There are two significant types of floor insulation – foil with an adjacent air cavity and bulk insulation such as blanket/batting, polystyrene insulation.
Reflective foil insulation is adhered to kraft paper, plastic sheet or bubble pack laminates. The insulation relies on having a reflective unvented air cavity which is readily achieved under laboratory conditions, but rarely in practice. The bubble pack insulation rarely increases Total R-value, as the inner bubble faces are not highly reflective and are spaced apart insufficiently.”
A vented cavity virtually negates any floor insulation value contribution from the foil. Further reduction in insulation can be expected as a layer of dust reduces the reflectivity of the foil facing the floor, often within months of installing the floor insulation.
Bulk underfloor insulation should be installed hard against the floor to prevent vented cavities that halve the Total R value (installed) and to stop rodents in the cavity.
Blankets/batting underfloor insulation should be supported by stapling, wires or mesh so they remain in contact with the floor. It should be cut overwidth for the cavity to prevent edge or joint gaps.
Under floor insulation blankets that absorb moisture (eg wool) should not be used in damp sub floor cavities.