I do think from your description that the failure of the drywall ceiling is a result of lack of venting between the top of the insulation material and the underside of the roof. For starters, I would poke around and see if you have pre-formed (often pink, but possibly blue) foam pieces with ridges tucked up under the roof. These are installed between the rafters in cathedral ceilings (which is technically what you have from an insulation point of view, no matter how garrett-y your rooms feel). If not, then that's your problem (or problems) in a nut shell. You can research this on the Owens-Corning website which has lots of instructions about venting cathedral-style roofs. And you can look over these pieces at any Home Depot/Lowes. It will make sense when you see them. You may need to remove, install roof venting and replace the wall/ceiling surfaces. The plaster may be damaged more from aggressive removal of adjacent surfaces than from roof deflection, but one can't tell without studying it. And you may also need additional venting on the kneewalls now that the attic has been converted into living space which generates a good deal of moisture. You can cut down on this by installing proper vent fans for the kitchenette and bathroom areas, but you will still need under-roof ventilation, if not soffit vents, as well. It sounds as though the attic work was done by DIY amateurs. And that brings me to the most serious point that occurred to me: is it possible your attic has been converted into living space on the QT, and without benefit of building inspections, permits, (and unfortunately, increased property taxes) etc.? It may seem like a persnickety issue, but the chief risk in my opinion is that your attic living space - if done sub-rosa -might not have sufficient, legal, means of egress in an emergency. If you never have an emergency, it might not matter, but in the case of fire, inadequate escape routes can lead to the death of occupants and firefighters attempting a rescue. This is a very serious risk, and not one I'd care to undertake; it has the potential for devastating financial, legal, and even criminal consequences. While you noodle on that risk, you can research the catherdral ceiling vent issues on line. If you think structural compromise might be the reason for the plaster cracking, then an old-house friendly structural engineer (and many are not) would be the one you need. Consultations with roofers, insulation contractors and plasterers will not resolve the central issues and may lead to unnecessary, off-point work at this stage. Aside from permitting issues, however it doesn't sound that dire, though ripping and venting and replacing the wall and ceiling surfaces won't be any fun in the heat of the summer. HTH, Molly~

I assume the spaces you are dealing with are under eave spaces rather than crawl spaces. A cathedral rafter configuration is ventilated with insulation baffles eave vents and ridge vents. I would finish the bottom of all rafters or none. An alternative to drywall on the rafters is fiberglass insulation with a foil facing stapled to the rafters. In any event eave and ridge vents are preferred for attic ventilation. This post was edited by Renovator8 on Mon, Sep 15, 14 at 9:38

My question isn't about safety for the cats, but if simply walling up the space would have any negative effects as far as the house goes. Do I need to add insulation behind the walls I put up? For the knee level walls, should there be anything put behind them? Do I need to be concerned about blocking anything that would effect the air/heating of the house?

A lot will depend on your ultimate goals for the space. If you want legal habitable space, you need to run all of the required infrastructure first. Like wiring. Or plumbing. Insulation is no where near the first step. By the time you are done with running the necessaries, you should know what a vented vs invented attic assembly is. And the pros and cons of each. You are way jumping the gun. Or just hire a contractor to do all of the necessary steps to convert the storage space to habitable space. Like shoring up the joists. And framing it out properly for the type of assembly that you choose. You don’t just throw some fiberglass at it and hope for the best. You can rot out your house if done improperly.