If you are staring at a gap in a wall, a tired old grille, or a building control note that mentions "free air space" and wondering what it means, you are not alone. Choosing the right size louvre vent seems simple until you try it properly, and getting it wrong can mean a draughty room or a damp problem under your floor. This guide covers what free air space actually is, the standard UK louvre vent sizes and what they suit, how the Approved Document C background ventilation rule works in plain English, and a worked example for estimating how many vents a wall might need, plus a short FAQ.
What Free Air Space Actually Means, and Why It Matters More Than Outer Size
A louvre vent is a ventilation cover with angled slats fitted into a wall, door or duct opening. The slats let air pass through while keeping rain, leaves and larger debris out, and many also have a flyscreen mesh behind them to stop insects getting in. When people ask "what is a louvre vent" for the first time, the outer dimensions are usually what they notice first, since that is what has to fit the hole in the wall or the cut out in a door. But the outer size is not the number that matters for ventilation performance or building regulations compliance.
What actually matters is the free air space, sometimes called free area. This is the real open area through the slats that air can pass through, and it is always smaller than the vent's overall outer size, because the slats and surrounding frame take up part of the opening. Free air space is quoted in square millimetres, and it is this figure, not the outer height and width, that gets compared against building regulations guidance. Two louvre vent covers with an identical outer size can have noticeably different free air space depending on slat design, material, and whether a flyscreen is fitted, so if you are choosing a plaster vent or door vent to meet a specific airflow requirement, always check the free air space figure rather than assuming outer size tells you everything.
Standard UK Louvre Vent Sizes and What They Suit
Most louvre vent covers sold in the UK come in a handful of standard sizes that line up with brick coursing, since most airbrick openings were built to fit a set number of brick courses in the first place. The three main sizes are 9x3 inch, 9x6 inch and 9x9 inch, corresponding to one, two and three standard brick courses. There is also a smaller 6x3 inch size for tighter openings, plus specialist shapes such as circular vents and long narrow letterbox style vents for jobs like door bottoms or duct outlets.
As a rough guide to free air space in this range, a 9x3 inch plastic louvre vent gives around 4400 to 7500 sq mm depending on the flyscreen type, a 9x6 inch plastic louvre vent gives roughly 9500 to 13800 sq mm, and a 9x9 inch plastic louvre vent gives roughly 15500 to 21900 sq mm. For an opening that will not need periodic access, a brown fixed flyscreen 9x9 louvre vent is a straightforward choice for a subfloor void or outbuilding wall, and a black louvre vent in the same sizing range works well where you want the cover to blend into a dark door or frame rather than stand out.
Aluminium vents in the same nominal sizes give somewhat different free air space figures again, since material and slat design differ from plastic. A metal option such as an aluminium 9x9 inch louvre vent suits spots needing more durability, while a fixed plastic vent is usually the simpler, lower cost choice for general background ventilation. Browse the louvre vent collection for the full range of sizes and finishes.
The UK Approved Document C Background Ventilation Requirement, Explained Simply
UK Approved Document C sets out background ventilation requirements for buildings, and one of the most common situations homeowners run into is ventilating a suspended timber floor. Damp, stagnant air trapped under a timber floor is bad news for the joists and boards above it, so the guidance calls for a minimum amount of free air space to keep air moving through that void.
The figure to work to is at least 1500 sq mm of free air space per metre run of external wall, or 500 sq mm per square metre of underfloor area, whichever is greater for your building. The guidance also calls for ventilation openings on at least two opposite sides, so air can move through the void rather than just entering and sitting there. This is usually called cross ventilation, and it matters as much as the overall free air space number, since one large vent in a corner will not shift air through the whole space the way vents on opposite walls will.
The same logic, more free air space means more airflow, applies to a room or duct rather than a subfloor void, but the specific regulatory figure depends on the room's purpose. Kitchens, bathrooms and utility rooms often have separate extract ventilation requirements alongside general background ventilation, so louvre ventilation for those rooms is not always a case of picking the biggest vent available.
Worked Example: How Many Vents Does a Wall Actually Need?
Working out the number of vents a wall needs means adding up the total free air space required rather than guessing, but a rough worked example shows how the numbers come together.
Take a small mid terrace house with a suspended timber ground floor. Say the external wall run available for ventilation adds up to around 12 metres between the front and rear elevations. At 1500 sq mm per metre run, that works out to a target of roughly 18000 sq mm of total free air space, split between the two opposite walls so air can flow through properly. Using 9x6 inch plastic louvre vents giving around 11000 sq mm each, that target needs roughly one and a half to two vents worth of free area per side, which usually means spacing out three or four vents per elevation rather than relying on one large opening. Step up to 9x9 inch vents and fewer units are needed to hit the same total, since each contributes more free air space.
As a rough practical takeaway, a small mid terrace house might need half a dozen decent sized vents or more spread around the perimeter to meet that figure, though the exact number depends on the wall lengths and floor area involved on your property. A longer wall run or bigger floor area can push the number up, so treat this example as a starting point rather than a fixed formula.
Frequently Asked Questions
What is the difference between a louvre vent and a plaster vent?
A louvre vent is the general term for a slatted ventilation cover fitted into a wall, door or duct opening. A plaster vent typically refers to one designed to sit within a plastered internal wall finish, so the frame is intended to be skimmed in flush rather than left surface mounted. Both rely on the same free air space principle described above.
Can I use a door vent instead of a wall mounted louvre vent?
Door vents work on the same slatted louvre principle and are sized for fitting into an internal door leaf rather than a wall, often to allow air to move between rooms or into a hallway. They come in similar standard sizes to wall louvre vent covers, so the same free air space logic applies.
Does a black louvre vent perform differently to a white one?
Colour alone does not change the free air space or airflow performance of a louvre vent cover. A black louvre vent, a white one, and a brown one in the same size and slat design will typically offer very similar free air space figures, so colour is a matter of matching the vent to your door, frame or wall finish rather than a performance decision.
Do I need to calculate free air space myself, or can I just guess?
For anything that needs to satisfy building control sign off, guessing is not a good idea. The figures in this guide are a helpful starting point, but they are not a substitute for checking the current Approved Document C guidance or speaking to a building control surveyor about your specific project. Every building is different, and getting the sums wrong can mean a failed inspection or a ventilation problem left unresolved.
This article is general practical guidance to help you understand how louvre vent sizing and free air space work. It is not formal building control advice, and requirements can vary by project. Always check the current Approved Document C guidance or consult a qualified building control surveyor before finalising ventilation for any project that needs building control sign off.
