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Is there a formula or rule of thumb to know how many soffit vents are requires in an attic space? Does every rafter bay at eave line require a vent? Location is in Ontario, Canada.
 

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Is there a formula or rule of thumb to know how many soffit vents are requires in an attic space? Does every rafter bay at eave line require a vent? Location is in Ontario, Canada.
In take vents(soffit) and attic vents are a system that should be balanced. The size of the attic dictates the exhaust vent size and that designates the intake vent size. It can't hurt to have more intakes then is required. I've seen soffits where the entire run is ventilated.
If the soffit area is a common one,where all the bays are intaking air from one large area, you don't need intakes in every bay. You would space the vents along the soffit area making sure they were evenly spaced and were sized correctly over all to satisfy the intake requirement.
If each bay is closed, then you would put a vent in each bay.
 

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Per code every joist bay needs to be ventilated and there needs to be a 1" air space between the roof sheathing and the insulation. This gap is usually obtained through the use of ventilation panels (foam inserts).

There is a formula for ensuring the proper open surface areas at each location, but if you install the vented vinyl soffit system the whole length, then they provide more than enough. A continuous ridge vent is also a must. make sure that the gap in the plywood at the ridge is at least 3/4" on each side of the ridge beam or 2" clear if there is no ridge beam.
 

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Our rules: Click on the “next section” to advance: http://publicecodes.citation.com/icod/irc/2009/icod_irc_2009_8_sec006.htm

The math: http://www.airvent.com/homeowner/products/intakeSoffit-specs.shtml

If punched soffit, figure 1’ deep for vapor barrier (1/300) as you probably have in the attic, for 4.68 NFVA per foot (minimum). http://files.buildsite.com/dbderived-f/owenscorning/derived_files/derived92755.pdf


Be sure to air-seal the attic first, I feel it’s more important than vents or insulation: http://www.jlconline.com/cgi-local/...e.storefront/4d17fa04053d6e9327170a32100a05c7

How to air-seal: http://oee.nrcan.gc.ca/residential/personal/heat/keeping-heat-in-chap4.cfm?attr=4 Check out the others pages while there.

Gary
 
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