Ventilation in Flats and Apartments

Flats are ventilated differently from houses, and the rules recognise that. A single-storey flat is given its own row in the ventilation tables: habitable rooms need background ventilators with a minimum equivalent area of 12,000mm2 each, and the kitchen 8,000mm2, where a dwelling on more than one floor is set 10,000mm2 and 5,000mm2¹. Extract ventilation to outside should be provided in kitchens, utility rooms, bathrooms and sanitary accommodation, either intermittently or continuously¹. And when work is done, the ventilation must not end up worse than it was: the Welsh rule is that "the ventilation in the building must comply with the standard set out in Part F of Schedule 1 or it must be no worse" than before the work².

The reason ventilation matters more in a flat is structural as much as regulatory. Many flats have windows on one side only, so there is no cross-flow of air to carry moisture and cooking fumes away. Wet rooms are singled out in the guidance: "rooms where steam will be produced (kitchens, bathrooms, utility rooms etc) should be provided with higher levels of ventilation, normally mechanical fans and windows, than other rooms where suitably sized window openings and background ('trickle') ventilators may suffice"³. This page sets out what the rules require across the UK nations, what each ventilation option does, and what a flat owner or tenant should expect to receive when the work is finished.

Why a flat's air is harder to change than a house's

A flat shares its fabric, its ducts and often its ventilation system with the rest of the block. That shapes everything on this page. Where a householder can simply fit a vent through an outside wall, a flat owner may face a facade owned by the freeholder, a duct shared with neighbours, or a system that serves the whole building from one plant room. The private rented sector's fitness checks in Northern Ireland state that "all habitable rooms should be ventilated directly to the open air by opening a window", and that all kitchens, bathrooms and toilets should have "enough suitable permanent ventilation by air brick or similar"⁶, which sets the floor a flat must reach regardless of who owns the walls.

Ventilation also interacts with energy work in a way that is easy to miss. Building regulations need adequate ventilation when installing loft insulation, for example, and householders are told to check with their local building control office⁷. Insulation lowers the amount of heat escaping and could cut fuel bills⁸, but the same airtightness that keeps heat in also keeps moisture, cooking fumes and carbon dioxide in. Guidance on converting traditional buildings puts it plainly: "it is important to ensure that adequate ventilation measures are in place both for the building and its occupants"⁹. A flat that is being tightened up for energy reasons needs its ventilation planned at the same time, not afterwards.

For a tenant, the picture is different again: the ventilation system is the landlord's to maintain, and problems with damp and mould that follow from poor ventilation are covered in our pages on damp and mould in rented homes and on asking a landlord for energy improvements.

The rules: Approved Document F and its Welsh, Scottish and Northern Irish counterparts

Approved Document F, Volume 1 "provides guidance on building ventilation, including building air quality and preventing condensation in a domestic structure"¹⁰, and it "sets standards for ventilation in new dwellings"¹¹. In a domestic property, the guidance requires that ventilation is circulated continuously throughout the whole building, that extract ventilation is used within an area of increased humidity or pollution, and that purge ventilation is possible¹⁰. Those three requirements, background, extract and purge, are the skeleton of every dwelling ventilation system, including a flat's.

Wales has its own suite. The 2022 amendment regulations changed the framework, and the Approved Document L Volume 1 2026 edition "gives guidance for compliance with the Building Regulations for building work carried out in Wal[es]"¹². The Welsh rule for existing homes is the one to hold onto: when Part L work is done, "the ventilation in the building must comply with the standard set out in Part F of Schedule 1 or it must be no worse" than before². The same principle appears in the consultation material: "the building, including the provision of ventilation, should not be made less satisfactory than before the work was carr[ied] out"¹.

Scotland's Building Standards Technical Handbook 2022 sets out its own extract rates: mechanical extraction capable of at least 30 l/sec intermittent above a hob, or at least 60 l/sec intermittent if elsewhere, or passive stack ventilation, with 12,000mm2 trickle ventilation in habitable rooms¹³. Northern Ireland's building regulations are under review, with a discussion document published in 2023 proposing changes that include continuous mechanical extract ventilation "where airtightness of less than 3m3/(h.m2)@50Pa is achieved"¹⁴. Where the nations differ, the work must follow the rules of the nation the flat sits in.

Minimum background vent sizes in flats

Background ventilators are the small slot openings, usually in the head of a window frame or through a wall, that let a slow trickle of air through the home permanently. The Welsh consultation tables give flats their own figures, and they are higher than the multi-storey figures:

Both figures come from the same table, and the kitchen figure rises to 12,000mm2 "where any bathroom has no window or external facade for a ventilator"¹. For dwellings with continuous mechanical extract ventilation, the requirement drops to a "minimum equivalent area of 5,000 mm2 for each habitable room in the dwelling"¹, a deliberate increase from the 2,500mm2 that applied before: the Welsh impact assessment records "the proposed policy change is for the size of background ventilators to be increased from 2500mm2 to 5000mm2 equivalent area" in habitable rooms⁴.

For existing dwellings, the checklist is lighter: habitable rooms under a mechanical extract strategy need background ventilators "having a minimum equivalent area of ... 4,000 mm2"¹. Where an existing habitable room already has at least 5,000mm2 of total ventilator area and a new habitable room is added, the guidance asks for "background ventilators of at least 10,000 mm2 equivalent area between the two rooms" and the same between the additional room and outside¹.

Two practical points matter to flat owners. First, ventilators "can be provided through any appropriate means", including through a wall, provided they give the equivalent areas described in Approved Document F, Volume 1¹¹, so a flat without a suitable window frame is not stuck. Second, where a room already has a wall ventilator meeting the minimum equivalent areas, "no further background ventilation needs to be added after replacing the windows"¹¹. The consultation also proposed raising wet-room ventilators: "If adopting intermittent extract, a background ventilator of 5000mm2 is now recommended compared to 2500mm2 currently"¹⁵, and increasing extension-related ventilators "from a minimum of 8000mm2 to 10000mm2 in both cases"¹⁵.

Natural ventilation and where it stops working in a single-facade flat

Natural ventilation is the default in the notional dwelling used for energy calculations, described simply as "Natural ventilation with intermittent extract fans"⁵. It works on pressure differences: air enters through trickle vents on the windward side and leaves through extract fans and vents on the other. In a flat with windows on one facade only, there is no other side, and the driving forces that make natural ventilation work are weak.

The guidance itself carries a warning on this. The consultation version of Approved Document F Volume 1 notes: "NOTE: The guidance for natural ventilation is only suitable for less airtight dwellings."¹ For highly airtight dwellings, expert advice should be sought¹. That has direct consequences for a modern flat, which is likely to be well sealed by design.

The whole-dwelling ventilation rate is set "by 0.3 l/s per m2 of internal floor area", counting all floors¹, so a flat's floor area determines how much air must move through it regardless of the system chosen. Purge ventilation, the ability to throw windows open and flush the home, remains a requirement in a domestic property alongside continuous circulation and extract¹⁰, and a room served by a new rooflight must be ventilated, with the rooflight able to provide both rapid and background venting¹⁶. But purge ventilation is occasional, not permanent, and it cannot substitute for the background supply a single-facade flat lacks the cross-flow to drive.

Mechanical extract ventilation: what a flat's fans must deliver

Mechanical extract ventilation (MEV) is the workhorse of flat ventilation. Extract ventilation to outside should be provided in all kitchens, utility rooms, bathrooms and sanitary accommodation, and it may be intermittent or continuous¹. The Welsh example specification for an existing dwelling shows what a full system looks like: "Natural ventilation system comprising 12 x 5000mm2 background ventilators (slot-type in head of window), 1 x 15 l/s intermittent fan in bathroom, 1 x 30 l/s intermittent fan in utility room", with the kitchen canopy reconfigured to duct to outside⁴. The continuous alternative in the same document is "Continuous mechanical ventilation comprising 7 x 5000mm2 background ventilators, 1 x 8 l/s continuous extract in bathroom, 1 x 8 l/s fan in utility room, 1 x 13 l/s extract fan in the kitchen"⁴.

The Scottish handbook sets the kitchen rates: mechanical extraction capable of at least 30 l/sec intermittent above a hob, or at least 60 l/sec intermittent if elsewhere, or passive stack ventilation as an alternative, plus 10,000mm2 trickle ventilation¹³. Northern Ireland's 2023 discussion document proposes that "continuous mechanical extract ventilation is suggested where airtightness of less than 3m3/(h.m2)@50Pa is achieved"¹⁴, which ties the choice of system to how airtight the flat has become.

In a block of flats, extract can be arranged dwelling by dwelling, each flat with its own fans, or block-wide, with a shared riser. Either way, the design must respect the three-part requirement: continuous circulation throughout the whole building, extract where humidity and pollution increase, and purge ventilation when needed¹⁰. A flat's extract system is also the one part of its ventilation that a household can hear, feel and maintain, so access matters: "Reasonable access should be provided for maintaining ventilation systems", including access to replace filters, fans and coils, access points for cleaning duct work, and access for general maintenance of plant¹.

Demand-controlled ventilation: matching the fan to the need

Demand-controlled ventilation is the principle of running mechanical extract only as hard as the dwelling requires, rather than at a fixed rate around the clock. The official documents in this area frame it through airtightness: continuous mechanical extract is suggested where airtightness of less than 3m3/(h.m2)@50Pa is achieved¹⁴, so the fan's duty scales with how little air leaks in naturally. The tighter the flat, the more the fan has to do, and the more worthwhile it becomes to modulate the fan rather than run it flat out.

The energy logic is straightforward. Every litre of air a fan extracts has been heated by the flat's heating system, and extracting more than needed throws that heat away. Guidance on traditional buildings makes the trade-off explicit: "While draught-stripping is valuable for energy efficiency, it does reduce the free air within the building and the practitioner should demonstrate how the proposals manage this"⁹. Demand control is one way of managing it: seal the flat, then supply and extract measured amounts of air rather than relying on uncontrolled leakage.

For a sense of what airtightness measures are worth, draught-proofing doors and windows combined with other measures "can be sealed to deliver around £85-95 in savings on your energy bills"¹⁷. No official savings figure for demand-controlled ventilation itself is published, so treat any such claim from a manufacturer as unverified against official data. What the official documents do establish is the trigger point at which continuous, controllable extract becomes the recommended approach¹⁴, and the hand-in-glove relationship between sealing a home and ventilating it deliberately⁹.

Heat recovery in a flat: centralised, ductless and single-room

Mechanical ventilation with heat recovery (MVHR) extracts air from wet rooms and passes it through a heat exchanger, transferring warmth to the incoming fresh air. The Centre for Sustainable Energy describes the typical arrangement: "These systems tend to be made up of concealed ducting (piping) in ceiling cavities that lead to the heat exchanger unit" housed in "a cupboard, utility room or sometimes a loft"¹⁸. That description is the obstacle for most flats: a flat rarely has a loft, and ceiling voids deep enough for ducting are a feature of new build rather than conversion.

Where a flat does have the space, the cold-loft advice from the same guidance illustrates the practicalities: "install the unit on a raised platform, insulate ducts and the condensate pipe and keep duct runs as short as possible"¹⁸. Short duct runs matter in a flat for a second reason: every metre of ducting through a ceiling is a metre that must be accessible for the cleaning and filter replacement the guidance requires¹.

Ductless and single-room heat recovery units are the flat-specific answer. A single-room unit fits through an external wall and recovers heat between the supply and extract air paths of one room, with no ducting to anywhere else. Published official guidance gives no performance data for these units, so no efficiency figure can be given here; what can be said is that they address the central problem of MVHR in a flat, the absence of routes for concealed ducting, at the cost of ventilating one room rather than the whole home. A flat considering whole-home MVHR should compare it with the alternatives on our pages about MVHR vs positive input ventilation and PIV unit vs dehumidifier for condensation.

Retrofitting a flat: insulation, windows and the airtightness penalty

Retrofitting is where ventilation problems are created, and the official guidance is unusually direct about it. Retrofitting includes "insulating roofs, walls and floors; replacing windows; improving ventilation; draught-proofing; installing efficient heating and hot water system; renewable energy e.g. solar panels"¹⁹. Note that improving ventilation is listed as a retrofit measure in its own right, not an afterthought.

The sequence matters. Draught-stripping "is valuable for energy efficiency, [but] it does reduce the free air within the building and the practitioner should demonstrate how the proposals manage this"⁹. Windows should "retain their full capacity to ventilate rooms" in all circumstances⁹. When windows are replaced, the trickle ventilator sizes in the tables above apply, unless an existing wall ventilator already meets the minimum equivalent areas, in which case "no further background ventilation needs to be added after replacing the windows"¹¹.

The financial side is not negligible. Draught stripping "qualifies for a lower rate of VAT, for example insulation fixed around windows and doors to reduce draughts", as does "insulation on walls, floors, ceilings and lofts"²⁰. Insulation measures of this kind appear in the Energy Company Obligation's reported data under "Other Insulation", which "includes the following measure types: flat & pitched roof insulation, solid floor insulation, window glazing, draught proofing and higher performance external doors"²¹. Home insulation, energy efficient windows and doors, and draught reduction reduce energy bills²².

Where a flat forms part of a building with a flat roof, roof work brings its own ventilation duties: "the roof will need to be ventilated"²³, and "through ventilation is then achieved by incorporating eaves venting"²³. A ventilation gap, usually 50mm, should be provided between the top of the insulation and the underside of the roof covering, with ventilation openings either at the eaves or upstand²⁵. A warm deck roof "avoids the need for ventilation and the roof covering can be re-applied over the top"²³. These are block-level matters, but a top-floor flat owner paying for roof insulation will meet them. Flats also face tighter permitted development limits than houses: for air source heat pump enclosures, "flats are limited to 0.6 cubic metres" where houses are allowed 1.5²⁷, and there are no permitted development rights for flats for single storey ground floor extensions. Consent issues for flats are covered in our page on leasehold consent for energy work.

Where to put intakes and outlets in a town or city

The siting rule comes from Approved Document F's own guidance: "ventilation systems should be designed to minimise the intake of external air pollutants, by locating ventilation intake[s] away from the direct impact of local pollution sources"¹¹. In practice that means thinking about what is outside the wall the vent goes through: a busy road at window height, a bin store, a loading bay or a neighbouring flue all push polluted air past the intake.

For a flat, the constraints are tighter than for a house. The facade is the only surface available, and it may face the street. Intakes and outlets on the same facade need vertical and horizontal separation so that extracted air is not drawn straight back in, and the extract terminals of a whole block must not discharge into a neighbour's intake path. No numerical separation distances are set in the guidance, so the design judgement sits with the installer and building control, guided by the minimisation principle¹¹.

Roof-level terminals are the block-wide answer, which is why centralised systems in apartment blocks route ducts to a plant area or roof. The same cold-deck logic applies at roof level: ventilation openings either at the eaves or upstand²⁵, with air still allowed to flow from one roof void to another where rooflights are installed¹⁶. A flat owner cannot control the block's terminal siting, but a buyer can ask to see where the intakes sit relative to the pollution sources around the building.

Testing, commissioning and the paperwork to keep

Ventilation work is tested, and the results are recorded. In Wales, the 2022 regulations "require mechanical ventilation air flow rate testing on existing dwellings which undergo notifiable building work"². For new dwellings, "an air pressure test should be carried out on every new dwelling"²⁸. The pressure test results must "be given to the building control authority not later than seven days after the final test is carried out"²⁹.

The owner's own paperwork has a deadline too. The installer "must, not later than five days after the work has been completed, provide to the owner sufficient information" about the system⁵. Keep that information with the flat's deeds or tenancy file: it tells the next owner or the landlord's agent what is installed, how it is set and how it should be maintained.

In England, notification can be avoided where "a building control body does not need to be notified" because the owner employs an installer registered with a relevant competent person scheme designated in the regulations, a route that covers certain types of services and fittings including fuel-burning appliances and replacement windows⁵. Maintenance access is a design requirement, not a favour: "Reasonable access should be provided for maintaining ventilation systems", including access to replace filters, fans and coils, access points for cleaning duct work, and access for general maintenance of plant¹. A flat whose MVHR unit is sealed behind a plastered ceiling does not meet that guidance, and its filters will not be changed.

How ventilation affects a flat's energy autonomy

Ventilation is one of the few places where a flat's energy independence and its air quality pull against each other, and the rules resolve the tension by requiring both. Sealing a flat saves heat, and draught-proofing combined with other measures can deliver around £85 to £95 a year in savings¹⁷. But the same sealing removes the uncontrolled air movement the flat relied on, and the official position is that the work must not leave the ventilation "less satisfactory than before the work was carr[ied] out"¹. A household that seals without ventilating gains on bills and loses on air quality, condensation and the fabric of the building itself.

The dependence that remains is worth naming. A mechanical system depends on electricity and on the manufacturer's fans and filters; a block-wide system depends on the freeholder or management company to maintain it; a ducted MVHR system depends on ceiling voids that a flat may not have. Against that, ventilation is one of the few systems a flat can own outright: a trickle vent needs no power, a single-room heat recovery unit needs only a wall, and the extract rates and vent sizes are fixed in published guidance that any householder can read and check¹.

For a tenant, none of the installation decisions are theirs, but the ventilation standards are still enforceable: habitable rooms ventilated directly to the open air, and permanent ventilation in kitchens, bathrooms and toilets⁶. For an owner, the practical independence test is whether the flat's ventilation can be understood, maintained and repaired without reference to anyone else's plant room. The pages on energy improvements in flats and energy independence when you rent or live in a flat take those questions further.