What to do with plaster ceilings
I am starting to airseal my attic. I have old cracking plaster ceilings.
The prior home owner dumped a ton of cellulose in the attic and I think that made the ceiling worse. Many of the keys are broken or loose, not too mention 100 years of gravity.
Should I cover them with drywall or leave them in place? I can take them down but that is not a good option as it is very messy.
In the mean time I will work on the many penetrations but before I add insulation or other barriers I want to make I am on solid ground.
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What i usually do (pretty much our specialty) is use plaster washers and long screws into studs, structalight basecoat where needed then hot mud and compound with either crackstop or fiba fuse. I hate laminating anything, i feel like it's cheating, it makes the moulding profiles gross and have weird exposure, lights and boxes fit weird, etc. In my personal house we are currently gutting the 100 year old lath and horsehair plaster on the ceilings for spray foam and we will replace that with regular 5/8 drywall.
If the ceilings are already questionable, be very cautious about rummaging around in the attic. I am bit unclear on what you mean by air sealing the attic, if the insulation is over the top floor ceiling, then I think you need to control the ventilation and the insulation placement.
A relative of mine had very old and cracking plaster walls that had already been rescued with glued on canvas. For walls that was a somewhat reasonable rescue approach. I would not try that for a ceiling. When they did finally face up to some renovations I know they mentioned finding horse hair in the mix, as well as a lot of broken lath. It would probably have been a good idea to test for asbestos, but the idea of 100 year old horse hair alone got them to suit up and dispose of the refuse carefully. Fortunately, they did not find the bonus surprise of menure being used as insulation. The only house I knew of that had that surprise, dated back to the mid 1700's. It was probably put in the mid 1800's for reasons unknown. Rags were more practical.
From the age and description, I am guessing the plaster is attached to wooden lath. If those are already failing in part, the weight of the plaster will tend to keep propagating key failure as the remaining keying takes the field load. Any moisture that might be accumulating in the plaster will serve to weaken it. Be very concerned if bathrooms are involved.
I had one client that woke up to a (fortunately empty) guest room with 75% of the ceiling shattered across the bed and floor. A leak in the attic had been stealthily soaking the back side, while the many years of oil paints masked the dampening on the bedroom side. This ceiling was set into wire mesh and the much tinier key structures just stripped off en masse.
It might be feasible to apply 5/8 sheet rock to the ceiling with long-ish screws. You will want to be into overhead joists at least 1 1/2 - 2". Different crews have their own screw patterns, but go for more rather than less without creating a break line like a saltine cracker.
The danger in this approach is not knowing how much failure is actually present. Check with a long straight edge to see if the ceiling has a bow or sag. This can be tricky since many old homes are saggy anyway. If the ceiling is sagging, it may mean too much of the supporting keying has separated already to safely add new dry wall. The act of pressing up the dry wall sheet and screwing it could push the remaining keying over the edge. Then all hell might break loose.
If seemingly stable you might consider 1/2" plywood first and then 1/2" dry wall to make an even more secure carrier of the insulation weight above. Same potential for disaster is present. I would clear a room and put down expendable 1/2" osb sheet to protect the floors. I would suggest making support trees to hold the sheets of ply or 5/8" tight to the ceiling at a of minimum two points across the 48" width. You do not want to be trying to hold up a 50lb sheet of drywall when an additional 100 lbs of plaster and base decide to break loose of their keying.
I would check around locally to see if you can find trustworthy crews that have done restoration of old homes, not renovation. Ripping out the plaster and starting over is clearly one way to fix it, but not necessarily the preferred way.
Unloading an entire attic of insulation with a vacuum system is one other path way, but I don't know if that service is available to you. Most frequently it is done to remove vermiculite insulation, but I don't see why it wouldn't be able to do cellulose. It will be a huge volume to deal with though.
Hope this gives you some avenues to explore.
Just as you said, just go over it with new drywall or blueboard and plaster. Just make sure you use long enough screws and make sure you hit joists. You don't want to start tearing it down with all that insulation above. You can remove any really bad sections and fill with a piece of drywall to make up the difference before going over the whole ceiling.
I'll offer a Contrarian opinion:
My last house was built in 1870, had the same problem, including blown cellulose above. My contractor said, "just let me tear it out." Forty-five minutes and the ceiling was gone, about the same time to put in new drywall. The most time-consuming part was bagging everything up and carrying it out to the dumpster. I felt like I came out way ahead.
Depends on what shape the lath is in. If you start taking down the plaster, you may end up with a room full of insulation if the lath gives way. As you say, the time is in cleaning up, then you still have to put up a new ceiling, so your just saving yourself some clean up time by just going over it. Unless you got some fancy molding to match up to and don't want to lose a 1/2 inch.