Our crawl space below the kitchens is nothing but clay soil earth, with 0.5% or so ventilation.But I cannot believe that nigh zero rust, mildew occurred. And the air had a great aroma...
Cobwebs ? yes, but what do they hurt??
I do not really know why..

Sardonic but not what I was looking for. I had a guy from the cleanspace company come out and spec what was needed and what it would cost. After inspecting my crawlspace, he showed me pictures of a floor beam that had some powder post beetle damage as well as some piers wicking water from the soil, rat urine and feces, rodent holes dug under the footing. He told me I would need to clean out all the debris, close the vents, install a drainage mat under neath a 20 mil vapor barrier, install a dehumidifier with a condensation pump and a new crawlspace door.

I asked if I needed to replace the insulation in the crawlspace and he said you don't need it because the encapsulation would keep the air at a constant temperature as well as the dehumidifier maintaining any moisture issues. I wouldn't have to worry about spiders, rats, mold, mildew, beetles or anything and I would be able to store items underneath the house and they would smell as clean as they were before they were put in. All for the total price of 14k.

Creating a conditioned crawlspace is favoured by Building Science Corp. See their site (linked below) for info on what's involved.

Here's another thread at GardenWeb on Cleanspace.

I would remove water-sensitive insulation as mould is likely in it and will be activated again.

Here is a link that might be useful: Crawlspaces