How many solar attic fans does my home need?

Solar panels on brown roof

A common question we get from consumers is: exactly how many solar attic fans do I need to install in order to ventilate my attic properly? 

The answer largely comes down to the size of your attic, and the elevation of your home. 

As a general guideline for estimates, attic size is the first consideration homeowners should consider. This will help you approximate how many solar attic fans are needed. It will also help you figure out which types of fans should be installed in order to achieve maximum airflow. It is important to bear in mind that different brands of solar attic fans vary in efficiency and size. This may entail different considerations when these estimates are made. For simplicity, this guide will use iSolar Solutions’ products to help homeowners estimate how many solar attic fans are needed. 

Figuring out your attic’s size

Some basic mathematics will help you figure out the size of your home’s attic. Simply multiplying the length and width of your home will give you an approximate estimate of your attic’s size in square feet. The efficiency of solar attic fans is measured in CFM (cubic feet per minute) of air moved. So understanding the size of your attic is a critical first step. 

Considering the elevation of your home

Is your home a one-storey bungalow or does it have multiple floors? Is it fully detached? These considerations will also play a role in determining how many solar attic fans will be needed to achieve optimal attic ventilation. 

Small and medium-sized Bungalows

If your home is a one-storey bungalow with an attic less than 1,000 sq ft. in size, one iSolar 20WSPF-FLEX fan is all your home will likely need. These fans have a peak efficiency rating of up to 400 CFM. Slightly larger bungalows between 1,000 sq ft. and 2,000 sq, ft. may require two 20WSPF-FLEX fans. These can be strategically located on the roof to optimize airflow. Typically, they should be spaced 15 feet apart. 

Larger homes between 1,000-2,000 sq. ft.

If your attic is 1,000-2,000 sq. ft. an additional consideration that needs to be factored is the number of existing passive vents on your roof, as the iSolar 20WSPF-FLEX can be easily retrofitted into existing vent holes. 

As mentioned before, it is generally advisable to install multiple small solar attic fans on larger roofs. This is because solar attic fans work best when spaced as far apart as possible (at least 15 feet apart). Larger, more powerful solar attic fans can also be used, but only one such larger unit would be recommended. In general, smaller, modular units like the 20WSPF-FLEX would be much more effective. 

The number of solar attic fans on any roof should never exceed ⅓ of the number of existing passive vents. Sometimes, the small number of passive vents on a roof does not permit the installation of many smaller fans. In these cases, the recommended solution is to install a single large fan such as the iSolar 40W-INT fan

For larger bungalows with attics that are 2,000 sq. ft. in size or more, a good guideline is to install either two to three smaller 20WSPF-FLEX fans, one to two medium-sized iSolar 18W-INT solar attic fans (rated at 810 CFM), or a single iSolar 40W-INT or 50W-INT fan that can push in excess of 2,000 CFM.

Homes larger than 2,000 sq. ft.

For two-storey homes with attics less than 2,000 sq ft., it is recommended to install either one or two 20WSPF-FLEX fans. Alternatively, a single 18W-INT fan will also work. For two-storey homes with attics between 2,000-3,000 sq. ft., two or three 20WSPF-FLEX fans, or one to two 18W-INT fans is advised. Larger two-storey homes with attics over 2,000 sq. ft in size are encouraged to install either two to three 18W-INT fans, a single large iSolar 40W-INT fan, or a 50W-INT fan.

Semi-detached homes or townhomes will typically install either one to two 20WSPF-FLEX fans, or a single 18W-INT fan. A full list of these estimates can be found in the FAQ page of iSolar Solutions’ website. 


In iSolar Solutions’ list of products, you will find our fans are classified as either MOD or INT fans. You may be wondering what these designations mean. 

MOD fans are considered modular, meaning that they can be coupled with a second fan if an attic requires additional airflow. MOD fans provide medium-capacity airflow, and feature adjustable solar panels to best capture the sun’s rays. They are also more than half the cost of more expensive INT fans. 

In contrast, INT fans are integrated, one-piece units which are much heavier, but also much more powerful. INT fans like the 40W-INT or 50W-INT are not designed to be modular, as typically only one such unit would be installed in a central position on the roof. An iSolar Solutions representative can help you figure out which of these options is right for your home. 

Get an accurate estimate

Achieving optimal airflow in your home’s attic requires an intricate knowledge of building science and roof design. The information presented here is intended to be a general guideline for homeowners to estimate the cost of installing solar attic fans. To find out more about what solar attic fan solutions are right for your home, please contact the experts at iSolar Solutions for more information.

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