Hi Mandy,
You'll probably end up with as many different oppinions as corresponds to the number of those that respond, but I did it this way..... and it seems to be working fine.
I used the ridgid sheet insulation, not the white styrofoam (made with the little pellets) Cut to fit tightly with-in the floor framing (press fit), painted the underside of the floor with the asphalt sealer, messy but does a great job, thus far, pressed the insulation in place, asphalt again around the edges for a good seal, went ahead and put a coat over the foam for a uniform look, who's going to crawl under and see???? but I did it anyway.
tear has been on the road for about a thousand miles total, about 50 - 75 on dirt, sand and gravel looks as good as the day I did it. Don't seem to get much being thrown up under the trailer, now to be clear I have not been in any real wet weather here in So. Cal, but I don't think I have any reason to worry about it.
Just the way I did it, and it is warm, I am an astronomer as my signature suggests, and the desert can get real cold at night, now I don't need to hear back from those in the Northern climates, I'm just saying that the insulated floor really does seem to make a difference, when it's well below freezing and the wind is blowing at 2:00AM
