Do Replacement Windows Have to Have Trickle Vents?

In most cases yes. Since Approved Document F was updated, the great majority of replacement windows fitted in England are expected to include trickle vents, and any competent installer will price them in as standard rather than present them as an optional extra.

The nuance that matters is that the requirement is not a flat blanket rule. What applies to your house depends on what your existing windows have, what mechanical ventilation is already in the property, and whether the building is listed or sits in a conservation area. From working with clients in Epsom, Esher and Leatherhead, we've found this is the single most common thing people are caught out by when comparing quotes, usually because one installer has included the vents and another has quietly left them out to look cheaper.

What changed, and when?

Approved Document F Volume 1, the 2021 edition, came into force in England on 15 June 2022. Wales followed on 23 November 2022. Any installation carried out from those dates has to be assessed against the updated guidance rather than what went before.

The reasoning behind the change is straightforward enough. Homes have become far better sealed over the last two decades, and the accidental draughts that used to ventilate a house have largely been designed out of them. Without a deliberate route for background air, moisture and indoor pollutants accumulate, and the visible result is condensation and then mould. The vents exist to reinstate ventilation that older buildings used to get by accident.

What if my existing windows already have trickle vents?

Then the rule is simple and there is nothing to debate. The replacements must have vents too, those vents must not be smaller than the ones being taken out, and they must be controllable either by the occupant or automatically.

That last condition catches people out more often than the first two. A vent painted shut during redecoration, or screwed permanently open because it rattled, does not satisfy the requirement, because being able to control it is part of what is being asked for.

What if my old windows had no vents at all?

This is where it becomes more involved, and where installers genuinely differ in their approach.

The governing principle is that ventilation after the work must be no worse than it was beforehand. Guidance from Local Authority Building Control sets out the routes available for demonstrating that.

• Fitting trickle vents to the new windows. The equivalent areas are a minimum of 8,000mm² for a habitable room, 8,000mm² for a kitchen and 4,000mm² for a bathroom.
• Relying on existing mechanical extract ventilation. Where continuous mechanical extract is already installed and working, a reduced provision of 4,000mm² per habitable room in the non-wet rooms may be acceptable.
• Agreeing an alternative provision with Building Control. Other arrangements can be accepted where they are demonstrated to work, which in practice means involving a Building Control surveyor before the windows are ordered rather than after they are fitted.

The practical reality is that the first route is by far the most common, because it is self-contained and does not depend on other equipment continuing to function years later. If somebody is proposing one of the other two, it is entirely reasonable to ask them to set out the reasoning in writing.

How much ventilation does a room actually need?

Background ventilation is only half the picture. Approved Document F also sets extract rates for the wet rooms, and fitting new windows does not remove the need for those to be working properly.

• A kitchen needs 30 litres per second where the extract sits next to the hob, or 60 litres per second where it is positioned elsewhere in the room.
• A bathroom needs 15 litres per second, which is more than a good number of older fans still manage to achieve.
• A utility room needs 30 litres per second, and in our experience this is the room overlooked more often than any other.
• A WC needs 6 litres per second, the lowest figure of the set but still a requirement rather than a suggestion.

The two halves work as a pair. Trickle vents let fresh air in and extract fans push moisture out, and doing one without the other rarely produces the result people were hoping for. You can read the full detail in Approved Document F on gov.uk.

Are listed buildings and conservation areas exempt?

Not exempt as such, but treated differently, and locally this matters a great deal.

Approved Document F recognises that historic buildings cannot always comply in full. Listed buildings, buildings within conservation areas, and buildings of traditional vapour-permeable construction are expected to comply where it is reasonably practicable, with the balance struck so as not to harm the character of the building or create problems for its fabric.

This is not a rare situation in Surrey. Epsom and Ewell alone has 21 designated conservation areas, along with around 30 Article 4 direction records on the national planning data register, and Elmbridge has separately designated conservation areas at Esher and at Church Cobham covering Mill Road, River Hill and Church Street. If your property sits inside one of those, the ventilation question and the planning constraints on a conservation area property need answering together rather than one after the other, because a solution to one can very easily breach the other. The Planning Portal is a reasonable starting point, though your own council's planning team is the authority on what applies to your street.

In our experience, a slim head vent set into the outer frame works better than a vent cut through the top of the opening sash on a period property, because the head version leaves the sightline and the ironmongery line of the sash uninterrupted, which is usually the detail a conservation officer is actually looking at. It is also the version we get called back to least often, since a through-sash vent puts the airflow path across the weather seal line and is the one that tends to whistle in a strong wind.

Will trickle vents make the house cold or noisy?

They are a far smaller change than most people fear, and the complaints we hear are usually about something fixable rather than something inherent.

• Heat loss through a trickle vent is modest set against the moisture problem it prevents, and it is very much smaller than the loss through the draughty windows being taken out.
• Noise is the more legitimate concern, particularly on a busy road. Acoustic trickle vents exist and are worth specifying on the front elevation of a house on a main road rather than accepting a standard vent everywhere by default.
• Whistling is nearly always a positioning or product issue rather than an unavoidable feature. If a vent whistles, it can usually be changed for one that does not.
• Closing them permanently defeats the entire point. We see this constantly, and it is the most reliable way to reintroduce exactly the condensation the vents were fitted to prevent.

What if an installer offers to leave them out?

Treat it as a warning sign rather than a saving.

An installer registered with a competent person scheme is self-certifying that the work complies with building regulations. If vents have been left out where they were required, a certificate is being issued against work that does not comply with what it claims. The problem then surfaces later, most often during a sale, when a buyer's solicitor asks for the paperwork and it does not match what is actually in the wall.

If you have inherited windows of uncertain age, it is worth establishing whether an existing installation was signed off before you need the answer in a hurry, because putting it right is a great deal simpler before you are under offer than in the middle of a chain. FENSA hold records of installations registered through their scheme.

So, do you need trickle vents in your new windows?

Almost certainly, and it is far better to plan for them than to discover them halfway through comparing two quotes. If your current windows have vents, the new ones need them and they cannot be any smaller. If they do not, your installer has to demonstrate that ventilation is no worse than before, and in the large majority of houses fitting vents is the cleanest and most durable way of doing that.

The exceptions are real but narrow. Listed buildings and properties inside conservation areas are assessed on what is reasonably practicable, which is a genuine conversation to have with your installer and your council rather than a loophole to lean on.

Our team has over 30 years of combined experience installing to these rules across Surrey, and we would much rather explain the requirement at the quoting stage than surprise anybody with it on fitting day. If you would like a quote that sets out exactly what is being fitted and why, get in touch.