Thermal Bypass Checklist package (50+ details).
We’re going to add insulation to his attic eventually so we want to put insulation blockers and air channels in the rafter bays. We want the most amount of insulation as far down as possible over the top plate of the wall. The insulation blockers should still allow the air to flow up under the roof sheathing and to the ridge so it cools the attic. One way to do this is by using ventilation channels. They’re a piece of plastic or piece of foam which is designed to be installed between the rafters. It leaves a space that the air can flow up and through. To do it right we must install a blocking panel at the very end underneath the ventilation channel and down to the top plate. This will stop any of the air from short circuiting the ventilation channel and going through the insulation where it can degrade the value of the insulation.
The blocking panel has already been cut to size to go between the top plate at the bottom and the underside of the roof sheathing at the top, and to fit between the rafters on either size. I need to carve out the shape of the ventilation channel. The cuts don’t need to be neat; as long as I’m within a half an inch the spray foam will fill the rest. The next thing to do is trim all the way around the perimeter a 45 degree angle off the face about a half an inch back. That’s going to leave a channel for the spray foam insulation to get a good bond between the insulation panel and the perimeter wood framing. Now I’m going to install the panel.