Insulation R value vs. foil facing
The layer fields on the professional Ubakus-Site allows to input also metal foils, so it should take the reflections into account.
But those reflections do only have a tiny effect if the subject is building insulation. Much more important is the fact that metal is a barrier for waterdamp.
And in general, wet insulation material decreases the insulation effect. So in reality, removing a metal foil could improve the insulation.