It is called radiant because of its design (not necessarily performance ).
Pretty much every heat transfer in our atmosphere is a percentage (.37 = 37%). If you only have 37% radiant heat transfer the remaining 63% is transmitted a different way.
Its efficiency rating is the percentage of heat transfer that occurs in relation to the other two.
Radiant heat is the only source of energy transfer that can occur through the air. Even though the molecules that make up standard air do conduct, the purpose/design of the particular heater is such that the majority of the energy consumed to create heat is transferred without convection or conduction. The heat transfer rate through air by conduction significantly diminishes with distance where it rapidly falls off within a very short distance of the radiating source. Unlike Radiated heat which can travel great distances and still deliver a huge percentage of its energy.
The radiant heater in a workshop will function with the doors wide open and the wind blowing. The amount of conduction/convection that occurs is considered a loss because it does not heat the subject because it is blown away.
HVAC units are designed to transfer their heat to the air through conduction and distribute it throughout the house through convection. The equipment is located outside of the living space in many cases and does not rely at all on radiant heat transfer. Radiant heat transfer from this type of equipment is a loss. That is why there is foil faced insulation on the inside of your furnace.
When the sun shines on your house and makes it warm inside, the energy comes from the sun through radiation. It is absorbed by the exterior roof (for example) that conducts the heat. Air on the other side of the roof picks up some of the heat through conduction and uses convection to transfer the heat to the ceiling below. At the same time, heat is radiated from the roof to the ceiling below directly through the air. Because the roof does not touch the ceiling, no conduction occurs at this point. The heat transferred to the ceiling conducts through the ceiling components where again conduction, convection and radiation heat the inside the house.
Is this clear as mud?
How heat transfers from the sun to the air inside your house is classified by the heat transfer method that produces the greatest percentage of heat transfer. This can be very difficult to determine and is the reason for considerable debate.
Is it better to insulate the attic or install a radiant shield to reduce HVAC cooling load ?
Radiant barriers are highly reflective and prevent radiant load from passing through.
Radiant barriers have a low emissivity, which means they do not absorb and re-emit heat energy well.
Radiant barriers however tend to be highly conductive so how and where they are utilized is critical.
Adding more insulation basically slows down heat transfer until the solar load (the sun in this case) moves below the horizon. If it slows it down enough (long enough), that heat never reaches the interior because it reverses direction as the solar load is removed (heat always moves from a higher temperature object to a lower temperature object). When the sun is out, heat energy moves towards the cooler interior of your house. When the sun goes down, that heat which is trapped in the installation changes direction and moves back towards the exterior.
Which one works the best for your application? You decide.
All objects on earth radiate heat at a rate relative to their emissivity (emissivity changes with temperature; Planks curve). The radiant heat transfer rate is the net emissivity of the two objects. In other words, there is no such thing as hot and cold, only warm and warmer.