While this is the most common question asked when comparing covers, the answer can be the most deceptive. The following, though rather lengthy, explains insulation and R-value. The higher the R-value, the slower heat will travel or conduct through a material. Better insulating materials tend to have better R-values. A cast-iron skillet handle heats up because of conduction through the metal.
Convection is the transfer of heat by physically moving the molecules from one place to another.
A campfire can warm you even if there is wind between you and the fire, because radiation is not affected by air. As it rises it heats the underside of the cover, then moves through the cover by conduction . Increasing the covers R-value reduces, or slows the rate of conduction. R-value is a term coined by the FTC in response to a need for a way to compare the relative effectiveness of various forms of insulation. As air within the test hot box is heated, convection takes place, causing the heat to rise.