Equivalent area in background ventilation

Equivalent area is the measure Approved Document F uses to size background ventilators, and it is not the hole you can see. It is the area of a sharp-edged orifice that would pass the same air flow as the ventilator at the same pressure difference, determined by test rather than by measuring the slot. A ventilator with a generous-looking free area can have a much smaller equivalent area once its grille, insect mesh, baffles and acoustic lining are accounted for.

That gap is where ventilation schedules go wrong, and it is where a noise condition and a ventilation strategy stop being compatible.

What equivalent area measures

The definition is performance based. A ventilator is tested under a known pressure difference, the volume flow rate is measured, and the result is expressed as the equivalent area in square millimetres of an idealised sharp-edged orifice giving that same flow. The test method sits in the European standard for the aerodynamic performance of air terminal devices.

Because it is a tested value, it is a property of the specific product, including the frame type it is mounted in and the length of the unit. Two visually identical trickle vents from different manufacturers can declare different equivalent areas, and the same product in a thicker frame can perform differently again.

The declared figure is what the Approved Document expects you to use. Not a calculation from the opening dimensions.

Why it differs from free area

Free area is geometry. Measure the smallest cross section of the opening, multiply it out, and you have a number that tells you nothing about resistance to flow.

Equivalent area is lower than free area for almost every real product, because air passing through a ventilator loses energy at each change of direction and each obstruction. The factors that reduce it are predictable:

• An insect mesh, which can take a surprising proportion of the flow capacity.
• A weather baffle or hood needed for rain and wind-driven water exclusion.
• A hit-and-miss closure plate, which restricts even when fully open.
• An acoustic lining and the folded air path that goes with it.
• The depth of the frame the unit passes through.

Older specifications and some older local authority noise conditions still refer to free area. Where a condition is worded that way, it is worth clarifying at the discharge stage which measure the officer intends, because specifying to free area when the regulations need equivalent area produces an under-ventilated dwelling that still complies with the condition.

Provision required per room

Approved Document F sets the requirement two ways at once for a dwelling using background ventilators with intermittent extract. There is a whole-dwelling figure, which varies with the number of bedrooms and the floor area, and there are minimum provisions for individual rooms so that no habitable room is starved while the total looks adequate.

The numbers have changed between editions and the current edition increased them, so read the figures from the current Approved Document F rather than from a product brochure or an older specification. Two practical consequences hold whichever edition applies.

First, the total has to be distributed sensibly. Putting the whole dwelling allowance into the living room does not satisfy the per-room minima.

Second, rooms with a single window carrying a single short ventilator are where a schedule fails. A 600 millimetre ventilator in a narrow bedroom window frequently cannot deliver the room minimum on its own, and the fix is either a longer unit, a second unit, or a through-wall ventilator independent of the window.

Acoustic ventilators and their trade-off

This is where planning and building control collide. A noise condition arising from a Noise Impact Assessment typically specifies internal levels with windows closed, and the acoustic consultant satisfies it by specifying glazing plus attenuated background ventilators with a stated sound reduction.

An attenuated ventilator achieves that reduction with a lined folded air path, and that path costs flow. For the same frame length, an acoustic unit commonly delivers a fraction of the equivalent area of a plain one. The arithmetic then forces a choice: longer units, more of them, or a mechanical strategy.

An illustrative comparison makes the point. Take a bedroom needing a given equivalent area under the current Approved Document. A plain ventilator in a 900 millimetre head might meet it in one unit. Swap to an attenuated unit of the same length with a meaningful sound reduction and the declared equivalent area may fall well below the requirement, so two heads or a through-wall attenuated unit are needed. These figures are illustrative; take the required area from the current Approved Document and the delivered area from the product's test data.

The sequencing error that causes most abortive work is fixing the window schedule and the glazing specification before anyone has checked the ventilation arithmetic. Do the two together, and raise it in the pre-application discussion where the site is noise constrained. The standing advice a consultee publishes sometimes states the measure it expects.

Interaction with overheating strategy

Background ventilators do not help with summer comfort. They are sized for moisture and pollutant removal at low rates, and the overheating requirements rely on purge ventilation through openable areas, which is a separate calculation using openable free area and the restriction imposed by any fall-protection device.

Where windows must stay shut at night for noise reasons, the overheating strategy has to come from elsewhere: shading, lower solar transmittance glazing, mechanical ventilation with a summer bypass, or reduced glazed area. An Overheating Risk Assessment run early tells you whether the acoustic glazing specification you are about to fix makes the summer condition unachievable.

And where a scheme already has permission and the ventilation strategy has to change, a condition tied to a specific submitted document may need a section 73 variation rather than a simple discharge.

Evidence from test data

A ventilation schedule that survives review contains six columns and one appendix.

1. Room name and type, with floor area.
2. The required equivalent area for that room from the current Approved Document.
3. The product reference and length proposed.
4. The declared equivalent area for that exact product and length.
5. Any acoustic performance figure where a noise condition applies.
6. The surplus or shortfall, so the reviewer does not have to do subtraction.

The appendix holds the manufacturer's test data sheets showing the declared equivalent area and, where relevant, the sound reduction. Officers and building control bodies accept declared figures from a test report. They do not accept a figure derived from the opening dimensions, and a schedule that quotes free area in a column headed equivalent area is the most common reason these submissions come back.

Browse the report types PlanScope can draft to see which documents a noise-constrained residential scheme needs.

This is general guidance, not legal advice. Check the position with your local planning authority or a qualified professional before relying on it. PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval.