The U-Value describes the rate of steady state heat flow through a single building assembly, such as a wall or a roof. It is the amount of heat flowing per hour through one square foot of the assembly, for every degree of temperature difference between the inside air and outside air, in units of Btu/h-ft²-F. The heat flow can be in either direction; it can be heat loss from the building or it can be heat gains. Each of the elements of the building assembly, such as sheathing and insulation, has its own conductance, or rate of heat transfer. The conductance is like the U-Value, and it has the same units, but it is only for a single element.
Masonry walls present some problems when it comes to calculating R-Values and U-Values. Many masonry walls are hollow, with cores and webs connecting the inside and outside surfaces. Furthermore, some of the cores are usually filled with reinforcing and solid grout. Over the face of the wall, there are significant areas that are solid and hollow, and each has a different thermal transmittance. The U-Value of a masonry wall can be calculated in much the same way as a framed wall, using a "framing fraction" approach, but this introduces some inaccuracies because masonry materials are more thermally conductive than wood studs. In the real world, heat flows sideways as well as through the wall, and the approach needs to account for that.