Is additional padding recommended when used on a concrete slab?

I've installed this product on both concrete and plywood floors and I did not add any additional padding other than what was already attached to the bottom of each plank and I was happy with both install types. Keep in mind that if you do add additional padding and the planks get too spongy, you could possibly sacrifice the integrity of the connections at the joints and cause the planks to start separating from each other.

Definitely not. Only add the black moisture barrier. Additional padding will void the warranty, as well as ruin the floor.

You may want to opt for a slightly thicker moister barrier over the concrete slab to improve sound and warmth.