I just bought a 1830 Post and Beam house. I love post and beams and I am currently exposing all the beams in one of the rooms. I currently have a plan for the walls and have begun that work. Come spring I am going to tackle the ceiling. Currently there is 3/8 sheet rock nailed to the joists. The joists are 4x6's and they are 24 oc give or take. My plan was to pull the old sheet rock down and put new 1/2 sheet rock on top of the beams. Then I would either put 3/4 plywood on top of the sheet rock to make it stiff enough that you can walk around in the attic with out having to guess where the joists are. Once the plywood is down then I will put the insulation (8 Inch Poly Iso boards) back on top of the plywood. My worry is that when I go to the attic to check on the slate roof, my walking around may crush the sheet rock that is between the joists and the plywood.

Why use sheet rock at all? There are some decent looking bead-board finish plywoods suitable for staining or painting that may be architecturally appropriate.

Or, you can lay down the 3/4" ply, and cut in blue-board to fit between the joists for a plaster finish.

Whatever you do, detail the plywood as an air-barrier using fiber reinforced mastic at the joints. Using t & g or ship-lapped plywood and acoustic sealant in the groove/lap make for a more reliable long-term air seal than butted-up flat-edged plywood. Then lap the seams of the iso with those of the plywood, and tape (or foam) seal the seams each layer of iso.

Roofing iso is higher density (2 or 3lbs/cubic foot) than most foil-faced good (1.5lbs nominal), and more ruggedized against the occasional walk-over. It is often reclaimed from commercial demolition & re-roofing, and can be had for a fraction of the cost of virgin-stock- typically a 70-80% discount, so it's worth looking for. Ratings on 2lb & 3lb iso tends to run ~R5.5/inch, compared to ~R6/inch for foil-faced 1.5lb iso, so if you were looking for R49-ish to meet code shoot for 9" rather than 8".

I'm way ahead of you on the insulation. I got a tractor trailer load of it. Only issue with it is it is tapered. Most of it is 4 inch tapered to 3 3/4 so its not a big difference. I am currently measuring it by the cord i have so much of it. I feel like the ply wood with a finished side would be pretty pricey. I plan to lay a house wrap over the plywood. Taping all the seams. Then lay two layers of the 4 inch poly iso using foam in a can at the seams. Currently the house has zero insulation in walls ceiling foundation or anywhere else. Its a dry layed rock foundation too. Im not exactly worried about meeting code for insulation in the attic at this point. 8 inches should be close enough until i get other things buttoned up. I'll have to look at the finished sided plywood and see how it will look. I like the plaster idea but i have no idea how good of a plasterer I am. I dont think ceiling applications are the best place to learn.

All I can find online is 1/4 inch. It would look really nice but would it hold up to light live load to inspect the roof and maybe some light storage.

The finish plywood it's supposed to be a replacement for the 3/4" structural stuff, only a less-compressible/crushable finish-side layer under the more structural 3/4" decking. For more money you could use 1x t&g bead board planking (the real stuff!), and put the structural plywood on top of that, but it's more labor intensive than the decorative plywood versions.

Housewrap tape (or any other tape, really) doesn't really stick to plywood over the long term unless you give it a bonding surface. A high quality acrylic latex primer painted on a bit wider than the intended tape works. But fiber reinforced duct mastic takes about the same amount of time as painting on a bonding surface, and seals at least as well.

I was going to house wrap the whole plywood deck when it is finished so the tape would be on the house wrap seams not the plywood seams. I haven't seen the finish ply wood. is it available at big box stores or would I have to go to a lumber yard?

Are you talking about the sanded plywood? Would 15/32 3 ply be thick enough? If there is no drywall then there can be some flex in the ceiling. How would you finish the seams? Tape and joint compound? Wouldn't that crack? The cost to do the ceiling is pretty expensive and the cheapest option so far look like drywall with plywood subfloor over it. This would pinch the drywall between the plywood and the joists. Would this crush the drywall?

I'm talking plywood or fiberboard decorative paneling.  Most of it is t&g, and you'd have to trim it so that the seams aligned with the joists. It is definitely NOT meant to structural, but it's paintable, and some of it is clean enough and a nice enough species to be stainable.

If you want a drop-dead-gorgeous authentic 19th century look you can still get bead board wainscotting in hickory or cherry, oak, etc, but it can be pretty price on a per-square foot basis.  I have some in-laws with an ~1880 vintage house where the ceilings in the formal dining & living areas were all done in pine bead-board spanning 6x6 joist beams, which has aged to a very rich medium-dark brown. When they were updating the place and building in on a master bath they used bead-board wainscotting on the walls and bead board ceiling- you'd never mistake it for original construction but it was true to the general look & feel of the place.

Biglurr54,

Have you considered 2x6 T&G pine, spruce or fir? It would take the place of everything and you could put the insulation on top of it. Looks great! You could pre finish the boards before installation.

There are usually saw mills that have that product in most areas. Where are you?

richm

I live in upstate NY in Albany. Im looking do keep costs down as best as I can. I think it might add up fast.

put a strip of 1/2" or 5/8" plywood in the center of the beams for the floor to bear on, then sheetrock to it. This will prevent crushing

Here is the latest plan. Will this work? Bead board will be alot more expensive than drywall. i can keep all the seams over the joists so there won't even be any taping to do.

To do this in 5/8 sheet rock proves to be substantially cheaper than to do it in bead board. The plan would now be 5/8 rock layed on top of the joists, 23/32nd OSB with tongue and groove on top of the sheet rock, 2 layers of 4 inch Polyiso foam on top of the osb.

If you install the plywood or OSB strips to bear the load per Bob Irving's recommendation it should work just fine.