Use a minimum of 50mm high density insulation below the pipes and 65 – 75mm of screed on top of the insulation. All installation work must comply with current Building Regulations. Note the screed drying time can be 4 to 6 weeks before the underfloor heating system is operated. The mixing valve needs to be set at minimum temperature when commissioning the system. Increase the temperature by 2 to 3 degrees everyday. This will avoid the screed cracking.