Cooling Load - Roof

The loads from the roofs and walls are conductive loads. Heat from the outdoors is conducted through the roofing or wall materials as it enters the space. If the problem assumes no radiation loads or does not take into account time, then the only load is the conductive load from the temperature difference between the outdoors and indoors, which is as shown below.

Cooling Load CLTD Calc for the Mechanical PE Exam

However, the heat effect from the roofs and walls is not this simple. The radiation from the sun onto the building and the time it takes for the heat to transmit through the materials must be taken in to account. In order to calculate the total effect of the difference between the indoor and outdoor temperature, the effect of the solar radiation onto the walls and roofs and the time factor due to the heat storage of the roof/wall material, the engineer should use the Cooling Load Temperature Difference or CLTD. These values can be found in the ASHRAE Fundamentals book 1997 edition and older. These tables are organized by latitude, roof or wall type, month and wall facing orientation direction. In addition, the CLTD is organized by the hour of the day. It is not the opinion of the author that you will need to look-up these values in ASHRAE 1997 and that these values will be given to you as part of the problem. It is only important to understand what CLTD is and how to use it when given it in a problem.

Cooling Load CLTD Calc for the Mechanical PE Exam

It is also important to note that the CLTD is a simplified approach to determining the heat load due to roofs and walls. In actuality the heat load due to the roofs/walls will also be dependent on many other conditions like the indoor conditions and the heat radiated from the inner wall/roof to the indoor space.

Heat Transfer R-Value for the Mechanical PE Exam