How Do Attic Fans Work?

Attic fans work primarily by augmenting the movement of hot air rising out of the attic. They do this by transmitting the mechanical energy (kinetic energy) from their rotation to the molecules of the air around them, thus increasing the rate of convection.

As attic fans promote the escape of hot air from the attic, they create a pressure drop. This pressure drop, in turn, will support the inflow of cold air into the attic. So, ultimately, an attic fan works by promoting the outflow of hot air and a corresponding inflow of cool air in the attic and living space.

Normally, hot air rises and escapes spontaneously from the attic as it becomes buoyant through thermal expansion. But this natural process is slow, and it will not get the attic cool enough in real time. This is where an attic fan will come in.

Instead of relying only on the natural ascension of heated air to cool the attic, installing an attic fan will hasten the process by intensifying the outward motion of the rising warm air.

Besides pushing heated air out, attic fans work by pulling fresh air into the attic space. They drive the inflow of cool air into the attic by creating negative pressure (negative pressure is a situation in which the pressure in the attic is lower than the pressure outside).

Air – whether hot or cold – adds to the overall pressure inside an attic. So, as the attic fans drive hot air out of the attic space, the pressure levels will drop, creating negative pressure. Air spontaneously moves from areas of higher pressure to areas of lower pressure. So, with the fall in pressure in the attic, cool air will flow in naturally. Ultimately, the outflow of warm air and the inflow of cold air are driven by attic fans occurring simultaneously.

Do Attic Fans Need Vents to Work?

An attic fan will not work alone – without vents or inflow/outflow paths, hot air will remain in the attic, and cold air will not come in. If heated air remains in the attic, it will ultimately seep into your home and drive up the temperature in the living space. With the temperature in the living space elevated, your air conditioner’s effectiveness will drop – an undesirable situation. Therefore, you must install intake and exhaust vents alongside your attic fan for proper ventilation.

Typically, an attic fan pulls air into the attic from the outdoors using the soffit vents installed in the eaves. The attic fan’s creation of negative pressure zones remains the primary catalyst for the inflow of cool air through the soffit vents.

Also, an attic fan will push hot air out of the attic through exhaust vents such as gable vents, ridge vents, and dormer vents. It may also push warmer air out through another attic fan. However, the vent is usually a gable vent when an attic fan is coupled with a true exhaust vent.

How Are Attic Fans Powered?

Most attic fans are electrically powered fans installed on roofs or gable walls to exhaust hot attic air to the outside. However, some are powered by the kinetic force of the wind.

An electric-powered attic fan may get power from your home’s main electricity supply. Alternatively, it can be powered by solar energy from the sun through PV panels. The solar-powered attic fans tend to work energy-efficient. In addition, since they are not connected to your home’s electrical supply, they do not cause any extra charges on your electricity bill.

Should the Attic Fan Work All the Time?

An attic fan does not have to run all the time. The essence of having an attic fan is to cool your attic down whenever the air in it becomes hotter than the outdoor air. So, going by this, your attic fan should only run when the attic becomes hot.

Besides, if you use an electric attic fan, you should not keep running constantly, or your energy bills may hit the roof.

When one is looking to buy a solar attic fan, the positive impact on your energy bills is a bonus, but this is not guaranteed. For one, the sun will not always be up in the sky. Second, even if your solar energy system comes with a battery, batteries have limited capacity, so they cannot endlessly power the attic fan.

The operation of a wind attic fan or wind turbine depends on the availability of wind. Since nature determines when the wind blows, it is unlikely that a wind turbine will run all the time.

Attic Fans With Thermostat

To keep attic fans from running all the time, some manufacturers include a thermostat in the structure of their fans. With a thermostat, the operation of the attic fan is automated.

When the attic temperature rises to a threshold point set by you, the thermostat senses the change and activates the fan. Then, the thermostat senses it and deactivates the fan when the temperature falls below the threshold.

Attic Fans With Humidistat

Some attic fans also work with a humidistat. In a way, the humidistat functions like a thermostat. But instead of sensing temperature changes, the humidistat senses changes in humidity.

So, when the relative humidity in the attic rises above a set threshold point, the attic fan comes on. Then, when the humidity falls below the threshold point, the attic fan stops.

Why Attic Fans Have Humidistat

Attic ventilation goes beyond just bringing the temperature down; it also reduces humidity. If your attic experiences sustained levels of elevated humidity alongside the rising temperature, many items in the attic might get damaged.

For one, the growth of mold and mildew promoted by humid conditions spell doom for the pieces of wood in your attic. But it goes beyond just wood. Mold and mildew can damage many other organic objects in your attic.

Besides promoting fungi that can damage your attic, high humidity levels will make the attic inhabitable. Unfortunately, humid air is not the healthiest for anyone’s lungs.

Attic Fans Without Humidistat and Thermostat

While some attic fans work with thermostats and humidistats, some do not. Some of those that do not have these sensors come with timed switches. You turn the attic fan off with the switch and set the period you want it to operate. When the time elapses, the fan goes off.

The timed sensor is also a good way to ensure your attic fan does not keep running on and on. However, since its operation does not depend on temperature changes, the attic may not be cool before the attic fan goes off. So, in the end, you have to verify for yourself that the attic got cool.

Types of Attic Fans

Wind Attic Fan (Wind Turbine)

Wind attic fans or wind turbines work and are powered by natural energy – the wind’s kinetic energy. As wind drifts by, it collides with the blades of the turbine. This collision, in turn, causes the turbine to rotate and pull hot attic air out, leaving space for fresh air to come in.

The larger the volume and the faster the wind, the more effective ventilation is with wind turbines. But this also means that the turbine cannot remove hot air from your attic in the absence of wind or with a smaller wind flow. This phenomenon places a question mark on the reliability of wind turbines in cooling attics.

Since the wind flow varies greatly, you cannot rely on its presence to power wind turbines to keep your attic cool.

Nonetheless, the upside of using wind turbines to cool attics is that you save money. Since they are powered by the natural energy of the wind and not your home’s electrical supply, you do not pay a dime.

Electric Attic Fan

Electric attic fans are the most common type of attic fans. They are connected to your home’s electrical supply, affecting your attic bills. Nonetheless, electric fans typically have a higher venting capacity than other attic fans.

More times than not, an electric attic fan will come with a thermostat. This way, the attic fan will not keep running and raising your utility bills.

In addition, the thermostat feature is convenient in the summer when the attic might be hot for longer periods. But even with a thermostat, the cost of operating electric-powered fans tops the cost of operating other attic fans.

Solar Attic Fan

Unlike electric attic fans, solar attic fans get their electrical energy from the sun’s solar energy. Therefore, they are not connected to your home’s electrical supply and do not affect your utility bills.

A solar fan may come with a separate solar panel or with the panel attached to it. The ideal solar panel angle and azimuth depend on the position where the fan will be installed.

With solar attic fans, abundant sunlight is vital. When there is enough sunlight, the fans will ventilate your attic effectively. However, the fans will be less effective with less intense sunlight, and if the skies are overcast, they will not work. This is why they are best used in regions that receive a lot of sunlight.

Of course, this may have you questioning the reliability of solar attic fans. But the situation is different from what you have with wind turbines.

Some solar attic fans come with batteries. The solar panels charge the batteries. So, the batteries power the fans when there is no direct sunlight.

Solar attic fans are less likely to come with thermostats than electric-type fans. Those without thermostats may keep running when the sun is out.

How Are Solar Attic Fans Different?

Eco-friendliness

Compared to electric-type fans, solar attic fans work on renewable energy. So, they are eco-friendly, and you do not have to worry about your home’s carbon footprint.

Longer Warranty Period

Besides their eco-friendliness, solar attic fans have longer warranty periods than electric fans. On average, an electric fan would have a 1-year warranty, but a solar attic fan can have a 25-year warranty.

They Thrive Better When It’s Hot

During hotter periods, attic fans running on electricity must work harder to cool the attic. Therefore, they will consume even more energy than usual. However, things are different with solar fans. Since solar fans run on solar energy, they thrive better during hotter (sunny) periods.

You Can Get Government Incentives

With solar attic fans, you might be eligible for government incentives, which reduce the overall cost of going solar. But no such thing exists with electric fans.

They Are Relatively Safer

Since they have higher venting capacity, electric fans are more likely to create intense negative pressure zones in the attic. When this happens, there is a chance that toxic combustion gases will be drawn into the house. This is not very likely with solar fans since their venting capacity is not very high.

They Do Not Come With Energy Consumption Cost

Of course, since solar attic fans can work solar energy-free, they come with no energy consumption cost. They also come with little or no operating cost – you may only have to spend a little fee on maintenance occasionally.

They Are More Expensive

While the operating costs of solar fans are meager, getting one is higher than getting an electric fan or wind turbine.

Their Venting Capacity Is Relatively Lower

The venting capacity of an electric fan is generally higher than a solar fan’s. Electric-powered fans are more likely to create greater negative pressure zones.

The Pros and Cons of an Attic Fan

Pros

They Keep Your Attic Cool

With poor ventilation in your attic, there will be heat accumulation when heat accumulates in the attic, the likelihood of damage to the attic’s contents, the attic itself, the insulation, and the roof increases. But with an attic fan channeling heated air out of your attic, none of these will happen.

Of course, by channeling heated air out of your attic space, an attic fan will create space for cooler air. Then, when cooler air comes in, the attic becomes cool.

Besides, without an attic fan, your attic space can get hotter than the rest of the house. If this happens, the unchecked heat accumulating in the attic will eventually leak into the rest of the house. Consequently, your air conditioning system’s workload will increase, raising your utility bills.

An attic fan will ensure that the rest of the house does not get hot since it will push out heated air and prevent accumulation in the attic.

They Protect the Roof

The accumulation of heat in the attic will eventually spread to the roof. When this happens, the roof will age faster than it should. You may notice this in the fading color, curling, warping, and blistering of the roof shingles and wood sheathing.

With an attic fan, there will be no heat accumulation. So, the roof is protected from accelerated thermal degradation.

They Help Reduce Humidity and Prevent the Growth of Mold

Warm air typically holds more water than cooler air. So, your attic might become more humid as it becomes hotter.

Moisture promotes the growth of microorganisms, including mold and mildew. So, as your attic becomes more humid, it becomes more conducive for mold and mildew to grow.

However, as attic fans bring fresh outdoor air while taking stale attic air out, there will be little or no rise in attic temperature. Consequently, there will be no mold growth.

They Eliminate and Prevent Ice Damming

In winter, houses with ineffective insulation and ventilation suffer ice damming. This happens when heated air leaks into the attic from the living space and melts the snow on the roof.

As the snow melts, it runs off towards the edge of the roof, where it refreezes into ice. This ice formed at the edge of the roof acts as a barrier. So, as more snow melts, it holds the ensuing water back, creating something like a dam.

The retained water may damage the walls and sheathing of your home if unchecked. But with an attic fan, there wouldn’t even be ice dams in the first place.

Cons

They Increase the Risk of a Roof Leak

Whenever you install a roof attic fan, there will be a risk of roof leaks. This is even more likely if the hole in the roof is not maintained properly or if the installation is done with inappropriate flashing. With gable attic fans, the risk is lower but not completely absent.

They May Pull-in Toxic Combustion Gases

As we mentioned earlier, attic fans, especially the electric types, can create intense negative pressure zones in the attic. These zones may then drive the flow of toxic combustion gases (such as carbon monoxide) alongside outdoor air into the attic and the living space.

Other Ways to Keep Attic Space Cool

Passive Ventilation

Passive ventilation involves the natural rise and escape of heated air from a hot attic, replacing the escaping air with cool air.

It is driven by thermal buoyancy and the movement of the wind, and it is only effective when passive vents such as gable, ridge, or soffit vents are installed.

Install an Air Conditioning Unit

Installing an air conditioning system is one of the most effective ways to cool a hot attic. However, it is also one of the most expensive options. The costs go beyond buying and installing air conditioners since their operation will affect your electric bills. But, on the bright side, the cooling is worth it.

If you ever decide to make your attic a functional space, then an air conditioning unit should be one option you consider for attic cooling. A mini-split ductless or portable air conditioner should do when getting an air conditioner for your attic.

You could even get an HVAC system instead of just an air conditioner. An HVAC system goes beyond just attic cooling since it can keep the attic warm in winter and remove humid air.

Fix the Cracks and Gaps

To ensure your attic stays cool, you must ensure no cracks, gaps, or holes. If you have such openings in your attic, heated air will seep in during the summer months and raise the temperature, leaving you with a hot attic.

Openings less than 0.25 inches can be effectively sealed with heat-resistant caulk. However, larger ones should be sealed with expandable foam spray.

Install Radiant Barriers

Radiant barriers dispel radiant heat before it enters the attic space. Such barriers are made of reflective materials (such as aluminum foil), which bounce the sun’s radiant energy back before it ever gets in the attic.

Other reflective materials used as radiant barriers include foil-faced plywood and oriented-strand board sheathing. Compared to aluminum foil, these two are more solid.

Install Reflective Roofing

The activity of reflective roofing is based on the same principle as that of radiant barriers – they both reflect solar energy away from the attic. The main difference is that the reflective material is installed on the roof for reflective roofing, not under it.

Improve the Insulation

Without proper insulation, most of your efforts to cool the attic may not be as effective as they should be. However, if you insulate your attic correctly, your heating and cooling costs may fall by as much as 20%. Also, the energy efficiency of your whole house will go up.

Insulation blocks the entry of heat into the attic by conduction. So, even when the sun’s roof and walls get heated, the heat will not be transferred into the attic space.

How to Insulate an Attic

There are many ways to insulate an attic. However, spray foam insulation is one of the most common ones for a good reason. It comprises isocyanate and polyol resin – two substances that expand swiftly and create countless air bubbles when they mix.

Spray foams work in 3 ways to block heat transfer. First, the spaces that come with the formation of air bubbles block heat transfer by conduction. Also, the bright color of the spray foam bounces radiant heat back by reflection. Then, the air bubbles ensnare heat gained by convection.

Besides high-tech foam, other materials like cotton, cellulose, mineral wool, and fiberglass may also be used in insulating attics.

Final Thoughts

The basic operation of an attic fan is to get heated air out of a hot attic while facilitating the inflow of cool air into the same attic. With enough makeup air (sufficient passive vents), proper insulation, and sufficient fans, attic fans do an excellent job keeping attics cool.

But you can make your attic even cooler by using attic fans alongside other means of attic cooling.

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