Dense-packed rafter bays need rigid vents.
The roof slopes in the ProHOME are dense-packed with 20 in. of cellulose. To provide adequate roof ventilation there is a ¾-in.-high air channel in each rafter bay directly under the roof sheathing. Dense-pack cellulose and cheap Styrofoam vent baffles don’t mix, so we use 7/16-in. OSB to create the air channel. The OSB is rigid enough to prevent it from being crushed when the cellulose is packed into place.
To create the channel, we start by extending the Zip System sheathing and tape above the top plate of the wall to the height of the rafter heel minus the depth of the air channel. This blocks the cellulose from spilling out into the soffit space. Next, notches are cut for each rafter. It only takes a few minutes to do it this way and it’s faster than cutting a piece of plywood for each space between the rafters later. Plus, no additional nailers are needed to hold plywood pieces in place.
The Zip System sheathing is run past the top plate so that it ends 3/4 in. below the roof sheathing. The wall sheathing acts as continuous blocking to prevent insulation from being blown into the soffit. Notches are cut for each rafter.
View with the rafters installed. The gap between the top of the wall sheathing and the upper surface of the rafter is the ventilation channel.
Once the rafters are set, 1×3 strapping is ripped for the ¾-in. nailers, which will be used to fasten the OSB plywood, and they are stapled to the side of each rafter. OSB is then ripped to the width of the rafter bay and secured with sheathing staples (see top photo). As a final step, any air gaps are sealed with Tightbond X-TREME foam.
This article first appeared on FineHomeBuilding.com by Tim Biebel. http://www.finehomebuilding.com/2017/08/02/ventilation-for-the-prohome-roof