TABLE OF CONTENTS Insulation on ductwork and other air distribution equipment serves to reduce heat gain to the supply air and to prevent condensation of ambient moisture. Insulation requirements are increased in cold-air distribution systems because of the lower supply air temperature. The higher temperature difference to ambient air increases the driving force for heat gain, and there is an increased chance of some surfaces being cooled below the ambient dew point and causing condensation. For most applications, insulation specified for the purpose of limiting heat gain will also be sufficient to prevent condensation.
To minimize heat gain and the resulting increase in supply air volume, cold-air distribution systems should be designed with thicker insulation than that used in standard designs. Duct heat gain is calculated by the methods given in section 4.5. Designers should evaluate the benefits of reduced heat gain and supply air volume with added insulation vs. the costs for each system. However, in general, where 1-in. (25-mm) external blanket insulation is used with conventional supply temperatures, 2-in. (51-mm) insulation is recommended for systems with nominal supply air temperatures of 40 F to 45 F (4 C to 7 C), and 1.5-in. (38-mm) thickness is recommended for nominal supply temperatures of 46 F to 50 F (8 C to 10 C).
Good workmanship in the application of duct insulation is essential for cold-air distribution systems. Complete coverage of ducts and air distribution equipment and a continuous, intact vapor retarder are necessary to prevent excessive heat gains and unwanted condensation.
