moisture is under control and visible growth has been treated, an air treatment such as Spore Stopper is designed to help reduce airborne mould spores and refresh the space.
Nor will any ventilation system fix damp caused by water entering the building. If the damp is confined to one wall, shows a tidemark low down, appears after rain, or the plaster is blown or salty, the cause is structural and needs a survey — from someone who inspects outside as well as in, and who does not only sell one remedy.
