OSB, I always roll. You really need to work the paint into all the voids. If you have to rent the sprayer, rolling is way faster than spraying..... and really no setup/cleanup with rolling.
You need better/more attic ventilation.
Where is the AC condensor located? Are you adding heat?
But no, insulation is not your best choice.
Eliminate any heat sources.
And ventilate.
yep, just like PolyB and pine shakes, as soon as it creates a disaster the companies will disappear and so will the warranties. But I think Pex is a better product. And it sure is cost effective.
One thing to be concerned about, often the compounds from the lath & plaster days and early drywall days used asbestos. I'm pretty sure every state/province has a testing facility.
What you describe, that plaster, was a fabulous, vapour barrier too.
You should need a permit and drawings. There are issues of fire separation which are far more critical than the foundation. But yes, you may have to dig down to verify the existing foundation OR your municipality may have plans from the original construction permit.
There are valid reasons to...