Underfloor Heating Vapour Barrier

The underfloor heating vapour barrier should be laid on the top of the insulation and sealed at the edges, this also forms a seal eliminating any of the warm screed floors coming in contact with the cold concrete sub-floor, should the insulation not be fitted tightly at all point,
The vapour barrier obviously eliminate and vapour rising upwards as well a it eliminates and chemical reaction of the aluminium foil and the floor screed.