I'm replacing an old LVP floor, which required an underlayment, with a new brand LVP that has an underlayment built-in. I'd think that to mitigate sound and vapor, a reasonable thing to do is to continue to use the existing ~2mm underlayment with the new floor.
But the manufacturer doesn't just say an underlayment simply isn't required, but explicitly also says that using an additional underlayment will void the warranty. I wonder if it will actually damage the flooring or if the warranty being voided is just a manufacturers way to weasel out of honoring the warranty.
Does anyone know if using a preexisting underlayment will actually somehow destroy the new LVP that has it built-in?