How to Size a Whole House Attic Fan

Thanks to high energy prices and a renewed interest in energy efficiency in the home, whole house fans are making a comeback in many areas. Sizing a whole house fan is a simple matter of measuring your home’s living space and applying a basic formula to find the CFM (cubic feet per minute) rating required to exchange the air in the house every few minutes. One additional calculation tells you what size of vents you’ll need in the attic to ensure proper circulation when the fan is running.

Step 1

Measure the width, length and ceiling height of each room in the house, including bathrooms and the kitchen but excluding closets. Also, measure hallways and stairwells. Write down the dimensions of each space.

Step 2

Calculate the volume (in cubic feet) of each space by multiplying the width, length and height. For example, a 10-foot by 12-foot room with an 8-foot ceiling has a volume of 960 cubic feet (10 x 12 x 8 = 960).

Step 3

Add up the volume of all spaces to find the total volume of air space in the house. Divide this number by 3 to find the recommended CFM rating for the whole house fan. For example, if the total volume of the house is 12,000 cubic feet, you should look for a fan rated for at least 4,000 CFM (12,000 divided by 3 = 4,000).

Step 4

Find the required ventilation area (in square feet) for your attic by dividing the fan’s CFM rating by 750. For example, a 4,000-CFM fan requires at least 5.33 square feet of total vent area in the attic (4,000 divided by 750 = 5.33). Note: This number represents the “net free ventilation area,” not the dimensions of the vent units. The net free ventilation area accounts for any louvers, bug screening and other obstacles to true ventilation; look for this rating when purchasing vent products.

About the Author

Philip Schmidt has been writing about homes for more than 19 years and is author of 18 books, including "Install Your Own Solar Panels," “PlyDesign,” and “The Complete Guide to Treehouses.” Schmidt holds an English degree from Kansas University and was a carpenter for six years before hanging out his shingle as a full-time writer and editor.