Re: Insulating a large cathedral ceiling
Cold Canadian, floors are different than walls and ceilings. In walls, there is a cold vertical wall and a warm vertical wall. The air against the cold wall cools and falls, the air next to the warm wall warms up and rises. The air that falls ends up next to the warm wall at the bottom and the warm air that rises to the top ends up next to the cold wall, The air is in constant circulation.
In a ceiling, the bottom surface is warm and the top surface is cold so there is circulation there as well. In both the walls and the ceiling, the added R3 for a 3/4" space is the maximum available. Increasing the air space does not increase the insulation value.
But in a floor, the top surface is warm and the bottom surface is cold. The air next to the top surface warms up and stays there, the air at the bottom cools down and stays there. For that reason, as the spacing between the boundaries increases, so does the insulation value.
So yes, it sort of works the same, only better. You just have to take in the dynamics that occur around the rim joist. The rim joist needs to be insulated and sealed if possible.