The PEX cross-linked polyethelene piping is attached to the reinforcing steel with a spacing averaging around 18" on centers. The insulation is preferably a 2" insect resistant EPS insulation placed over a 6 mil vapor barrior. The tubing should be more equally spaced and preferrably not more than 12" on centers. The correct installation procedure can be found at http://www.radiantdesigninstitute.com