I’m replacing some white cedar siding shingles, it’s about a 20 sf section of the bottom dozen rows.  

Shingles were installed directly against OSB sheathing.  No house wrap or moisture barrier.  OSB is in good condition.  

I’ve been adding tar paper under the replaced shingles, trying to tuck it 1/2” or so under existing shingles.

Just occurred to me to ask if this is a good idea, bad idea, doesn’t make a difference?

My instinct was that adding a moisture barrier is good but doing it spotwise like I am may be useless at best, or trapping moisture and encouraging rot at worst.