A whole house fan and an attic fan get confused constantly, and they solve different problems.
One pulls cool outside air through your living space. The other only ventilates the roof void above it.
A whole house fan pulls cool evening air in through open windows and exhausts it into the attic, cooling the living space directly. An attic fan only exhausts hot air from the roof void, which reduces heat gain through the ceiling. For direct cooling, the whole house fan wins easily, though it needs open windows and cool outside air.
What each one actually does
| Whole house fan | Attic fan | |
|---|---|---|
| Location | Ceiling, usually a hallway | Roof or gable |
| Air source | Open windows in the house | Attic vents |
| Cools | The living space directly | The roof void only |
| Typical airflow | 2,000–6,000 m³/h | 800–2,500 m³/h |
| Needs windows open | Yes, always | No |
| Works when outside is hot | No | Partly |
| Effect on cooling bills | Large, when conditions suit | Modest |
A whole house fan only helps when the outside air is cooler than inside, which usually means evenings and early mornings.
Sizing a whole house fan
The rule of thumb is based on air changes rather than floor area alone.
- Calculate the house volume in cubic metres.
- Target 15 to 20 air changes per hour for effective cooling.
- Divide the result by 60 if working in cubic metres per minute.
- Check the attic has enough net free vent area to exhaust that airflow.
- Confirm the open window area can supply it without excessive velocity.
- Finally, check the noise rating, since these fans are close to living space.
The attic vent area matters more than people expect. An undersized exhaust path pressurises the roof void and reduces airflow badly.
Sizing an attic fan
Attic fans are sized on the roof space rather than the house.
A common target is 10 to 15 air changes per hour for the attic volume. Meanwhile intake ventilation matters as much as the fan, since a fan without adequate soffit vents will pull air from the house instead.
An attic fan with insufficient intake venting can draw conditioned air out of the house through ceiling gaps. In heating climates it can also depressurise the house enough to affect combustion appliances, so intake area is a safety matter rather than a performance one.
When each one is the right choice
- Cool evenings and a large temperature swing suit a whole house fan.
- Hot humid climates with little night cooling suit neither particularly well.
- A very hot attic above poorly insulated ceilings suits an attic fan.
- Homes with air conditioning benefit more from attic ventilation than from a whole house fan.
- Homes without air conditioning benefit enormously from a whole house fan.
- Finally, better attic insulation often beats both for the same money.
That last point deserves emphasis. Insulation works every hour of every day, whereas fans work only under the right conditions.
Using them together
They are not alternatives in every house, and they can complement each other.
A whole house fan handles evening cooling of the living space. An attic fan handles daytime heat build-up in the roof void. Meanwhile both depend on adequate ventilation area, so check the vents before adding any fan.
Our attic ventilation sizing guide covers the vent area calculation, and our roof exhaust fan range covers the hardware.
Safety and installation points
Both fans move large volumes of air through a building envelope, so a few points matter.
Fit an insulated, sealing damper on a whole house fan, otherwise it becomes a heat leak in winter. Check combustion appliance safety, since strong exhaust can affect flue draught. Meanwhile confirm the ceiling opening is properly framed and the fan is isolated to prevent vibration transmission.
For residential ventilation requirements, ASHRAE Standard 62.2 sets out the accepted rates.
House ventilation FAQ
Does a whole house fan replace air conditioning?
In mild climates with cool evenings, often yes. Where nights stay warm and humid, it cannot, since it can only supply air at outside temperature.
Do I need windows open?
Always. Without open windows the fan cannot draw air, and it may pull air from unwanted places such as flues or crawl spaces.
Is an attic fan worth it?
Modestly, and only with adequate intake venting. Improving attic insulation usually delivers more comfort per unit of spend.
Can either fan cause problems in winter?
A whole house fan can, if its damper does not seal. Fit an insulated sealing cover, since an open ceiling aperture is a significant heat loss.
A whole house fan cools the space you live in. An attic fan cools the space above it. Check your ventilation area first, then choose the one that matches your climate.
Choose the right roof exhaust fan
Longwell manufactures 22 roof exhaust fans in-house. Send airflow, static pressure and voltage — an engineer replies with a matching model, performance curve and price within one working day.
