Sizing of roof vent


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Sizing of roof vent

  1. 1. Sizing Roof vent for your home.ridge vent size per sq. ft.Lets do some is 40 feet long by 30 feet deep, or 1,200 sq ft. This is the attic area whichincludes a 2 foot overhang, not the house area.Required ventilation is 1:300 if 50% or more is near the ridge, 1:150 if soffit ventsonly.1,200 sq ft / 300 = 4 sq ft / 2 = 2 sq ft. If *at least* 2 sq ft of net free vent area is atthe ridge, then we can use the 1:300, otherwise we need to use the 1:150 rule.Okie dokie, the ridge is 40 feet long, or 480 inches long less 21 x 1.5 = 31.5 inchesfor a total open area of 448.5 inches.The ridge vent slot in the sheathing is 3/4 inch to each side of the ridge board, or 1-1/2 inches wide total. 1.5 X 448.5 = 672.75 sq inches.672.75 sq inches / 144 = 4.67 sq ft That makes it doable, now, though, what is thenet free vent area of the ridge vent itself?This will be given in sq inches per linear foot.The net free vent area of the ridge openings are 672.75 sq inches / 40 = 16.8 sqinches per linear foot. Thus, as long as the given sq inches of ridge vent are lessthan 16.8 inches per linear foot, that 3/4 inch cut out to each side of the ridge isgoing to be adequate.Anyone want to check my math and make I did it right? Thanks.Re: ridge vent sizeThe minimum and maximum width of the slot are not usually specified by code butby the manufacturers installation instructions. The code will let you figure out theminimum square inches of net free vent area NFVA) that you need.
  2. 2. Sizing Roof vent for your home.The manufacturer makes a vent that has a certain amount (in square inches) of netfree vent area per lineal foot of ridge. This sets a limit on how narrow the slot canbe, so that the vent and not the slot is the limiting factor on air flow through thevent. The vent itself will be made to a certain width and only extends to cover theroof a few inches on either side of the ridge. The manufacturer will specify amaximum slot width which, if not exceeded, should ensure a watertight jointbetween the vent and the roof covering.Lets go back to that minimum slot width again and see how narrow it could be.Ive seen typical ridge vents with NFVA as low as 9 sq in per ft of ridge and ashigh as about 18-20. Take that low flow vent at 9 sq in/ft. If you use that one, youneed at least 9/12 or 3/4" total width of a slot (clear of the ridge board) or 3/8" oneither side of the ridge. This is to ensure that the vent, and not the slot, is limitingair flow. Take a higher flow vent at 18 sq ft/in. On that one you need 18/12 or 1.5inches minimum total width (clear of the ridge board) or 3/4" on each side if theridge. Cutting slots wider than the minimum based on the NFVA of the vent youare using does nothing to increase ventilation; it increases the chances that the ventwill leak.Check on Jerrys math:2006 IRC 806.2 has the requirements.Quote:§RR806.2 Minimum area. The total net free ventilating area shall not be less than1/150 of the area of the space ventilated except that reduction of the total area1/300 is permitted, provided at least 50 percent and not more than 80 percent of therequired ventilating area is provided by ventilators located in the upper portion ofthe space to be ventilated at least 3 feet above eave or cornice vents with thebalance of the required ventilation provided by eave or cornice vents. As analternative, the net free cross-ventilation area may be reduced to 1/300 when avapor barrier having a transmission rate not exceeding 1 perm is installed on thewarm-in-winter side of the ceiling.Lets assume a gable roof and our ventilated space (inside the attic) is 40 feet long(at the ridge) by 30 feet wide (eave to eave) or 1200 sq ft.
  3. 3. Sizing Roof vent for your home.Using 1/150, the required total NFVA is at least 8 sq ft. Using 1/300, it is at least 4sq ft.Lets assume worst case of NO soffit vents and either no vapor barrier or one withan effective perm greater than 1. If we cut a slot along the entire length of the ridgeinside the gable end walls (assumed to be 40 feet), we would need a minimumclear slot width (8 sq ft / 40 ft) x (12 in/ft) = 2.4 inches (total on both sides of theridge board). For this to work, wed need to find a vent that provided at least 29 sqinch NFVA per foot of ridge. (Note - some manuf say you couldnt do thisconfiguration with their vents, because their install instructions specify use of theirvent in conjunction with soffit vents.)Lets assume a more typical case where we have soffit vents and they provide atleast 50 percent of the required NFVA. Now we can use 1/300 which requires 4 sqft total. We are going to meet that by providing 1 sq ft at each soffit and 2 sq ft atthe ridge. Our slot running the full length of the ridge (inside the gable end walls)can be reduced to (2 sq ft / 40 ft) x (12 in/ft) = 0.6 inches (total on both sides of theridge board). For this to work, wed need to find a vent that provided at least 7.2 sqinch NFVA per foot of ridge and run it the full 40 feet.Lets say we want to use one of those low profile ridge vents with the filter inside,that provides NFVA of 9 sq in/foot. For this vent we would need a slot 3/4 incheswide and (2 sq ft x 144 sq in/sq ft) divided by (9 sq in/foot) = 32 feet long. Use oneof those higher flow ridge vents (NFVA of 18 in/foot) and you need a slot 1.5inches wide (3/4" on each side of ridge board) and only (2 sq ft x 144 sq in/sq ft)divided by (18 sq in/ft) = 16 feet long. In theory, you could use that higher flowvent on a 3/4 inch slot if you ran it for 32 feet. BUT code says you must follow themanufacturer’s instructions and they will tell you how wide of a slot to cut for theirvent.