At its most basic, a BUR consists of multiple layers of felt or synthetic plies, bonded by tar or other adhesive and covered with a surface layer of metal or gravel-impregnated composite material. Layers may be assembled on location or laid in sheets. Built-up materials require flashing, as do shingles and other types of roofing materials.
Presuming that your cellar, walls and windows are perfectly insulated, the difference in temperature between indoors and outdoors (D) times the area of your roof in square feet (A), times the period in hours (T) divided by the R-value of a material yields the number of BTUs your furnace must produce to keep your house at the same temperature over that period (T). R-values are enhanced in foam, polystyrene, loose fill and other materials that capture or seal still air in their substance. Even the paper on wallboard retards the passage of heat, albeit with an R-value of only .06.
The R-value for a BUR depends on the materials used in its composition. The average R-value of a 3/8-inch BUR composed of felt, adhesive and covering averages only about .33. In a Colorado wood frame home, for example, where attic and roof insulation would need to total 38 to eliminate most heat loss, a BUR might provide little, if any, benefit compared to a cedar shake, which has an average R-value of .97. The BUR is laid over a layer of insulation that can be customized for climate. R-values for building materials such as polystyrene and foil-faced polyisocyanurate range from 4 to 6 per inch. When these materials make up the base ply, a BUR can provide two-thirds of the R-value needed in the roof, according to Colorado Energy.
Traditional asphalt (.44) or wood (.98) shingles may have minimally higher R-values than BURs, but build-ups perform better on roofs sloping less than 33 degrees. This feature is especially useful in cold climates where ice dams can form under and between shingles in air pockets. The BUR and its newer cousin, the membrane roof, provide solid layers that keep out moisture. The lower R-values in their upper plies can be more than offset by a thicker layer of foam in their base.