If you are considering re-roofing a property it’s a good time to address an issue that most people overlook. Attic ventilation. A properly ventilated attic can increase the energy efficiency of the HVAC, fight heat and moisture in the attic and will most likely increase the life of the new shingles.
From our experience most apartments, particularly older properties, have inadequate and often no attic ventilation. In the recent past, it has been an afterthought at best. Just install a few “airhawks” and be done with it. So, let’s look at how to do it correctly.
The International Residential Building Code (IRC) call for a minimum of 1 square foot of Net Free Area (NFA) for every 150-square foot of attic space. Let’s say we have a building that’s 100 feet long and 20 feet wide. 100×20= 2,000sf. To determine how much NFA we need 2,000sf / 150sf = 13.33sf NFA. Now that we have our NFA in square feet it’s helpful to convert that to square inches, as that’s how most all ventilation products are rated. So, 13.33 x 144 = 1,920 square inches.
Now that we have our NFA figured out in square inches we need to discuss where to install this ventilation. It’s the recommendation of the Roofing Assembly Ventilation Coalition (RAVC) that an attic ventilation system be equally balanced between intake and exhaust. Put simply, the exhaust ventilation is at the peak of the roof and the intake ventilation is at the eaves (or lower edge) of the roof. Going back to our example then we would need 1,920 square inches / 2 = 960 square inches of exhaust and 960 square inches of intake ventilation.
Knowing the NFA of exhaust and intake ventilation that we need, let’s look at some common forms of ventilation for our project. For the exhaust ventilation well use GAF/ELK Cobra Ridge Vent. This product offers 18 square inches of NFA per linear foot so doing the math 960/18 = 53 linear feet of ridge vent. For the intake ventilation we have James Hardie Perforated Soffit. This product offers 5 square inches of NFA per linear foot. 960/5 = 192lf of vented soffit. This intake ventilation will be split equally on both sides of the building so as the hot air rises in the attic it will draw in fresh air through the intake ventilation at the eaves.
Now this is just one example of a building using a particular type of intake and exhaust ventilation. While these are both great options, there are many situations in which one or both of these examples will not work. Maybe the roof is built without an overhanging eave or there is not sufficient roof ridge to use a ridge vent. We have a host or products that we can tailor to specific situations.
A well-ventilated attic makes for a healthier building. So, have Falcon Construction give you an assessment on yours.