U value is a measurement given to the thermal insulation qualities of a wall with the lower the figure the better . As a guide a new well insulated home will have a U value of around 0.25 and a solid wall home will generally be around 2.05 to 2.10 !!
The U value is therefore a measure of how quickly heat escapes through a structure to the outside and as a result also how much carbon is leaking through the walls as well. In todays hard pressed times with ever rising heating bills it is very important to have a home with a low U value if you wish to keep those winter bills down.
The Building Regs state that if the walls are worse than 0.7 in U value you have to fit insulation if adding or replacing a layer to an external wall . The target to try and hit is 0.3 but they will accept a higher figure if 0.3 is not achievable due to the property features etc. An average installation would be around 0.35 U value which still represents an improvement of over 80% in thermal efficiency of the wall.