||04-05-2013 12:02 PM
10 inches of blow in cellulose that would hold moisture with the air movement through it, unless it is truly, successfully densepacked, which is extremely difficult with that much depth, which would give you the same performance as spray foam as far as air movement, with less r-value.
If breathability of the roof deck is the main concern, put 1x2" spacers attached to the rafters where they meet the deck, and put fanfold there, and apply the sprayfoam to that. It will allow plenty of air movement behind the insulation, give you your best finished product.
In this given application, any negatives applied to spray foam also apply to cellulose and then some. Read the entire link that you liked from gary's post above. It goes into detail about the exact issues I am discussing.
No one argues that it is a dumb idea to shove a bunch of fiberglass batts into a cathedral ceiling with no provision for ventilation air to remove moisture that leaks in from the interior. The results are often catastrophic (Photograph 6). No matter how good we are at air sealing the interior gypsum board ceiling we can’t quite get it “perfect enough” consistently to stay out of trouble. We can get lucky on one in 10, but that is not a high success rate. I mean it is possible to get a blindfolded drunk to cross Niagara Falls on a high-wire without a net, but it wouldn’t be a good idea.8 Note that not only did we insulate the roof cavity, but we also did our best at air sealing as well with caulks and foams and whatever, and it still did not work. So, what makes us think that we can dense pack a cathedral ceiling and get it to work without any attempt at air sealing?
So, what should we do with these flat roofs and cathedral ceilings that can’t be vented and insulated in the “typical” way? Simple, follow the building code. Add insulation on the top of the deck to elevate the roof deck temperature above the dew point of the interior air vapor mix (Figure 1) before dense packing or take the interior ceiling down and install an air impermeable insulation. Spray foam on the underside of the roof deck (Figure 2). The thermal resistance of the insulation needed to be added above the roof deck or in the form of spray foam on the underside of the roof deck is dependent on climate and the interior moisture load. Tables and a map in the International Residential Code make it easy for you.10 The building code also lets you know what a “desert” looks like and where “deserts” are located and under what conditions you don’t need to vent and can in fact dense pack. But, if you are not in a desert with no top side vapor barrier, just say no to dense packing a roof.
And I am aware this is not a cathedral ceiling per say, but given the HIGHEST point is only a foot, it should be treated as one for the most part.