Re: R values for foil-backed board
I was talking about the emissivity or the foil, not its R-value. The emissivity is how much radiated energy it will either absorb or how much it will radiate. The lower the emissivity, the less energy it will absorb or radiate. This does not affect the energy it will accept from direct contact through conduction.
Think of a wood stove. These are often a flat black in color. As you get close to the stove, you feel the heat in two ways, the primary heat you feel is the radiation. This is long wave energy (infrared or IR) that is contacting your skin. The other heat you feel is the hot air. If you take a piece of aluminum foil and put it between your exposed skin and the stove, you will feel much less heat, especially as you get closer to the stove.
When these long waves strike a surface, they must be either absorbed or reflected. Since the aluminum foil has a very low emissivity, the side of the aluminum foil that is next to the stove is reflecting most of the radiating heat that is striking it, that is it is not absorbing it. What little radiation is absorbed combined with the heat transfer from direct contact with hot air is heating up the aluminum foil, but on the skin side, since the aluminum foil radiates very little energy, your skin does not feel the heat from it.
The radiation from the stove heats the air around the stove. There is also a little heat transfer from the air due to direct contact of air molecules with the surface of the stove. So the air is heated by radiation and conduction. The warm air rises and moves about the room. As it moves across surfaces of the room, it transfers heat to them via radiation and conduction. The movement of the air is convection. The surfaces of the room are also being heated directly by the stove through radiation alone.