There are such values, called R-values.  They define the resistance to heat loss per unit time, temperature difference and area (i.e., higher is better).  These are English units, hour-degreeF-ft^2/BTU.  An R-value of 20 means that 100 sq. ft. of wall separating a 20 deg F temperature difference for 8 hours passes 800 BTU of heat.  Typical recommended R-values (ref. 1) for use in residences are 4 to 60, depending on climate and location in the house.  For walls the range is 11 to 28.  In the ref. 2 web site you can find R ratings for various insulation materials, in R-value per inch of insulation thickness, and thicknesses required for various recommended R-values.
What's the rate of heat loss through a domestic well-insulated wall per sqm when the heat differential is 15C?