Do I need to prime the subfloor before installing Ultralight?
Ultralight Insulation Boards
Typically, you will not need to prime before installing Ultralight insulation boards.
Primers help encapsulate dust to improve bonding with the thinset you’d apply to lay the boards. But on brand new or clean subfloors this may not be necessary. When in doubt, consider a solvent-free floor primer like Ardex P51 or Mapei SM. If your subfloor is not level or has a lot of residue, such as glue remnants from an old vinyl floor recently removed, you may need to apply a thin coat of leveler to smooth the subfloor in preparation for your project.