Trickle vents and Approved Document F: what landlords must know

Quick answer: Since 15 June 2022, Approved Document F of the Building Regulations requires that replacement windows do not reduce a home's existing ventilation. Where the old windows provided background ventilation, the new ones must provide at least as much — which in practice means fitting trickle vents on most window replacements. For landlords, this closes off a common route by which a window upgrade quietly causes a condensation problem.

Replacing old windows with sealed double glazing improves warmth and cuts noise. It also removes the unplanned background ventilation that old draughty frames provided — and if nothing replaces that airflow, moisture from normal living has nowhere to go and condenses on the next-coldest surface. Approved Document F is the rule that stops this happening. This guide explains what it requires and why it matters for a rental property.

What Approved Document F requires

Approved Document F sets the ventilation standards for dwellings in England. The version that came into force on 15 June 2022 introduced a specific requirement for replacement windows and doors: the work must not make the building's ventilation worse than it was before.

In concrete terms:

• If the existing windows had trickle vents or other background ventilation openings, the replacement windows must provide at least equivalent background ventilation. That almost always means trickle vents in the new frames.
• If the existing windows genuinely had no background ventilation openings at all, the replacements are not strictly required to add them — but the guidance encourages doing so, and good installers now fit them by default.
• The requirement applies to the room, not just the window: the installer has to consider whether the room still meets the background ventilation standard after the work.

Replacement window work is notifiable under the Building Regulations. Most installers are registered with a competent person scheme (such as FENSA or CERTASS) and self-certify compliance, issuing a certificate. That certificate is worth keeping.

Why it matters for landlords

Condensation appearing shortly after a window replacement is a recognised pattern, and it puts the landlord on the wrong side of two things at once:

1. The building has been made worse. A window upgrade that removes background ventilation without replacing it is a building deficiency the landlord introduced.
2. The "lifestyle" defence does not survive it. GOV.UK guidance is already clear that damp and mould is a building problem, not a tenant behaviour problem. A tenant who developed condensation the winter after the windows were changed has an obvious building cause to point to.

For social landlords, condensation mould is within Awaab's Law scope now, and Phase 2 adds excess cold from 30 November 2026. Making sure replacement windows are compliant is a cheap way to avoid creating an Awaab's Law liability.

Practical steps

• When commissioning replacement windows, specify trickle vents unless there is a documented reason not to, and keep the competent person scheme certificate.
• For windows already replaced without trickle vents, get a ventilation assessment. Retrofitting trickle vents may mean new sashes; an alternative is to add continuous mechanical extract to the wet rooms and, in a house with loft access, a PIV unit — which provides the whole-home supply that missing trickle vents would otherwise have contributed to.
• If a tenant reports condensation after a window upgrade, treat the window work as a likely contributing cause and investigate the whole ventilation strategy, not just the one room.
• Do not rely on advising the tenant to open the windows. That is not a substitute for compliant background ventilation, and it is not a lawful response to a ventilation deficiency.

What the three types of ventilation do

Approved Document F expects a dwelling to have all three of:

• Background ventilation — the slow, continuous trickle that carries away moisture and stale air between window openings. This is what trickle vents provide.
• Extract ventilation — mechanical extraction at the source of moisture in kitchens, bathrooms and utility rooms, so that steam and cooking moisture leave the building rather than spreading through it.
• Purge ventilation — the ability to air a room rapidly, usually by opening a window wide, for example after a spill or to clear paint fumes.

A property can fail on any one of these. The most common failure after a window replacement is loss of background ventilation, because sealed units without trickle vents remove the incidental airflow the old frames leaked. Extract failure is the next most common — fans that are undersized, ducted into a loft instead of outside, or disconnected.

Common problems inspectors find

• Replacement windows with no trickle vents and no competent person scheme certificate on file.
• Trickle vents fitted but painted shut, or so noisy or draughty that occupants have taped over them.
• Bathroom extract fans discharging into the roof void, causing damp timbers and mould on the underside of the felt.
• Kitchen "extract" that is a recirculating cooker hood with a charcoal filter, venting nothing outside.
• Airbricks blocked by a later extension, a raised patio, or insulation.

For a landlord, any of these is a building deficiency to record and remedy — not something to raise with the tenant.

What to specify when replacing windows

• Trickle vents in every replacement window, sized to the room, unless there is a documented and defensible reason not to.
• Humidity-controlled trickle vents where noise or draught complaints are likely — they open and close automatically with indoor humidity, which reduces the incentive for occupants to tape them over.
• A written confirmation from the installer that the room still meets the Approved Document F background ventilation standard after the work.
• The competent person scheme certificate (FENSA, CERTASS or equivalent) on completion — file it with the property records.

If the installer says trickle vents "aren't needed" or "spoil the look", ask them to put the ventilation assessment in writing. A reputable installer will not have a problem with that.

Talking to tenants about trickle vents

Tenants close trickle vents because of draughts and noise, then the condensation they were preventing appears. The productive framing is not "you must keep these open" — it is to design the ventilation so it does not collapse when one vent is shut: humidity-controlled vents, continuous extract in the wet rooms, and a PIV supply where there is a loft. If a tenant reports a vent whistling or letting in cold air, treat it as a maintenance item to fix, not a behaviour to correct.

Trickle vents are necessary, not sufficient

Trickle vents provide the background supply of outdoor air that mechanical extract and PIV both depend on to work. They are one essential part of a working ventilation strategy — but on their own they will not clear an established condensation problem. If a property already has visible mould, treat the trickle vents as step one and read the condensation control solutions guide for the rest, or find a verified ventilation specialist to assess the whole property.