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Old 05-30-2010, 11:59 PM   #16
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Roof Ventilation


Given the size of the dormer vents (2- 20"X20") minus the angled slat framing (about ) equals 800sq.in. This much concentrated volume of incoming or outgoing air (negative or positive), depending on the airflow direction, will cause havoc with either the supply (soffit) or exhaust (hip ridge) vents.


A balanced ventilation system provides about half the
total ventilation area at or near the ridge, such as a ridge vent, and the remaining area at the low points, such as soffit or eave vents (see Figure 2). Balancing ventilation in this manner allows forair intake to occur at the low points and exhaust to take place at the high points. Air movementfrom low points to high points is aided by natural convection. From: http://www.fureyco.com/content/image...ng_The_Air.pdf

Pages # 616, 617 talk a little about gable vents, mainly because they cause turbulence and promote inefficient air flow when used with soffit and ridge vents. http://books.google.com/books?id=Z8a...0vents&f=false

A little different read with gable/power vents: http://www.ronhungarter.com/ventilation_repairs.html

With the ridge vents at either 12sq.in., or 18sq.in.per lineal foot, Im surprised they used a picture with the soffit vents shown. If they are 8X16, they give 65sq.in.NFVA. These could be spaced 7 apart and still give the required 9sq.in.per foot for the soffit intakes. They appear to be 4-5 apart which may be to get more supply because of the high amount of ridge vents (possibly 110-130) because you gain so much more with all the ridge hips compared to a single gable ridge. They really should have used a continuous soffit vent (9sq.in.per foot) to get full supply in each rafter bay. This would eliminate any ice dams by removing the heat loss from below over the outside walls with ventilation. Existing, the ice dams could still form anywhere between the soffit vent locations.

Be safe, Gary

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Old 06-02-2010, 03:17 PM   #17
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Roof Ventilation


I just checked out that hip vent product more carefully and wanted to make you and everyone else aware that it only provides 12 square inches of net free air for the ventilation specifications, versus the 18 square inches that the ridge vent product, Shingle Vent II provides, but it does look like a well constructed and designed product.

Ed
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Old 06-02-2010, 08:41 PM   #18
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Roof Ventilation


Thanks for that, Ed. I was reading the one below also, as a hip which it wasn't..... On a side note--- as a hip runs diagonally and the pitch changes with elevation, there is more than 12" per foot in each bay, 17" actually, PLUS the rise length per foot(4/12 = 1.25" per foot, etc.) So 2' on center bays get about 3'+ of ventilation........

Be safe, Gary
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Old 06-03-2010, 03:33 PM   #19
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Roof Ventilation


Oint well taken Gary.

But for clarifications sake, so as not to confuse any home owners out there reading and misinterpreting that aspect, Gary is referring to the horizonal distance, from a left-right measurement, which would be determined by the pitch of the roof and the hip conversion charts.

To accurately determine how many square inches of NFVA you would actually be recieving, it is necessary to determine the true length of the hip or ridge vents being installed and apply the proper NFVA of specified exhaust for each application.

Hats off to you for that astute observation though.

Ed

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