How to Calculate Roof Ventilation

It is recognized that nearly 90 percent of the houses in North America have poor roof ventilation. This is because most people are ignorant about the fact that roof and attic ventilation can bear upon the life of their complete house. The right ventilation in the attic area permits an uninterrupted flow of air from outside through the attic and aids to bring down the temperature in the living area. It maintains equilibrium between the intake and exhaust of air.

There are some norms set by the Federal Housing Administration that specify minimum of one square feet of area of ventilation for every 300 sq. ft. of attic area. For instance you may need 4 sq. ft. of ventilation for an attic area of 1200 sq. ft.

Therefore, prior to deciding about the vents and fans for your attic, you need to work out the area of ventilation necessitated for your roof.

If you have provided the vapor barrier in your ceiling, you have to allow 1 sq. ft. of free ventilation area for every 150 sq. ft. of area of attic, otherwise you need the double of this amount of ventilation. To find out the exact amount of ventilation required for roof of your house, you can follow these simple steps:

 Material and Tools:

  • Tape measure
  • Scribbling Pad
  • Pen
  • Calculator
  • Ventilation Calculation Chart

Step 1- Calculate Square Footage Area of Attic

Measure the length and width of roof of your house in feet with a tape measure from eave to eave. Also, add to it the garage area, if that is attached to house. To calculate the area of attic, multiply the length and width measurements to work out the square footage area of the attic. For instance, if your roof has length 80 feet and width of 30 feet, the entire square footage is 80x30 = 2400 sq. ft.

Step-2- Calculating Net Free Ventilation Area

Divide the total square footage area of attic by 300. We have 2400 sq. ft. area in our example. Divide 2400 by 300 and you get 8. So the recommended net free ventilation area required for your roof is 8 sq. ft.

Step 3- Converting Net Free Ventilation Area to Square Inches

Convert the net free ventilation area into square inches by multiplying it by 144. Please note that all vent products are generally having a rating in square inches. 8 x 144 equals 1152. You require 1152 square inches net free ventilation area.

Step 4- Divide the Total Amount of NFA Required Equally Between the Intake and the Exhaust Vents

Work out the ratios of intake vent-holes and the exhaust vent-holes. The exhaust vents are to be put near the peak of the roof. You may use the ratio of 50:50 for inlet to exhaust. For a ratio of 50:50, you will need 1152 divided by 2= 576 sq. inch for each inlet and exhaust vents.

You can confer with your local building department for their recommendations on deciding and establishing attic vents.

Step 5-Find Out the Number of Vents

Find out the number of vents for each inlet and exhaust. You have to select the standard size of vent in square inches and then divide the square inches of NFA by the standard size of vent, say it is 60 sq. inch for SSB960 vent. For equal ventilation for inlet and exhaust:

Number of inlet vents = 576 divided by 60 =9.6, round up to 10

Number of vents for exhaust also will be 10 in this case.