Conduction through exterior walls and roofs is calculated using conduction time series (CTS). Wall and roof conductive heat input at the exterior is defined by the familiar conduction equation as
(31)
where
= conductive heatinput for the surface n hours ago, Btu/h
= overall heat transfer coefficient for the surface, Btu/h·ft²·°F
= surface area, ft²
= sol-air temperature n hours ago, °F
= presumed constant room temperature, °F
Conductive heat gain through walls or roofs can be calculated using conductive heat inputs for the current hours and past 23 h and conduction time series:
(32)
where
= hourly conductive heat gain for the surface, Btu/h
= heat input for the current hour
= heat input n hours ago
, 
etc. = conduction time factors
The conduction time factors can be used in Equation (32) and provide a means for comparison of time delay characteristics between different wall and roof constructions. Construction heat gains calculated for walls or roofs using periodic response factors (and thus CTS) are identical to those calculated using conduction transfer functions for the steady periodic conditions assumed in design cooling load calculations.