The Uvalue describes how well an element of a building, such as a wall, window or door, conducts heat. The higher the Uvalue, the more conductive the element is, and therefore the less desirable it is if your goal is to insulate. The Uvalue for walls is more complex to calculate than the Uvalue for windows, because a window or door is a single element, whereas a wall comprises multiple elements (such as studs, insulation, sheathing and drywall), all of which have separate Uvalues.

List the Rvalues of every component of your wall. The Rvalue is a measure of thermal resistance used by the building industry. Knowing the Rvalues is the easiest way to derive the Uvalue. You can get the Rvalue from the manufacturer for items such as insulation. For simple, common items such as studs or sheathing, the typical values can be found in tables published in books or online.

Create an "area weighted average" Rvalue. This represents the proportion of space occupied by each element of your wall, multiplied by the element's Rvalue. For example, if every 18 inches of wall is composed of 16 inches of insulation and 2 inches of framing studs, then your area weighted average Rvalue equals 16 times the Rvalue of the insulation divided by 18, plus 2 times the Rvalue of the framing stud divided by 18.

Add the Rvalues of all the other standalone elements of the wall  such as brick or exterior sheathing  to the area weighted average. This sum represents your total Rvalue.

Calculate the reciprocal of the total Rvalue by dividing 1 by it. U is the reciprocal of the Rvalue. This can be expressed as U =1/R.
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References
 "Insulation Handbook"; Richard Bynum; 2000.
 Photo Credit drywall, sheetrock image by Greg Pickens from Fotolia.com