Heat gain from exterior surfaces is affected by the rate of conductive heat transfer through them. In order to determine this rate, the engine uses conduction time series (CTS) values. Each exterior wall and roof has 24 different CTS percentage values, summing to 100%, that indicate how much of the heat stored in the envelope has come through after a given amount of time.
| Conduction Time Factor, % |
 CTS for Light to Heavy Walls |
|
| Hours | ||
| Key: |
 |
Metal Wall Panel, Sheathing, R-11 Batt Insulation, Gyp Board, U = 0.074. |
 |
Brick, Sheathing, R-11 Batt Insulation, Gyp Baoard, U = 0.066. | |
 |
8” HW Concrete, R-11 Batt Insulation, Gyp Baoard, U = 0.076. | |
CTS values are derived through a complicated process that examines the material properties that make up each construction. For the purposes of cooling, the U-value of a construction is also used (for heating, only the U-value is used and not CTS values). The weight of a construction (also derived from material properties) also plays a role in determining the thermal storage characteristics of a space.