Ventilation for damp, with the numbers nobody gives you
Government guidance names ventilation as one of the three fixes and no consumer page tells you what adequate means. Part F publishes a rate for every wet room in the house. Here they are, plus the two ventilation defects that pass a visual check.
By The Damp GuyLast updated
The short version
"Improve ventilation" is advice you cannot check. There are published numbers: 15 l/s for a bathroom, 30 l/s for a utility room or a kitchen with a hood venting outside, 60 l/s for a kitchen without one, 6 l/s for a WC. Two things are worth checking before you buy anything: whether your trickle vents are shut (they are designed to be open) and where your extractor fan actually discharges. A fan venting into the loft runs perfectly and moves the problem into your roof.
What adequate ventilation actually means, in litres per second
Approved Document F, 2021 edition. Minimum intermittent extract rates, by room.
| Room | Rate | Note |
|---|---|---|
| Kitchen, hood extracting outside | 30 l/s | The lower figure depends on the hood actually going outside. |
| Kitchen, no hood or recirculating | 60 l/s | A recirculating hood filters grease and returns the steam to the room. |
| Utility room | 30 l/s | The room a tumble dryer or a drying rack usually lives in. |
| Bathroom | 15 l/s | The single most checkable number in this table. |
| WC | 6 l/s | Sanitary accommodation without a bath or shower. |
There is a whole-house figure too. The whole-dwelling ventilation rate scales with the number of bedrooms, running from 19 litres per second for a one-bedroom home to 43 for a five-bedroom one, and the sum of continuous extract should meet it.
That scaling is worth sitting with, because it turns a very common accusation into arithmetic. Houses often "become damp" when the household grows. Each person puts six to 12 litres of moisture into the air per day, and the ventilation a dwelling was designed with was sized for a number of bedrooms, not for whoever ended up living there. A home ventilated for two and occupied by four has a shortfall by design.
The two defects that pass a visual check
Both of these look fine. One of them is actively making things worse somewhere you are not looking.
One: the fan that vents into the loft. Somebody fits a bathroom extractor, runs the flexible duct up through the ceiling, and stops. Every check you would think to make passes. The fan is powered, it makes the right noise, the mirror clears. What it is doing is taking the moisture from every shower in the house and delivering it into a cold, unheated void full of structural timber. If you have condensation or mould in the loft and a bathroom fan directly beneath it, look at the duct before you look at the roof covering.
Two: the trickle vents everybody shuts. Background ventilators are intended to normally be left open. They sit at least 1700mm above floor level precisely so incoming air mixes before it reaches head height and does not feel like a draught. Almost every occupant closes them at the first cold snap, which removes the background ventilation the dwelling was designed around, in the season it was designed for. It costs nothing to go round and open them.
Neither of these is exotic and neither needs a survey to find. They are worth ruling out before anybody sells you a system, because a house with its vents shut and its fan pointing into the loft has not yet tried ventilation.
PIV, described by people who are not selling it
Positive input ventilation is marketed hard as a cure for damp. Its limits are conceded in its own sellers' literature.
The mechanism is straightforward. A unit, usually in the loft, draws external air through a filter and introduces it continuously, gently pressurising the house so that stale moisture-laden air is pushed out before it can condense on cold surfaces. For a condensation problem in a house with poor air circulation, that is a real intervention and not a gimmick.
Now the limits, all of them taken from companies that sell PIV or damp treatment. It will not stop rising or penetrating damp. It cannot compensate for a poorly insulated loft, because air introduced into a cold space can condense there instead. Rooms can feel cooler in winter, since loft air is cold and not every unit includes a heater. And the filters clog, reducing airflow over time unless somebody maintains them.
We found no neutral source on PIV at all, which is itself worth reporting. Every page describing it is owned by somebody with an interest in the answer, and that includes the pages saying it works. Read the limits above as the floor of what its own industry admits to, and note that the first two of them are about the building rather than the machine.
Why this is the intervention the guidance actually names
Ventilation is not one option among several. It is one of three things the government's own damp guidance points at.
The guidance for rented housing providers, published by the Ministry of Housing, Communities and Local Government with the Department of Health and Social Care and the UK Health Security Agency, names three interventions: ventilation, heating and the building fabric. It also names inadequate ventilation among the fundamental causes of damp and mould, alongside building deficiencies and poor energy efficiency, and frames all of them as causes rather than as things the occupant is doing wrong.
And it makes the point this whole site is built on: "simply removing surface mould will not prevent the damp and mould from reappearing". Wiping it off, painting over it and running a machine are all ways of changing what a wall looks like. Ventilation is one of the few things on the list that changes what the building does.
Which is why it is worth checking yours properly, with numbers, before spending on anything else. A dehumidifier has a running cost forever. Heating has a running cost forever. Opening the vents you already have costs nothing, and if the damp is not condensation at all, none of the three will touch it and you need the diagnosis first.
About this resource
One sourcing limitation, stated rather than hidden. The extract rates and the whole-dwelling range are from Approved Document F Volume 1, 2021 edition, for use in England, but the published PDF does not render to text for automated reading, so the figures here are taken from a ventilation industry restatement that cites that edition. They are consistent across the sources we read. If a number matters to a decision you are making, go to the Approved Document itself rather than take it from us.
The PIV section is drawn entirely from companies that sell PIV or damp treatment, because no neutral source on it could be found. It is reported as an industry position, and only the limits those sources concede are repeated here. No unit, brand or installer is named, and nothing on this site earns a commission.
The moisture-per-person figure and the "band-aid" framing come from Michael Taylor, an adjunct academic at Flinders University researching mould and indoor air quality. The cause wording is quoted from the government guidance for rented housing providers, updated 1 April 2026. There are no installation costs on this page; work costs are handled separately on this site with their own sourcing.
This site does not survey, treat or quote for work, and it cannot tell you what is happening in your building. If your vents are open, your fan goes outside and the problem is still there, that is the point to get an inspection from someone with no interest in selling you the treatment.
Common questions
Should trickle vents be open or closed?
Open. That is not a preference, it is what they are for: background ventilators are intended to normally be left open, and they are positioned at least 1700mm above floor level specifically so the incoming air has mixed before it reaches you and does not read as a draught. Closing them is the single most common way a house quietly loses the background ventilation it was designed around, and it is usually done in the same month the condensation starts.
Does opening windows reduce damp?
It moves one of the two variables, and it moves it fast. A window swaps the moisture-laden air in the room for drier air from outside, which is the quickest change you can make to a room without buying anything. What it does not do is warm your cold surfaces, and it throws your heat out along with the moisture, which is why almost nobody keeps doing it through the month they most need to. It also does nothing whatever if the water is arriving through the wall rather than being generated inside it.
Do I need an extractor fan in my bathroom?
Approved Document F sets a minimum intermittent extract rate of 15 litres per second for a bathroom, 30 for a utility room, 6 for a WC, and 30 for a kitchen where a cooker hood extracts to outside or 60 where it does not. Those are numbers you can check yours against. Most people do not know a number exists, which is how a bathroom ends up with an ornamental fan that runs, makes a noise and moves almost nothing.
Where does my extractor fan actually go?
Follow the duct. A fan that discharges into the loft rather than through the roof or the wall is a genuinely common defect, and it passes every casual check because the fan runs and the mirror clears. What it has done is move a bathroom's worth of moisture into an unheated space full of cold timber. If you have condensation in the loft and a bathroom fan below it, look there before you look anywhere else.
Do PIV units work?
For condensation, by the accounts of the companies that sell them: a PIV unit draws filtered air, usually from the loft, and introduces it continuously so that stale moisture-laden air is pushed out of the house. Those same companies concede the limits. It will not stop rising or penetrating damp. It cannot compensate for a poorly insulated loft, because warm air pushed into a cold space can condense there. Rooms can feel cooler in winter, as loft air is cold and not every unit has a heater. And filters clog, which quietly reduces airflow.
How can I quickly dry out my damp house?
You can dry the air quickly and you cannot dry a building quickly, and conflating the two is how people conclude the ventilation "did not work". If the source is closed, moving air and gentle warmth will bring a room back over days. If the source is still open, you are ventilating a space that is being resupplied. Before doing anything, establish which of the three kinds you have, because two of them are not moisture-in-the-air problems at all.
What will draw moisture out of a room?
Extraction at the point it is produced, background ventilation everywhere else, and a dehumidifier where you cannot have either. The order matters: a household generates six to 12 litres of moisture per person per day, and the cheapest place to deal with it is at the kitchen and the bathroom where most of it is made. Research on mould and indoor air quality is blunt about the alternative, describing a dehumidifier used in place of proper extraction as "a band-aid for a deeper problem".
My house got damp after the family grew. Is that our fault?
It is arithmetic, and government guidance is unusually direct that it is not a lifestyle failing. Each person adds six to 12 litres of moisture a day, and the whole-dwelling ventilation rate in Part F scales with bedrooms, from 19 litres per second for a one-bedroom home to 43 for a five-bedroom one. A house ventilated for two people and occupied by four has a design shortfall, not an occupant problem.