Interested in learning the pros and cons of attic fans?
Attic fans can be useful to ventilate problematic attics but there are a few downsides.
In this guide, you will learn…
- The top pros and cons of powered attic fans
- Why attic fans may protect your asphalt roof shingles
- Attic fans sometimes lead to carbon monoxide problems
- How attic fans can dramatically lower the temperature of your attic
- Sometimes attic fans raise HVAC running costs (rather than lower)
- Attic fans sometimes prevent ice damming
Let’s get into the details of the pros and cons of attic fans!
What Are Attic Ventilation Fans?
Attic fans are fans that remove warm attic air to the outside --- these fans are installed on the roof or gable of a home.
They are intended to help reduce the temperature of the attic and to eliminate moisture buildup. Attic fans are either powered by the home’s electrical system, solar powered, or wind powered.
Almost all attic fans (except turbine and solar) are controlled by a thermostat. The thermostat is adjustable by the homeowner, and the attic fan will turn on when the attic hits that temperature.
For example, if the thermostat is set to 95 degrees Fahrenheit (a common setting), when the attic reaches 95 degrees, the fan will turn on until the temperature drops below 95.
The most common type of attic is the electrically powered fan. These attic fans are hardwired into the home's electrical system. They are either installed on the roof as a roof mounted fan, or it is installed vertically on the gable wall of the home. A gable attic fan uses the the existing gable vent opening to blow out hot attic air.
Solar powered attic fans use the energy from the sun to power the fan. The solar panel will either be integrated with the fan, or there will be a separate solar panel that is attached by a cable to the fan.
Solar attic fans do not usually have adjustable thermostats, and simply turn on 24-hours when the sun is out. Solar fans are good for homes that get a high amount of sunlight.
Turbine attic fans have wind turbines that are simply moved by the wind. The turbines are usually made out of metal, and they are a good choice for homes that get a high amount of wind.
Read Also: Venting Without Soffits? (Is It Possible?)
What Are The Main Factors Of Attic Fan Performance?
There are some critical advantages to having an attic fan such as keeping your attic cool and removing moisture --- but there also some hidden benefits that many owners do not realize.
But how well an attic fan performs depends on some external factors such as....
Is your attic well insulated? A poorly insulated attic will still leak interior warm air into the attic during winter (hot air rises), and during summer hot air will radiate heat downwards into the home like a furnace.
Before even considering installing an attic fan, it would be a good idea to make sure your insulation is adequate.
Does your attic have the proper amount of makeup or intake air? For an attic fan to work properly, there has to be an adequate amount of passive venting for it to pull air from.
If there isn't enough passive venting in the attic from the gable vents, soffit vent, single passive vents etc. then the attic fan will pull in conditioned air from the interior of the home.
What Are The Pros?
There are numerous advantages to installing an attic fan, but below are the most important benefits...
1. Asphalt Roof Protection
Probably one of the biggest and little known benefits of having good attic ventilation is that it can help prevent premature aging of your roof. Many attics that I inspect just have old and degraded insulation, or missing insulation, and conditioned air leaks into the attic.
Also, there are usually issues with the passive ventilation system. There are vents under the eaves called soffit venting which are commonly missing, inadequate, or even painted over.
The easiest sign for me that an asphalt shingle roof has prematurely aged is the color of the shingles.
As you can see in the picture below, this roof covering is actually only seven years old, but based on the fading, it appears to be more like a 20+ year roof! Since standard roofing shingles are made from asphalt, which is derived from crude oil, when it heats up, it can literally “cook the shingles” on your roof.
The premature aging will show up in the faded coloring, as well as blistering and curling of the shingles. According to Building Envelope Consultants, not only can a poorly ventilated attic accelerate shingle aging, it can even warp the wood sheathing of roof, leading to shingle bucking and ridging.
Read Also: Protect Your Roof With Attic Fans?
2. A Cooler House
Besides protecting a roof, lowering your attic temperature during warm weather will make your home overall more comfortable. A hot attic will create temperature differences in the home, with the second level of a home being much warmer than downstairs.
During hot weather, the sun will heat up the roof, and this heat will transfer down to the attic through the wood framing and wood sheathing. According to Oregon State University, the attic acts like a “heat trap” similar to how a car heats up when the windows are closed on a hot day.
And this extraordinary heat, sometimes up to 150 degrees, can still transfer heat to the home’s interior long after the sun has gone down.
In addition to helping cool down the home, a cooler attic may help reduce your air conditioning costs.
If you have a very hot attic, this heat will leak into the home, causing your air conditioner to work harder, and to run more often, which may dramatically increase your a/c costs. Besides the increase energy usage, having your A/C run more often will prematurely age your air conditioner.
The average life expectancy for an air conditioner is about 15 years, but if it is run hard, it may only last 6-8 years.
Read Also: The Best Roof Mounted Attic Fans
3. Stop Mold Growth
A poorly ventilated attic can lead to mold growth, during cold or warm weather months. During hot weather, you will have the A/C cranking out cold air in the home’s interior, and there will be hot air in the attic.
When these two different temperatures meet, condensation and moisture can form in the attic. When there is darkness, moisture, and a food source (in the form of wood) mold growth can occur.
An attic vent fan can help reduce moisture in the attic, and thereby inhibit mold growth.
4. Eliminate Ice Damming
Ice damming is a strange phenomenon of a poorly ventilated attic, and it only happens during cold weather. According to Attic Insulation Toronto, ice damming can have numerous causes and result in major issues.
If the attic is poorly ventilated and insulated, the warm air can leak into the attic from the home’s interior, and this can cause snow to melt on the roof. When this snow re-freezes, it turns to ice. And since the part of the roof closer to the eave is the coldest, ice forms on the roof edge.
And when additional snow melts, it falls back down the roof to this mass of ice which now acts like a dam, holding back proper roof drainage. This “dam” can cause water damage to the roof sheathing, and to the home’s wall — damage to wood joists, drywall etc.
This water intrusion into the home can also lead to mold growth.
What Are The Cons?
There are some potential negatives to installing an attic exhaust fan. Here are some well known possible disadvantages to attic fans…
1. Carbon Monoxide Risk
If you have natural gas (or propane) burning appliances such as a water heaters or furnace, then installing a powered attic fan can possibly lead to carbon monoxide poisoning. (NOTE: This consideration doesn’t apply to solar powered or turbine attic fans because they are less powerful.)
According to the Home Improvement Contractors of Staten Island, the main reason that is happens is that powered attic fans can create a negative air pressure in the home’s interior. When the home’s interior air becomes negative, it can literally pull the water heater (or furnace) combustion gases into the home, rather than venting to the exterior.
An easy way to find out if your home has a negative air pressure is to partially open a window when the attic exhaust fan is running. If air is being pulled INTO the home, then it is under negative pressure.
However, this risk can be mitigated if you follow the manufacturer directions of the powered attic fan. It's also important to always have a carbon monoxide detector on each level of the home if you have gas-burning appliances or an attached garage.
Attic fans are rated by CFM of cubic feet per minute, which is how much air the fan pulls in one minute. The attic fans needs to have a minimum amount of “makeup air” or intake air which is the vent area that the attic fan pulls air from.
If there isn’t enough venting in the attic for the attic fan to pull air from, it will start “pulling air” from the home’s interior, through the wall cavities, and drywall ceilings.
Read Also: How To Install A Solar Attic Fan
2. May Increase Energy Costs
There have been studies performed that argue that attic fans do not decrease the energy usage of a home’s HVAC system.
The authors stipulate that the energy required to run the attic vent fan offsets any gains by increasing attic ventilation as a result of the fan. In addition, the thermostatic attic fan may be pulling conditioned air from the home’s interior, thereby decreasing even further any energy gains as mentioned by the U.S. Energy Department.
Also, factoring the cost of the attic ventilation fan (and installation), it may take in excess of 30 years to regain the costs through increased energy efficiency because of increased attic ventilation.
3. Roof Leak Risk
Any time you install something onto a roof, there is a possibility of a roof leak. The probability may be increased if improper flashing is used, or if the roof penetrations are not properly maintained. Of course, a gable attic vent fan minimizes this risk because it isn’t installed on the roof but vertically on the gable wall.
According to FloridaDisaster.org, most attic fans (and other roof penetrations) are not designed to prevent water intrusion during hurricanes. Attic fans are designed to keep out “normal” rainfall, but during hurricanes, the power of the wind can push water up the roof in sheets.
Are Attic Fans Worth It?
The problem with making a definitive statement on attic fans is that every house is different. There are different climates all around the country, some climates are more beneficial to attic fans.
If you have good continuous soffit venting and a ridge vent, then any type of attic fan may be totally unnecessary. However, in my experience, most homes do not have the proper passive ventilation. Many times vents are painted over, insulation covers soffit venting in attic, and the ridge vent is obstructed by roofing felt.
The arguments against the energy efficiency of attic fans may be countered with the use of solar attic fans or even turbine attic fans. Also, every house has different levels (and quality) of attic insulation and air sealing, which can dramatically affect the performance of attic fans.
In addition, many of the complaints from professionals about attic fans may be a result of improper installation such as inadequate venting for makeup air.
There are definitely positive testimonials from many users of attic fans around the country. But there are also many negative sentiments from contractors and other professionals about attic fans --- so attic fans are a somewhat controversial topic in the home industry.