Insulating underlayments and uncoupling membranes share some key characteristics like providing crack isolation benefits, but they are separated by their primary functions.
Insulating underlayments, like WarmlyYours ThermalSheet, provide a thermal break between a subfloor and the heating elements, which can help prevent the concrete from acting as a “heat sink” and increase energy efficiency. ThermalSheet has a 1.5 R-value, and it offers crack isolation of up to ¼”. This can be particularly beneficial if you are trying to heat a room above a cement slab or above an unheated garage.
Uncoupling membranes, like the Prodeso membrane, are great for protecting relatively fragile floor coverings like tile from foundational movement. They also allow for faster installation times than many other types of floor heating.