I am planning to install ceramic tiles with a "Warmup" heating element beneath them. My first step is to install 1/4" insulation boards on the concrete floor using non-modified thin-set. The floor previously was covered with "peel and stick" tiles that were installed in the late 70s. When I removed the tiles, the glue residue is still evident (the cement floor is tacky when you walk on it). My questions are: - must I remove the remaining adhesive from the floor. If so, how do I best do this? - Is there a type of thin-set that will work well without removing the adhesive?