Attic Ventilation Channels
Baffles are needed to prevent the insulation from filling the space between the ceiling and the roof sheathing along the eave edges.
The R-60+ of CertainTeed Insulsafe blown-in fiberglass insulation that will be installed in the attic will loft up over 2 ft. deep. The top of the rafters is only 14 in. above the outside edge of the wall plate, so baffles are needed to prevent the insulation from filling the space between the ceiling and the roof sheathing along the eave edges and possibly falling into the soffit area. The baffles will create vent spaces and prevent windwash (energy-robbing convective short-circuiting air moving through the insulation.)
We started thinking about attic ventilation before we installed the rafters. The solid TimberStrand blocking that was needed between the TrusJoist rafters blocked all airflow, so we notched them.
Rather than notching them with a big ‘V’ cut that Weyerhaeuser shows in the roof framing instruction guide, we made a modified ‘V’ cut and flattened off the bottom, so it was more of a slot with shoulders to support the top flange of the rafters.
Since the roof is framed with TrusJoist I joists, the baffles are easy to fabricate and install. The flanges are about 1 1/4 in. deep, so we just need panels that span between the webs. Some left-over pieces of 3/16 in. luan plywood were put to good use.
Bruce ripped the luan pieces at 23 1/4 in. wide — just a little narrower than the space between I joist webs — and cut them about 30 in. long. Two students cut notches in the luan panels so they would fit around the web stiffeners. Installation was fast. The panels were slipped between the I joist webs, slid down to contact the I joist blocks, and stapled to the underside of the TJI top flange.
It was important to install the baffles now before the MemBrain vapor and air barrier is installed on the ceiling. This saved the extra effort of crawling on our backs in the attic to install the baffles.