A modern villa is far tighter than an old one, the windows stay shut for months, and air conditioning recirculates. Where does the fresh air actually come from?
Air conditioning cools air; it does not necessarily renew it. A modern villa with sealed windows, good door seals and recirculating split units can go a long time without any designed supply of outside air, and the result is rising carbon dioxide, accumulating odours and humidity that has nowhere to go. Ventilation is a separate system with its own design, and the beliefs below are what lead to it being left out entirely.
How to test a claim like this
Ask one question of any house and follow it through: where does outside air enter, how much of it, and where does stale air leave? If the answer involves opening a window in August, the house has no ventilation strategy. If the answer is a fresh air connection to the air conditioning, ask for the designed quantity and whether it was measured at commissioning. If the answer is extract fans in bathrooms, ask what replaces the air they remove.
Myth 1: Air conditioning brings in fresh air.
Reality: Most split units and many ducted systems simply recirculate the air already in the space across a cooling coil. Fresh air is only introduced if the system was designed with a fresh air connection, a dedicated outdoor air unit or a separate ventilation system. Cooling and ventilating are different functions that happen to use similar equipment.
How to check it yourself: Ask the mechanical designer for the fresh air quantity in litres per second and where it enters the system.
Myth 2: A house leaks enough air naturally.
Reality: Older buildings with loose windows and doors did. A modern villa with sealed glazing, insulated walls and weather stripped doors is much tighter, and that tightness is deliberate because it reduces the cooling load. Tight construction without designed ventilation simply moves the problem from energy to air quality.
How to check it yourself: Ask whether any air tightness testing was done and what ventilation provision the design assumed.
Myth 3: Extract fans are a ventilation system.
Reality: A bathroom or kitchen extract removes air, and that air has to be replaced from somewhere. Without a designed supply path, the replacement is pulled from wherever the building leaks, which can mean drawing air from a car park, a riser, a drain void or a neighbouring unit. Extract without supply is half a system.
How to check it yourself: Identify the designed supply path for every extract point, including undercut doors and transfer grilles.
Myth 4: More ventilation is always better.
Reality: Outside air in a Saudi summer is hot and in coastal cities humid, so every litre has to be cooled and dried, and that is a real load. Ventilation is designed to a rate appropriate to the space and its occupancy. Over ventilating raises the energy bill and can introduce more moisture than the system can handle.
How to check it yourself: Compare the designed rate with the applicable code requirement rather than asking for the maximum.
Myth 5: Humidity will look after itself if the air conditioning is running.
Reality: Air conditioning removes moisture only while the coil is cold and air is passing over it for long enough. An oversized unit that cycles rapidly removes little moisture, and a house left unoccupied with the cooling off in a humid city accumulates moisture. Mould in a closed villa in Jeddah is usually a humidity problem rather than a leak.
How to check it yourself: Measure relative humidity in the rooms that matter, and ask the designer how the system controls it rather than just temperature.
Myth 6: Filters are about keeping the equipment clean.
Reality: Filters do protect the coil, and they also decide how much dust reaches the occupants, which matters in a climate with regular dust events. Filter grade, accessibility and the discipline of changing them are design decisions. A high grade filter in a system not designed for its resistance reduces airflow and makes everything worse.
How to check it yourself: Confirm the filter grade, that access for changing it exists, and that the fan was selected for that resistance.
Myth 7: Ventilation can be added later if there is a problem.
Reality: Adding ducted fresh air to a finished villa means opening ceilings, finding a route, finding an intake position away from contamination sources and adding capacity to equipment that was sized without it. Doing the same work during construction is a fraction of the cost and does not involve removing any finishes.
How to check it yourself: Settle the ventilation strategy at design stage and keep the route and intake position on the coordinated drawings.
What this changes in your decision
- Ask for the ventilation strategy as a separate document from the cooling design.
- Confirm the fresh air rate, the intake position and how it is filtered and conditioned.
- Identify a designed supply path for every extract point in the house.
- Require measured fresh air volumes in the commissioning record, not just design figures.
- Plan filter access and a replacement routine into the building maintenance schedule.
Domira builds and supervises projects across Saudi Arabia, and this is the level of detail its site teams work at. Send the drawings or the site, and Domira will tell you what the specification has to say before anyone prices it.
Frequently Asked Questions
Do air conditioners bring fresh air into a house?
Most do not. A typical split unit recirculates the air already in the room across a cooling coil and a filter. Fresh air arrives only if the design includes a fresh air connection, a dedicated outdoor air unit, or a separate ventilation system. In a tightly built modern villa this needs to be designed deliberately rather than assumed.
How do you ventilate a house when it is too hot to open windows?
With mechanical ventilation: a controlled quantity of outside air drawn in through a filter, cooled and dehumidified before it reaches the rooms, with a matching extract path. Heat recovery between the incoming and outgoing air can reduce the energy penalty. The quantity comes from the applicable standards for the space and its occupancy.
What causes mould in a closed villa in a humid city?
Moisture that has nowhere to go. A house left closed with cooling switched off accumulates humidity from the air, from the ground and from any water ingress, and surfaces stay above the temperature and humidity range that keeps mould away. The usual answers are maintaining a minimum level of dehumidification, ventilating, and fixing any actual water source.
How often should air conditioning filters be changed?
It depends on the filter grade, the system and the local dust conditions, and in a region with frequent dust events intervals are shorter than manufacturers assume. Follow the equipment guidance as a starting point, then adjust based on what the filters look like at the first few changes. Make the access arrangement part of the design, because an unreachable filter never gets changed.
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