Installing insulation boards over a Concrete Subfloor
When installing over cement, screws and washers are not necessary. You will first use a trowel to put down a modified mortar. Then you will stagger the boards and press them together.
In a dry area, use an alkali fiberglass mesh tape. Use a modified mortar over the top. Install your heating system accordingly.
In a wet area, use a polymer sealant and adhesive between the boards. Let dry (this usually takes about 20-30 minutes). Once dried, install your heating system accordingly.