Should I use primer with the Loose Wire System?
If the heating element is being laid directly onto a plywood or concrete subfloor, use the primer supplied. Leave to dry for 2-3 hours until tacky, then lay the heating element. If an insulated or standard tile backer board is being used, no primer is needed. We recommend you use Warmup Insulation Boards.
Check out the below installation video that discusses how to install the Loose Wire System in either scenario: