Reflective insulation works with the physics of reflective air spaces.
An air space alone does not have much thermal resistance. However an air space coupled with a reflective insulation will have significant resistance to the transfer of heat. Generally, the bigger the air space, the better the performance. If a building element such as a metal roof or wall cladding was installed directly onto reflective insulation with no air spaces, the thermal performance of the system would be reduced as there would be no reflective air spaces between the building element and the insulation to create additional resistance to the transfer of heat.
It is important to ensure a minimum 20mm air space (or the air space recommended for your system) on both sides of the reflective insulation to maximise it’s thermal performance.