Insulating a furnace plenum

[Rick Vollmer]

The plenum for the furance/a.c. is located in a vented crawl space of my home. It is supported by the floor joists. I was able to insulate under the bottome and most of the sides and ends with Reflectix insulation. However due to condensation drain pipe and other running pipes, I cannot get the Reflectix on the top.

My question is: Can I run R-19 insulation into the joists cavitiy (which I will do anyway) but then I'm thinking of running unfaced R-13 under that portion of the joists that can rest tightly on top on the plenum.

Will this work or can there be a problem of some sort that I cannot think of?

I hope I expained my question so you can understand it.

As usual, many thanks for your help