What is the U-Value? It is: Also known as the U-factor or coefficient of heat transmission, a measure of the rate of non-solar heat loss or gain through a material or assembly. U-values gauge how well a material allows heat to pass through. http://www.daviddarling.info/encyclopedia/U/AE_U-value.html
autodesk.de recently carried a detailed post on formulas related to the U-value.
To get the U value of a wall structure, add the sum of the heat transfer coefficients for the wall types specified thermal resistance R T by this value to be used as a divisor.
