Ahhh, the big question! Well, both methods are totally acceptable, and there can be advantages for both. Customheat 160 watt kits are designed for both under screed and on top of screed installations. Points to consider when deciding to install your floor heating under the screed or on top of the screed (in the tile glue): The first thing to consider is your finished floor height, at the point where you enter the room. For example, if you have no screed bed or less than 4mm screed bed at the entry to your room, then you will most likely install on top of the screed bed, i.e. your heating will sit in the tile glue. If a screed is fairly uniform in height then that's great, but if you have a lot of screed at one point in your bathroom and only a small depth of screed at another point in your bathroom, then you can get uneven heating in certain areas. In-glue installations (on top of screed) will have a faster heat up time from around 20 minutes. Under screed installations will have a slower heat up time from around 45 minutes as there is more material to heat through. However, this can result in a longer cool down period.