I have a project that the customer has decided he wants a rat slab poured. Does the slab require wire as it will be used as an a/c plenum only and what thickness should the slab be.

Others say gravel, poly + foam, then 2" of fiber reinforced concrete.

I have had them where they only want a rough slurry of lean concrete just to cap the ground.

I personally wouldn't want the dank, dark, dingy and dirty crawl space to be my plenum...that's the air you will be breathing after it passes thru the furnace/air handler...not for me thanks

Dank goes away with the air flow - and the others are pretty much true of any duct.

Thanks guys, after talking it over with him he has decided to just use sealed poly on the ground --without the concrete. Will be a sealed foundation with the ductwork though. He decided that if the code didn't require concrete why go to the extra expense. The poly is required.

Be sure to insulate the crawl space walls quite well and include forced ventilation with conditioned air year round. And especially, make sure the footings have a good drain system. I would suggest drain pipe on the inside as well as the outside of the footing.