Most homes in our area come up short on attic ventilation. There are several benefits to proper ventilation(air flow) in attic spaces.

1)                          Proper ventilation will keep the attic cooler in the summer and this saves you money on air conditioning bills.

2)                          A cooler attic will make the upper story living space more comfortable during the hot summer days and nights.

3)                          In the winter, proper air flow in the attic helps to reduce ice damming and condensation both of which can lead to damage to the house and mold growth.

4)                          Roofing shingle manufacturers require a 1/300 ratio of the square foot of ventilation to the square feet of roofing. If your attic doesn’t meet this ratio any claims for failed shingle materials can be denied. 

We have found that the 1/300 ratio mentioned above works well for most homes in this area. Keep in mind that attic ventilation should be split somewhat equally between the under eave (soffit) intake vents and the roof top exhaust vents.

As an example a typical 2 story Naperville home has an upper roof section of approx. 1600 square feet. This square footage is divided by 300 to arrive at 5.33sq. ft. of ventilation. This 5.33sq. ft. is then divided in half which comes to 2.67sq. ft. This number is then multiplied by 144 to get to approx. 340 sq. inches.

            The square roof vents that are on most homes in this area have a net free vent area (this is the actual area for the air to flow through a vent) of approx. 50sq. inches. So if we divide the 340sq inches needed by 50 we arrive at approx. 7 vents needed on the roof top. If there are 4 existing roof top vents we only need to add 3 more vents.

            Soffit ventilation is a little trickier because the net free ventilation of the existing soffit ventilation is more difficult to determine. Existing aluminum covered soffits usually have far too few vented panels because these types of panels only offer 8-10% net free vent area. So a soffit with 3 vented panels that are 1sq foot only accounts for approx. 36sq inches of ventilation. And that’s only if the wood soffit above the panels is actually cut out(we often find this is not the case) and the attic insulation is not blocking the air flow(also a common condition). These factors often lead us to ignore the existing vented panels. Most times in a retrofit application we install 8” x 16” soffit vents to the eaves of the home. These vents have a net free vent area of 60sq. inches. So in our example above we need to add 340sq inches divided by 60 is approx 5. The roof top and soffit vents should be spread out somewhat evenly; so, instead of 5 soffit vents we would install 6 vents with 3 in the front soffit and 3 in the rear soffit.