Ok, so some of you all have probably been following my project and it is going well but slow. I intend to blow cellulose in some walls but decided to do the attic as well since my original blown fiberglass has settled and is about 2-3 inches thick in places. This is putting a strain on the second floor AC and heat in the winter. I added a ridge vent on the existing house last summer which has helped. I also want to insulate the attic ceiling instead of the floor because my second floor HVAC system is in there. The attic is not very tall so adding the extra air space to be conditioned is not a big deal considering the added insulation value. Also, the ridge and soffit vents simply can not produce enough air flow to get the air out of the attic fast enough to keep it cool.

Options

1) option one is to install baffles at 1 inch between the roof deck and where I am going to insulate. Would probably be foil faced rigid foam. Would allow air flow in through the soffits and up to the ridge vent. I have a very similar setup in my addition where I nailed 3/4in foil under the 2x4 trusses and sealed them. Allowing the air to move up and out. The foam is warm to the touch but not burning like the roof deck use to be. For the existing house it is all 2x8 trusses so nailing the baffles to the bottom of the trusses is not an option. The baffles will have to go between the cavity. I would then nail rigid foam or OSB to the bottom of the trusses. I would then dense pack cellulose into the spaces between the baffle and OSB/rigid foam. Would be about 6 inches of cellulose.

2) Would simply nail the osb or rigid foam to the bottom of the 2x8 trusses. I would seal the holes to the vented soffit. I would then dense pack cellulose into the cavity all the way up against the roof deck and it would stop the air flow up to the ridge.

My thought about option 2 would be less work and if my house ever needs reroofing I will be getting them to install rigid foam over the deck anyway. I have about a 1 foot overhang on the sides of the house by using option two it would allow the corners of the rooms to be insulated quite a bit more . I am afraid of leaks years down the road ruining the cellulose and the foam baffle would at least direct the leak to the soffit vent. With option 1 I would also keep a radiant barrier.

Lots of good info here: http://www.greenbuildingadvisor.com/blogs/dept/musings/how-build-insulated-cathedral-ceiling

Read that and a few others. I think what I am going to do is have the rigid foam on the bottom of the rafters and have the cellulose between the roof and foam. This way the rigid foam will control the house moisture from entering the cellulose and the ridge vent will still allow drying.

Make sure you are clear as to where your air barriers are. Ideal is barriers on both sides. Taped XPS foam or OSB works well. Wraps - not as well.

Posted By kogashuko on 13 Jul 2013 11:27 PM
Read that and a few others. I think what I am going to do is have the rigid foam on the bottom of the rafters and have the cellulose between the roof and foam. This way the rigid foam will control the house moisture from entering the cellulose and the ridge vent will still allow drying.

To be code legal and to avoid punky roof decking you'll need a minimum of 1.5" air channel (2" is better) between the roof deck and the cellulose.  Cellulose is fairly air retardent, and while it would wick moisture from the roof deck (moderately protective, in moderate climates) it renders the soffit-to-ridge venting useless.

In sufficiently moderate or warm climates you can get away with simply dense-packing the cellulose against the roof deck, as long as the rigid foam you have on the interior is semi-permeable (2" of EPS would be fine, but foil faced goods of any type would not.)  More on unvented approaches here.  Take note of table 3.  

There are some cellulose vendors (notably National Fiber) who claim that dense packing cellulose to ~4lbs density in unvented rafter bays is adequate on it's own even in zones 5 & 6, but it doesn't simulate as very safe using WUFI.  Similarly there are those claiming half-pound foam works fine on it's own everywhere too, but that isn't borne out in the series of simulations in that RR-1001 paper.

But with 2-3" of Type-II or Type-IX EPS on the interior you would have a fairly stiff class-III (but not class-II) vapor retarder on the interior, and with full-depth cellulose plenty of seasonal moisture buffering capacity, which should work fine in most of the lower-48.  You probably won't want to go below 0.5 perms or above 3 perms on the foam layer in any lower-48 climate, but the colder the climate, the lower permeance you'd need.

Was looking at putting a vapor permeable barrier like house wrap over the ridge and soffit vents. This way any moisture that came in off the roof deck could have an avenue to escape that isnt covered with tar paper. Not really what it is designed for but I can tell you this past week there is little cool air being blown out of my ac vents during the 3-6 pm window. The attic is just way too hot for the HVAC up there. I simply cant spend hours up there stapling something in to give it a 2 inch air gap.

As for the link I would love to read it. For whatever reason it will not open tonight.