I have read a lot of the articles about installing vent baffles to make it so you can add as much blown in insulation as possible. As we know, the space is tight where the roof hits the wall. Is it ok to install the baffles only where the soffit vents are located but entirely seal the other spaces where the rafters hit the top plate?
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
In an attic where the ventilation at the eaves is not continuous, it is fine to only place baffles where necessary as long as they can supply the required amount of air, and are spaced so that there are no dead areas where air does not circulate.
Malcolm, in other words, it's OK for blown in insulation (be it fiberglass or cellulose) to completely fill a rafter cavity from attic all the way to ceiling if there's no existing vent to block. Is that accurate? This wouldn't somehow lead to moisture building up on the rafters would it?
Yes. It's alright to block the bottom of the bays in an attic where there is no ventilation path and the insulation will only be in contact with the ceiling for a short length. That area can dry by diffusion to the surrounding open attic. If the insulation was deep enough, and a shallow roof slope meant the insulation was going to be in contact with the sheathing for more than several feet, it's probably not a good idea.
I will only be blowing in 12" of cellulose.
It seems to me that if the soffit vents are open to the attic and they are spaced around the perimeter somewhat equally I wouldn't have to install baffles in every rafter bay.