Do I need 2 attic fans?

Quick Answer

Whether you need 2 attic fans depends on several factors, including the size of your attic, your climate, and your goals for ventilation. In most cases, 1 powerful attic fan is sufficient, but 2 attic fans can provide better cooling and ventilation for large attics. Installing 2 attic fans on opposite ends of the attic creates a cross flow of air that helps remove hot air more efficiently.

What Does an Attic Fan Do?

An attic fan, also known as a whole house fan, is installed in the attic and pulls hot air out of the attic, creating a negative pressure that draws cooler air in through open windows in the house. This exchange of air cools the entire house by removing built-up heat in the attic.

Attic fans are useful for reducing air conditioning costs in hot climates and provide whole house ventilation. They are typically installed on the attic floor to pull hot air up and out through vents in the roof or gable ends. Powerful attic fans can exchange the entire volume of air in a house in just a few minutes.

When Are 2 Attic Fans Better Than 1?

Here are some situations when installing 2 attic fans may be preferable to just 1:

– For large attics over 1500 square feet – A single fan might not be powerful enough to provide sufficient airflow throughout a very large attic. Adding a second fan improves ventilation.

– For long rectangular attics – Long, narrow attics benefit from having a fan at each end to create a cross breeze that flushes hot air out.

– If there is an attic division or blocked vents – Installing a fan at each end of a divided attic or on either side of blocked vents can help ensure good airflow throughout.

– For zoned cooling – Some homes have zoned HVAC systems with separate thermostats. Dual attic fans with separate controls can target ventilation to different areas of the home.

– As a backup – Having two attic fans provides redundancy in case one unit fails or needs repair.

Disadvantages of 2 Attic Fans

While dual attic fans have benefits in certain situations, there are also some downsides to consider:

– Increased cost – Installing two units costs more upfront than a single high-capacity attic fan. There are also additional electrical and ducting costs.

– More roof penetrations – Each fan needs its own vent cut into the roofline, which creates potential for leaks. Proper flashing during installation prevents leaks.

– Conflicting airflows – Fans positioned poorly can have airflows working against each other, reducing efficiency. Proper placement is key.

– Dual noise – With two units running, there will be more fan noise coming from the attic. Proper sizing the fans and using insulated ducting reduces noise.

– No smart control – Two units controlled independently lack the smart optimization of a single variable speed or WiFi-enabled fan to regulate airflow. Upgrading to “smart” fans adds cost.

Best Practices for Installing 2 Attic Fans

To maximize the benefits and minimize any downsides of dual attic fans:

– Size the fans appropriately – Determine the necessary CFM (cubic feet per minute) rating for each fan based on your attic size and climate. Oversizing can shorten fan life.

– Position at opposite ends – Install the fans as far apart as possible to create a cross breeze. Avoid positioning them near each other.

– Use thermostats – Have each fan controlled by a separate thermostat to target different zones or turn off one fan as needed.

– Install proper ducting – Use insulated, smooth metal ducting the same diameter as the fans to reduce noise and airflow restrictions.

– Get optimal roof venting – Work with a roofing contractor to determine the number and placement of vents that maximize airflow based on the fan positions.

– Wire into one switch – Having both fans wired into a single switch lets you choose to run one or both as needed.

– Maintain airflow balance – Keep attic vents clean and clear of obstructions so the fans don’t create negative pressure and draw air from the house.

One Powerful Fan May Be Enough

For many homes, a single high-performance attic fan in the range of 3000-5000 CFM provides sufficient ventilation to keep the attic and living spaces cool. Large attic fans can have a 30+ foot range for air circulation and cost much less than installing two smaller units.

Variable speed attic fans are ideal for customizing airflow based on changing conditions throughout the day. They can run on lower speeds to minimize noise as needed.

Here are signs that a single attic fan provides adequate ventilation:

– Attic temperature is within 10-15 degrees of the outside air temperature.

– The attic humidity level is around 70% or less. Higher indicates insufficient airflow.

– There is no moisture buildup on the roof deck and insulation. Excess moisture can cause mold.

– On hot days, the house temperature decreases within about 15 minutes of running the attic fan.

– There are no big differences between attic temperatures at opposite ends, indicating good overall airflow.

If a properly sized, high-quality single attic fan positioned centrally provides enough cooling and ventilation for your attic, there is no need to install a second unit.

Key Factors in Deciding How Many Fans You Need

The main considerations when determining if one or two attic fans are right for your home are:

– Attic size – The larger the attic, the more powerful the ventilation required. Over 2000 sq ft often benefits from two fans.

– Roof and attic layout – Long rectangular attics and divided attics lend themselves to dual fans for crossflow.

– Climate conditions – Hot, humid climates benefit more from maximum attic ventilation using dual fans.

– Budget – Installing two fans costs more upfront but can pay off long-term in energy savings.

– Noise tolerance – More fans equals more noise, which can be minimized but not eliminated.

– Zoning needs – Separate fans can better target different zones in zoned HVAC systems.

– Ability to balance airflow – Preventing backdrafts or short cycling requires careful placement and ducting of dual fans.

– Local codes and regulations – Building codes may limit the number of roof vents allowed for dual attic fans.

Considering these factors together will determine if one or two attic fans best meet your home’s needs and provide the most cost-effective solution.

How to Size Attic Fans Properly

Attic fans are sized according to CFM, which is the volume of cubic feet of air they can move per minute. To size attic fans:

1. Calculate the attic square footage by measuring length x width.

2. Multiply the square footage by 0.7 to get the minimum recommended CFM for ventilation.

3. Add 10-20% more for a safety factor depending on the climate. Hot, humid conditions need more ventilation.

4. Choose fans that meet or slightly exceed the target CFM. Oversizing more than 20% reduces effectiveness.

5. For dual fans, size each to ventilate 50-70% of the total area based on placement.

For example, a 30×40 ft attic is 1200 sq ft. The minimum ventilation CFM would be 1200 x 0.7 = 840. Add 20% for hot climate: 840 x 1.2 = 1008 CFM. For dual fans, size one ~600 CFM and the other ~400 CFM.

Proper sizing balances sufficient airflow with reasonable noise and energy use. Overpowered fans are loud, stressful on roofs, and waste energy.

How Much Does Adding a Second Attic Fan Cost?

Typical costs for attic fan installation:

– Attic fan unit: $100 – $300 each

– Roof jack/vent: $100 – $300 each

– Wiring and switches: $20 – $100

– Ducting: $5 – $10 per foot

– Labor costs: $200 – $1000

For a single attic fan, expect total installed costs of $500 – $1500 depending on the fan CFM and complexity of the installation.

Adding a second attic fan of similar size and ventilation needs will generally double the costs for the additional unit, roof jack, wiring, switches, ducting, and incremental labor to install.

So installing two mid-range 1500 CFM fans would cost around $2000 – $3000 total. Costs are higher for multiple larger, more powerful fans.

Getting quotes from local HVAC professionals is the best way to estimate actual costs for your specific project.

Are 2 Attic Fans Better Than 1 Larger One?

Is it better to have 2 smaller attic fans or 1 bigger one? Consider the following factors:

– Cost – 2 smaller fans often cost more combined than 1 oversized fan due to doubled labor and materials.

– Ventilation area – 2 smaller fans can cover a larger roof area, which helps for large attics.

– Redundancy – Having 2 smaller fans provides backup in case 1 fails.

– Noise – More fans means more noise, but larger single fans can get noisy at higher speeds.

– Airflow control – With 1 fan there is more ability to adjust airflow using a variable speed fan.

– Installation – 2 smaller fans requires more cuts in the roof and potential leaks.

For attics under 1000 square feet, one properly sized fan is usually adequate and simpler to install and operate. Over 2000 square feet, dual smaller fans may better meet ventilation needs.

In the 1000-2000 square foot range, choose based on layout, climate, and noise tolerance. Get professional input on the best plan for your specific home.

Typical Attic Fan Sizes

Here are some typical attic fan sizes and corresponding CFM capacities:

– 14 inch – 1600-2400 CFM

– 16 inch – 2200-3000 CFM

– 18 inch – 2800-4200 CFM

– 20 inch – 3000-5000 CFM

– 24 inch – 4500-7000 CFM

– 30 inch – 6000-8000 CFM

Measure the roof opening where you plan to install the fan rather than the fan itself, which is larger.

For dual fan setups, a combination like a 16 inch and 14 inch fan or 20 inch and 18 inch fans can provide good airflow while minimizing noise and costs.

Always get a fan rated for at least the minimum calculated CFM but don’t oversize more than 20% beyond that.

Roof Venting Needs for Attic Fans

Attic fans pull air in through lower attic vents and push it out through an upper roof vent. For proper airflow:

– The total net free vent area should equal 1:300 ratio of vent area to attic square footage.

– At least 50%-80% of venting should be at the upper roofline where fans exhaust hot air.

– There should be soffit or low vents allowing air intake to balance the fan outflow.

– Vent openings should be evenly spread out, not clustered.

– Vents must not be blocked by insulation or roof framing.

– Powerful fans may need a larger roof jack than the fan diameter to minimize backdrafting.

Consult local codes for venting requirements when adding attic fans. Meeting the 1:300 ratio is key to preventing moisture buildup.


The decision of whether to install 1 or 2 attic fans depends on your attic size, climate, budget, and specific home layout and ventilation needs. For many homes, one properly sized high-capacity attic fan provides sufficient airflow. Two attic fans are better for large, multi-zone, or difficult to ventilate attics to improve cooling, remove humidity, and balance intake and exhaust airflow. Have an HVAC professional assess your attic and provide quotes to help choose the best option. With good placement, ducting, and roof venting, dual attic fans can work together to maximize ventilation benefits.

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