Foil insulation serves many useful functions and are increasingly being used under roofs and in framed walls for thermal insulation, vapour and condensation control. Both new build and refurbishment.

Foil insulation can be powerful radiant heat barriers (with airspaces), reflecting 97% or emitting 3% of all radiant heat.

“R-value” means “resistance to the flow of heat” and foil insulations are always expressed as a total R-value for the entire roof/ceiling, wall or floor, determined by a calculation process that I will leave to others.

A good analogy is a cooked chicken wrapped in foil where the chicken stays hot, not because of reflection as conduction/contact has occurred on the metal surface, rather the outward low emitting surface. Most heat is lost by conduction where the chicken sits on the bench.

Another simple test to prove this is to heat your oven to say 200 degC, open the door and you will feel a blast of radiant heat. Now hold some mineral wool (quilt) insulation, say an R1.5 (75 mm thick) against the door.
After a few minutes (depending on your ovens insulation!) you will find the heat radiating at you from the mineral wool which is now quite warm to touch. Now place a sheet of foil, eg cooking foil against the mineral wool.
You will immediately find there is little or no radiation and even though the foil is warm to touch, there is only this low 3% emission or radiation from the foil surface.

And it will stay that way!