Once that happens, the whole premise of dead air, or trapped air, goes out the window and the R-value starts to go down. The fit is very, very important. That's probably more information than you wanted to know, but if you're going to invest a lot of money in insulation, you might as well have it fit right. Expandable foam, or blown cellulose, would seem to fit the cracks just right. Ken Rascoe of Plattsburgh says New York State has established R-38 as the minimum value for ceilings in new constructions or remodeling.
Standards change slightly depending upon the heating system.
They're a little more stringent for electric heat than they are for oil or gas. It depends a lot on what type of insulation you're buying and that's why the R-values are used. Styrofoam is usually and R-4 per inch, but not practical in all situations. Urethane is around R-7 or 8 per inch, once it stabilizes, which happens within the first year. The old vermiculite, that isn't used much anymore, was only R-2 per inch, which meant you had to have a lot of it.