If installing this product in a sun room with particle board subfloor, is the 6 mil Poly Film Underlayment suggested (per the instructions)? I have read that you should avoid a vapor barrier with a wood subfloor but due to the setting and the instructions, it seems like the underlayment is a good idea.

When installing COREtec Pro Plus Rigid Core Vinyl Planks over particle board, underlayment is always recommended. However, our advice for you would be to reach out to your particle manufacturer and seek their recommendation on which underlayment to use and follow their guidance on how to install vinyl over the particle board. You may also read our installation instructions that are available on the "Installation" tab on the product page for more details.