Insulation on undercarriage?

So I have a 1990 delightfully maroon colored interior van that I’m going to put a mattress and yes, a diesel heater into. I’m taking out the captains chairs and these are areas where there was not insulation or carpet.

Question 1- is other than losing an inch of height, is it so bad to just put a new carpet on top of this old stuff? I’m just thinking an area rug?

I’m not looking for aesthetics at all, just functionality. The carpet is dirty, but it doesn’t smell nasty in here or anything.

Other question – underneath this bare metal it looks like it would be easy to just spray some foam on the undercarriage to add some insulation to the underside of the van – is that a bad idea?

There are four holes from where the bolt went through which I’m going to seal off, but otherwise everything is pretty intact.