Walls and Ceilings - Sheetrock for ceiling

5/8" rock is always recomended for ceilings because it is less flexable and it is easier to get a good flat ceiling with. 1/2 rock is often used on ceilings by builders because it is cheaper/easier to hang.

I don't know much about cellulose isulation but assume it is similiar or the same as the blown insulation often used in attics - which I've never noticed affecting 1/2" rock on 2' centers.

How will you access the cavity between the ceiling and floor?

I've read through the several threads about ceiling sheetrock thickness recommendations, but I'm not sure what to use in my situation. The joists are 16OC, the ceiling will be sheetrock with plybead glued/nailed over it. The idea is to have dry cellulose blown inbetween the joists when the sheetrock is up so our second floor can be closed off in the winter and just the first floor heated. Dry cellulose isn't heavy, but it does have some weight. So, 1/2" or 5/8" for the sheetrock?
Thanks

We're going to hang the sheetrock, then we'll drill a 2" hole to feed the cellulose 'blower' tube into. At least that's how we'll do it if we have the insulation contractor who did the wet-cel in our open walls do it. I have to check at Home Depot and see how their DIY blown cel machine works- I'm assuming it's more or less the same way, though I don't know if the tube is long enough to feed to the end of the cavity. Anyway, then we'll plug the hole and proceed to hand the plybead.

The house was built in 1865 and I don't know if there is a single right angle or perfectly level plane in the place! I think achieving a perfectly flat ceiling is pretty much impossible, so we're OK living with some slight... undulations.

5/8" is pretty standard for ceilings, 1/2" for walls. Keep in mind that with an old house you don't make things square, you make them match what's nearby.

honeychrome Id for sure put up a 4 mil poly there then the 5/8 drywall. So you have a good V/B to that first floor .For when you close off the room above it like you said

Thanks for the Ed. Poly was part of the original plan, but I'd completely forgotten about it.

OK, so now to complicate things.... We've decided to leave the second floor joists partially exposed and just sheetrock between them. Not exactly 'correct' for the small farmhouse this is, but I think it will look pretty good and actually give the low ceilings at least the appearance of being a little bit higher. So.... the joist are about 2x8" actual, 16"OC more-or-less. Should I use lengths of 2x2 nailed to the studs to screw the sheetrock strips into? And leaving the joist exposed seems like it will make blowing the cellulose in a little problematic... maybe I ought to use foam board insulation strips instead?
Opinions....

I assume you mean to have 2x2 nailers attached to the floor joists to secure the drywall in the cavity. R-13 only requires 3.5" which would still leave you about 4" exposed. I don't think there is a lot of R value in the styrofoam.

Yes, 2x2 nailers.
So you mean go with the cellulose anyway? AFAIK, the tube to blow the cellulose in is about 2" in diameter, so I suppose if the cavity is at least 3" it ought to fit..

I was referring to batts - rolls of fiberglass insulation.

Ah, gotcha. Cellulose won out against fiberglas for the walls, but I wonder about for the ceiling....

And now I'm also wondering about the plastic for a vapor barrier, not least because cutting it to fit between the joists would be an additional PITA. Our primary heat is a wood stove, which keeps things pretty dry, and there is the additional 'air space' of the second floor, so the difference in temp bewteen the two sides of the ceiling/floor won't ever be as great as that between the inside/outside...

I would think that the paper facing on the batts would be enough of a vapor barrier.