Insulation, Radiant and Vapor Barriers - old insulation?

Hello, I have a question regarding existing insulation, I live in the New York City area and my house was built in 1957 and it seems the insulation in the attic is just as old, I was wondering if I should remove it completely or just add to it with new. the floor joists are 2x6 and the existing insulation takes up about 3" of that.

You can insulate over it.

Thanks for the reply. It seems I would probably lose some R value since I would be putting a lower value insulation on top of the existing but the time and effort saved might just be worth it.Would you agree?

If you are referring to the "Law of Diminishing Returns", this states that when the cost of doing insulation exceeds the amount saved within a reasonable period, the installation becomes cost ineffective. In your case, this does not apply. The reason for it is that it is the least expensive to add insulation to existing insulation in your attic than anywhere else in the home. And the bouyancy of heated air. So the costs and the characteristics of heated air out performs all other applications and therefore cost effective.

Almost all insulation is light weight because the thermal effect is created by trapping small pockets of air inside them. There may be some compression when installing insulation over old insulation, but the reduction of R-value would be negligible and would still make the installation cost effective.

You could remove the old stuff and air seal the attic with a 2 componant foam then add blown cellulose insulation up to code for retrofitting, in canada which is cold like new york can get the ideal is to get R-40
which is about 12 inches

Air seal the attic? What are the implications on attic ventilation?