As part of my understanding on the movement of warm air I am watching for places where "it matters" and our practice of blowing a cap of cellulose over fiberglass may be one we need to modify. As I have done and recommended to others, which doesn't mean it is the right way, before the cellulose is blown in, a batt of fiberglass is folded and stuffed into the end of each rafter bay just under the new baffles to block any cellulose from filling our soffits. Here's the problem, at least in cold country.
With an existing layer of fiberglass against the ceiling below, the air against the sheetrock is being warmed by the heat conducting through. But here is the difference. That warm air wants to go nowhere. Since it does not rise on its own, sealed under a cap of cellulose it would be quite happy right where it is. However, is it really sealed? The moving force that circulates that heated air up into the attic is the invading cold air and that folded batt of fiberglass we used to block the cellulose from filling up the soffits has now provided a path for that very cold air to infiltrate our layer of FG and force that warm air up through the cellulose cap, all-be-it a much more difficult task given the density of the cellulose.
A better solution might be, to either build a solid block to both keep the cellulose out of the soffits and eliminate and extra cold air infiltration, or to pull back the existing FG just inside the batt blocking the soffit and dense pack the new install right the sheetrock. If that dense pack sealed from the baffle to the ceiling, plus across the attic floor, there would be minimal cold air infiltration to either the FG next to the ceiling or the attic space above. The baffle of course would still be doing its job.
A simple modification of how we are currently doing this install, but perhaps one that improves the results. Any thoughts?