how much space do I need under the roof deck to ventilate it properly? House is 2X6 rafters and ceiling joists on 4/12 pitch. There is about 4 inches between the deck and top plate on the exterior wall.
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That four inches oughta be enough!
;)
After I finished joking with myself, I punched some buttons - good question and not sure that I have the answer. With rafter bays at 16" oc and spanning 18' and a ratio of 150/1 you'd need a cross section of about 24sq inch. if the span is 14' then a cross section of 18sq inches works.
So, in general, the two inch soffit vent strip and 2" ridge per side is enough.
But that would imply a 2" deep airflow space above the insulation, yet most I ever see is an inch, which leads me to think of other questions I don't have answers to.
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The materials industry has come up with a 'one-size-fitz-all' answer to your question which does not even take into account the calculations Piffin did--Raft-R-Mate¯ and similar Styrofoam¯ vent-channel inserts generally give you less than a vertical inch of effective vent space, once the insulation get shoved up against them.
Dhuh.
Mininum I would recommend is to nail nominal 2x2 strips along the upper corners of each rafter bay and then nail homosote board over that. That'll give you a 1½" clear vent channel, which is pretty standard for up here.
The problem is that cuts your rafter bay insulation down from 5½" to 3½", which ain't so great, so you'll need to lay that silvered-mylar bubble-wrap VB under your insulation to help get the R-value back up again. One company (Reflectix) claims R-10 for their bubble-wrap; I kinda doubt it's really that high but it does help a lot. I had a similar problem in the lower roof of my gambrel and that's what I used.
Dinosaur
How now, Mighty Sauron, that thou art not brought
low by this? For thine evil pales before that which
foolish men call Justice....
Edited 7/11/2009 5:51 pm ET by Dinosaur
I plan on using foam and blocking off a 2" airspace and filling under that as full as possible. The intersection of rafter and joist prevented me from blowing insulation all to the eaves. I took foam and nailed a 2 ft piece on the bottom of the rafters to keep from filling the bays so the air can't flow.
Sounds like you've got it under control....
Dinosaur
How now, Mighty Sauron, that thou art not broughtlow by this? For thine evil pales before that whichfoolish men call Justice....