In 1988 ASHRAE Research Project 472 worked to correct these oversights with the introduction of a classification system for walls, roofs, and zones.
Additional research in Thermal radiation and appliance heat gain with respect to CLTD data was also completed shortly after the original publication of the method.
These variables include, building material of the envelope, thicknesses of the building materials, day of the year, time of day, orientation of the surface (e.g. wall or roof, 90 degrees or 180), and wall face orientation (cardinal directions, i.e. N, NW, S, SE, etc.
[1] The respective tables of data were generally developed by using the more complex transfer function method to determine the various cooling loads for different types of heating.
This factor is used to represent the temperature difference between indoor and outdoor air with the inclusion of the heating effects of solar radiation.
Depending on the properties of the building envelope, a delay is present when observing the amount of heat being transferred inside from the outdoors.