Design your wall or roof assembly to see how materials interact.
Pay special attention to Radiant Barriers which behave differently in summer (Heat Down) vs winter (Heat Up).
Reflectix and other radiant barriers work by reflecting radiant heat, not just slowing conduction. For them to work, they MUST face an air space.
Roof Deck -> Air Gap -> Reflectix -> Insulation
The foil surface faces an empty air gap (at least 0.75"). This allows the foil to reflect heat back up (summer) or emit very little heat down.
Sandwiched (Zero Gap)
If you sandwich Reflectix between two solid layers (like roof deck and fiberglass) with no air gap, its R-value drops to ~1.1 (just the plastic bubbles). It becomes a conductor, not a reflector.