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Question DetailsAsked on 9/2/2017

sjr
What is the best way to install intake attic ventilation vents without using soffits.

I'm replacing a wood shake roof with Asphalt shingle, in Denver, CO. The Soffits are small and covered with insulation. I want to add insulation and a new roof.

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OK - if you have soffit covers (horizontal covers under the rafters, looking like this -


http://www.specialtydesign.com/blog/w...


They should have good ventilation openings (screened for insects), to let air into the eaves - which are what I think you are calling soffits. Eaves are the openings into the attic, usually in the spaces between rafters or top chord of trusses, like this - where the red airflow inlet arrow comes into the house.


http://www.finehomebuilding.com/2013/...


Generally, the open area of the eaves or eave vents should equal (in square footage) at least 1/300th of the square footage of the roof over the house, spaced out along all the eaves on both sides of the house. 1:150 or 1:100 ratio has been demonstrated to make a significant difference in attic temps. For instance, say your roof is 30x50 feet in dimension and a 6:12 slope. A 6:12 slope has (by pythagorean theorem from geometry - remember that ?) a length 12% longer than the horizontal projection, so a 30x50 footprint roof would have 1680 SF of roof area - calling for at least 5.6 SF of eave opening and ideally more like 11-17 SF. Using the 5.6 SF code minimum, assuming 25 two foot rafter bays along each long side of the house, would give about 0.11 SF of open area per rafter bay. With open 22.5" rafter bays (2 foot rafter spacing) this would mean a gap of only 3/4 inch over the blocking at each rafter bay - a pretty small area. If you have solid blocking between the rafters with no airgap above, then you would need air holes through the blocking totalling 0.11 SF or about 16 square inches. This would be 5 - 2" diameter holes every rafter bay.


To get 1:150 or 1:100 ratio for better ventilation, generally holes through blocking does not cut it - you need 1-1/2" to 2-1/4" air gap above the blocking.


You say your "soffits" are covered with insulation. If you actually mean the soffits that is not a very useful place for insulation (between open air and the roof) unless you have a roof icing problem due to wall solar heating getting in through the soffits.


If you actually meant your eave vents are insulation sutffed, then what you needf to install are eave baffles - they look like this, and act to keep the insulation from blocking the eaves at the wall, like this and should provide ventilation under essentially the entire roof area - not just a narrow slot in each rafter bay.


http://www.homedepot.com/p/ADO-Produc...


http://www.energyauditingblog.com/how...


http://www.ventmasters.com/attic-vent...


The airflow from the baffles then flows up under the rafters to ridge vents with similar open area, to vent the warm air from the attic to the outdoors. yes this means a colder attic in winter, but also removes the moisture which builds up from the house, so you lose some energy but avoid mildew/mold/rot in the attic. If in a windy part of the Denver area (which inclludes most of it) I would recommend snow guard type ridge vent, to prevent snow and rain blow-in which can wet attic insulation and in serious cases leakage into the house or rotting attic framing.


You can find a lot more discussion on roof ventilation and attic insulation in the Home > Roofing and Home > Insulation links under Browse Projects, at lower left.



Answered 1 year ago by LCD




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