I would like to insulate my HVAC ducts in my house. They are 6" diameter. I have a lot of 16" wide left over R13 paper faced insulation from other wall jobs. Can I use this to insulate these ducts by simply stapling it up into the joist space creating an insulated air pocket around the ducts with the floor joists and insulation? Will the vapor barrier on the insulation be a problem? Can I also wrap the big rectangular feeder ducts coming off the furnace with the same insulation?

Not a good idea to use this stuff.
While it may seem to be its not just the insulation factor you need to think about.

Duct insulation has a thin metal vapor barrier on its outside surface. Because of the difference in temp on the inside vs. the outside of the metal ducts the vapor barrier is critical as water can and will condense on the surface of the ducts. You may not see this if the duct is not currently insulated but once it is any exposed areas will become cooler during the summer and condensation will occur.

The metal vapor barrier on duct wrap is sealed with metal foil tape to prevent any moisture in the air from collecting within the fiberglass material. As it gets closer it cools down and changes back into water. If this happens which it will with wall type insulation mold will develop as well as the lowering of the R value when the insulation becomes wet.
You simply cannot seal the paper vapor barrier well enough to prevent this from occurring.

Some folks box in the ducts then wrap them or fill the space with insulation. Once this is done the box is then wrapped with plastic and covered with sheetrock. This will work but its a lot of work and the payback in savings is little.

What you can do to save some ** is to seal all the joints on the ducts in the delivery system. Both the supply and the return ducts. This loss can add up fast and you will see some savings on energy although little when you get the propane bill.

To help save some money on your energy bill install a humidifier on the furnace. Raising the humidity level in the home during the winter will help you save money by keeping the thermostat set lower for the same amount of comfort. As humidity goes up our natural cooling of our bodies (sweat) does not evaporate as fast and we feel warmer at lower temps. This is why cooling a house in summer is more about taking away moisture in the air then dropping the temps.

Hope this helps!

Thanks for the info. I'll not use the batts.

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insulating HVAC ducts

Once the ducts have been installed, and the trunks and runs established, it's time to finish off the heating system by insulating the ducts. The degree to which you insulate your ducts depends on local building codes for energy efficiency, and whether the ducts are in conditioned or non-conditioned areas. If a duct runs through an unheated crawl space, an unheated basement, or garage, it must typically be insulated to R-4 or greater. If ducts are long or poorly insulated, they often result in cool blows, or gusts of air that have lost the desired temperature. In such a case, running the heat can actually cool a room rather than heat it. The same can be said for cool conditioned air, which must maintain its temperature or fail to cool the room at the other end.