This. I recently fitted some lvt flooring which is about 5mm thick.

As it was going onto concrete subfloor, I decided to add some fibre board under to help it warm things up a little. The instructions said not to do this but I thought the extra warmth would be needed. Well it's fitted now and does have a slight flex to it as you walk on it.

Tbh I wanted a waterproof flooring as it's in the kitchen so the thicker planks weren't really considered, also didn't want to use the glue down stuff as it seemed harder and longer to fit.

I'm kind of regretting it now, but it's not too bad, I just wouldn't do it this way again. I do feel I was right in that the lvt backing is no where thick enough to help take the cold out of a concrete floor.

Really they should make these lvt about 10mm thick to keep the strength in it.