Venting the roof is standard in new construction, to protect the roof structure from moisture and excessive heat. They did the fiberboard chute so they could densely pack blown-in cellulose. Otherwise the insulation wouldn't have a good surface to push against (you don't want the insulation to touch the roof sheathing).

On typical homes with blown-in ceiling insulation the insulators will staple cardboard against the lower few feet of roof framing, creating a similar vent space from eave to ridge.

This home's ceiling R-value of 82 is about 2.5 times standard.

Thanks for the details.

Do you know why this isn't done for walls as well? It seems like the cellulose is packed directly against both the interior and exterior surfaces of the walls.

Is it simply that hot air rises, and so it's needed more for the roof?

It is done for some walls, but it's much less common.

Part of the issue is that, yes, hot air rises, so a lot more moisture will find its way to the roof than to the walls.

Part of the issue is that roofs are (traditionally) sealed much more thoroughly than walls (because rain falls on roofs), so the moisture has a harder time getting out. The ventilation channels provide an egress for the moisture below the sealed roof surface.

A big part of the issue, at least in many climates, is snowfall. If it's cold outside and warm inside the house, any snow accumulated on the roof is melted from beneath and not from above. As the slush and water try to run down the roof under the unmelted snow, ice dams can form that prevent effective drainage; water pools and leaks down through the roof. A ventilation channel under the roof surface keeps the surface at or near outdoor temperatures, so the heat of the house doesn't melt the snow from beneath.

Ah, yes -- I hadn't thought through the worse outcomes for snowfall.

Thanks!