Measurement Guide

Th NAHI
®
Forum Winter 2014 | 9
Here’s a Handy Shortcut
While it’s good to know where the attic ventilation calculations come from – especially if your customer asks for an explanation – you may be pressed for time
in some instances and cannot afford to go through the 5-step calculations just reviewed. Good news. Here’s a handy shortcut to the math.
Attic sq. ft. ÷ 2 = sq. in. of intake NFA and exhaust NFA needed @ 1/150 ratio.
2
Go back to Step 1. We used a 2,000 sq. ft. attic as our example. Now apply the shortcut.
• 2,000 sq. ft. ÷ 2 = 1,000 sq. in. of intake NFA and 1,000 sq. in. of exhaust NFA. You’ll notice that in Step 3 we calculated 950 sq. in. of NFA intake and 950
sq. in. of exhaust NFA was needed. The shortcut overestimates a little but that’s OK.
Hopefully you now have a better understanding of how to apply code requirements to actual projects. And if you’re in hurry just remember the “divide by 2”
shortcut. RAVC member companies have resources on their respective websites to further assist you with calculating balanced attic ventilation.
By Paul Scelsi, Air Vent, Inc. Scelsi is Chairman of the Roof Assembly Ventilation Coalition, whose mission is to be the leading
authority and technical resource on ventilated roof assembly design and performance.
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1
In part, the IRC states: Enclosed attics and enclosed rafter spaces formed where ceilings are applied to the underside of the roof rafters shall have cross ventilation for each
separate space. …The minimum net free ventilation area shall be 1/150 of the area of the vented space. …Exception: The minimum net free area shall be 1/300 of the vented
space provided one or more of the following conditions are met: 1) In Climate Zones 6, 7 and 8, a Class I or Class II vapor retarder is installed on the warm-in winter side of the
ceiling. 2) At least 40% and not more than 50 % of the required ventilating area is provided by ventilators located in the upper portion of the attic rafter space…with the balance of
the required ventilation provided by eave or cornice vents. Check local codes for any differences between the IRC and local municipalities.
2
For the 1/300 ratio divide attic sq. ft. by 4.
A D V E R T I S E M E N T
Did you know...
A nail or screw that splits wood will not hold the load it is designed to hold?