I would use foam!

i am using 2" foam to seal the building , but for the whole roof cavity, or to achieve required R value, it is over my budget. so i am thinking cellulose.

I don't think even wet applied cellulose will stay upside down. As far as I know, it was designed to go vertical not horizontal. At least I have never heard of that.

I just thought of a possibility. Hang the drywall. Then do it like they retrofit old houses with dry cellulose. Since it's horizontal, you would need to drill a hole at both ends between the rafters to be able to fill each section. Would need to use an L shape blower head. Before hanging drywall, measure how much sf is between the rafters. This way the installers can measure how much they blow into each cavity to make sure it's full. I've never heard of doing this....just throwing it out. Ya never know tho.

I sure wish this site would let you modify a mess after it's been posted!!!!!

Have you priced sprayed polyurethane on top of the roof deck, followed by an elastomeric coating and granules? If not now, in a couple of years. Better than spending money on half-measures.

A flat roof on a rental home I owned sprung a leak in the winter. When the ceiling suddenly fell down, it took a sink with it. Fortunately, no one was there at the time! Turns out it was one of three homes that an insulating company had "improved" by cutting a dozen holes in the roof deck, then pumping in insulation. All three roofs had to be redone.

what do you recommend, i dont want to do any experiments. should i just seal it of with 2" closed cell and than use batts for rest of cavity? are there any other options?

what about just stapling thicker, 7 mil transparent poly or something similar and than fill in cavities, so the poly would hold it in place before.

That might work. How much room between the foam and bottom of rafters? If it's not alot, then the weight of the cellulose shouldn't make it sag very much. Too much sag(over fill) and you might end up with wavey sheetrock. .....hmmmm another thought.....hang 2 sheetrock layers.....fill that in....then hang two more....etc.....last one would need to be blown in thru hole.

Fiberglass batts installed alone in the cavatie allows air infiltration that lessens the r-value. The claimed r-value is not real world. You have solved the infiltration issue with the foam, so why not go with high density batts and not worry about it. You could go with a blown-in blanket system with fiberglass or the same method with cellulose, but i think that would defeat the purpose of using the foam.

Here is a link that might be useful: blown-in blanket