Underfloor heating/issue with builder

[SJPurdy]

I struggle to understand is how masking tape can be enough of an insulation to stop the excessive heat buildup to transfer through it and cause it to burnout!

It is not the tape that traps the heat but when tape is put loosely over the wire joint then there will be a trapped air filled void which will then itself be surrounded by SLC or addy. Where the SLC or addy is in direct contact with the heating wire (joint) then heat will flow through it away from the wire. However where there is a trapped air pocket around the wire(joint) then the heat does not flow through the air very well (compared with the SLC or addy) and thus bit of wire(joint) might overheat.