attic venting and insulating

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Old 12-09-06, 08:47 AM
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attic venting and insulating

I have a lack of insulation in my roof behind drywall I really do not want to tear out. The house is a cape with half walls and drywall installed on the bottom side of the roof rafters. Here is the catch the rafters are 24 – 30 inches on center on a 6-12 or better pitch. (I guess that is why it handles the snow load up here) I have access at both the top and bottom of the cavities so I intend on sealing the bottom of the cavity with rigid insulation and blowing cellulose in from the top. Now my concern, I want to make sure I have ventilation from the bottom of the cavity at the side attic to the top attic and plan on sliding foam vent protectors (the ones usually used at the edge of attics to prevent insulation from blocking eave vents) down the cavity and stapling them to the underside of the sheathing. Now here is the question. Do I need to make the venting the full 24-30 “wide or can I run a 16” vent up each cavity and call it good???
 
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Old 12-09-06, 09:45 AM
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You need to span the 24-30 width for two reasons, capillary (suction) break and the low vapor permeability of roofing materials. You can use one 16" baffle and cut another in lengthwise to create two 8" wide baffles, one of which you can use with the 16" baffle to span the width of the cavity.

http://www.eere.energy.gov/buildings/info/documents/pdfs/26447.pdf

Page 4 on this site gives some insights on knee walls you might find useful.
 
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