The primary difference between Equivalent Area (EA) and geometric Free Area is that Equivalent Area is a measure of a ventilator’s actual aerodynamic performance, while Free Area is simply a measure of its physical opening size. In the UK, particularly under the Building Regulations Approved Document F (Ventilation), the EA is the required, more accurate metric for sizing background ventilators, such as trickle vents, because it reflects the true airflow efficiency.
Understanding the UK Ventilation Metrics: Equivalent Area vs. Geometric Free Area #
When grappling with Building Regulations Approved Document F in the UK, particularly concerning background ventilators like trickle vents, you’ll inevitably encounter two distinct, yet often confused, terms: Equivalent Area (EA) and geometric Free Area. As a responsible property developer or refurbisher, understanding the profound difference between these metrics isn’t merely an academic exercise; it’s essential for achieving compliance and, more importantly, ensuring genuine, effective ventilation.
Equivalent Area (EA): The Gold Standard for Airflow #
The EA is the modern, scientifically sound metric for assessing a ventilator’s performance. It acknowledges a fundamental truth: a hole’s size is not the same as the air that can actually pass through it.
What Exactly is EA? #
Equivalent Area is a measure of the aerodynamic performance of a ventilator. It factors in all the real-world impediments to airflow, such as the air friction and resistance created by the vent’s internal components—think louvres, baffles, and vital insect screens.
- Definition: The EA is the theoretical cross-sectional area of a perfect, sharp-edged circular orifice that would allow the exact same volume flow rate of air to pass through it as the ventilator itself, under an identical applied pressure difference.
- UK Standardisation: In the UK, this performance is rigorously tested according to standards like BS EN 13141-1, typically at a pressure difference of 1 Pascal (Pa).
- The Regulatory Imperative: The current UK Building Regulations (Approved Document F: Volume 1 – Dwellings, 2021 edition) mandates that all background ventilators be sized using their Equivalent Area, often denoted as mm2 EA or mm2 EQA. This shift happened because EA provides a demonstrably more accurate indication of the airflow delivered to the room.
For new dwellings, the minimum requirements are clearly set in EA:
- Habitable rooms and kitchens must achieve a minimum of 8,000 mm2 EA.
- Bathrooms have a slightly lower minimum requirement of 4,000 mm2 EA.
Choosing a ventilator based on its EA therefore guarantees compliance and a minimum level of necessary airflow to combat poor ventilation, damp, and mould.
Geometric Free Area: The Simpler, Less Accurate Measurement #
The geometric Free Area is the older, simpler concept. It’s what you might calculate with a ruler: the raw, physical size of the opening.
What is Geometric Free Area? #
The Free Area is simply the total unobstructed cross-sectional area of the physical hole or aperture.
- Definition: It’s measured in the plane where the cross-sectional area is smallest and at a right angle to the air direction. It’s essentially the pure, unhindered physical space available.
- Calculation: You calculate it by multiplying the width and height of the hole and subtracting the physical area of any non-functional components, like the surrounding framework.
- The Flaw in the Measure: Critically, the Free Area does not take into account aerodynamic drag. It completely ignores the resistance and turbulence created by internal components such as baffles, grilles, or cowls. Therefore, a ventilator with a large geometric Free Area can still deliver poor airflow because of a convoluted internal design.
Why EA is Always Less Than Free Area #
A universal rule in ventilation is that the Equivalent Area will almost always be less than the geometric Free Area for any practical ventilator (i.e., any product that isn’t just a simple, perfect hole in a wall).
Think of it this way: the Free Area is the size of the road, but the EA is the actual number of cars that can get through it every minute. If you put speed bumps, traffic lights, and tight turns on that road (the louvres and baffles of a vent), the physical size of the road stays the same, but the traffic flow (the airflow) drops dramatically.
The relationship is governed by the discharge coefficient (Cd), where EA is essentially Free Area multiplied by Cd. Since the discharge coefficient for a complex vent is typically well under 1, the EA is reduced.
| Feature | Equivalent Area (EA) | Geometric Free Area |
| Measurement Focus | Aerodynamic Performance (Actual Airflow) | Physical Aperture Size (Geometric Space) |
| Airflow Resistance | Accounts for all internal resistance and turbulence. | Ignores internal resistance and turbulence. |
| UK Regulation Metric | Required for sizing background vents under Approved Document F. | An older, less accurate figure, generally insufficient for compliance. |
| Relationship | Always less than the Free Area (for a typical vent). | Always greater than the EA (for a typical vent). |
The VENTI Perspective: Going Beyond the Hole #
As a leading supplier of residential ventilation systems, VENTI’s mission is to empower you to breathe freely. We champion Equivalent Area because we know that effective ventilation is the bedrock of a healthy home. Choosing a vent purely based on a large Free Area figure is a common, yet critical, pitfall that can lead to inadequate air changes, leaving properties vulnerable to damp, condensation, and ultimately, mould growth.
For example, our continuous decentralised extract systems like the ARIA range or the ductless heat recovery units such as FLUXO and AUREN are designed to provide controlled, continuous, and measurable air movement that far surpasses the unpredictable, performance-sapping nature of a poorly specified trickle vent based on Free Area. These systems are about enhanced air quality—filtering pollutants and particulate matter—not just moving air around a blockage.
When specifying for retrofit or new build projects, always demand the EA figure from your supplier. This data is the only reliable metric to ensure that the ventilation capacity meets the specific requirements of the Building Regulations and, crucially, the actual needs of the occupants. A simple measurement of a hole just doesn’t cut it when it comes to healthy living.
Always insist on the Equivalent Area (EA) figure when specifying background ventilators to ensure regulatory compliance and guarantee genuine, effective airflow for a healthier home.