Insulation, Radiant and Vapor Barriers - Crawlspace insulation question?

I just purchased a 1942 capcode that has an addition which was built in 1960. After checking the crawlspace under the addition, I found the there was NO insulation under the floors. The addition is built directly over dirt, but the dirt is covered with heavy duty black plastic. So I have started a project to insulate the floors using un-faced pink fiberglass insulation. I have secured the insulation in place by using cord attached by nails crisscrossing the underside of the floor joist every 2 feet.

My question is, after I am done insulating the floor, should I consider stapling a plastic barrier to the bottom of the floor joist? I would think this would help trap warm air. But I am unsure if there would be any unwanted side effects, or if it is even a good idea...


Josh

poly on the floor will be fine

There will ineed be unwanted side effects of attaching poly.

However, other breathable materials can do the same thing: think Tyvek or even building paper.