What percentage of nominal thickness is assumed for duct insulation to determine R-value?

The percentage of nominal thickness assumed for duct insulation to determine R-value is significant because it impacts the assessed thermal performance of the duct systems. In HVAC applications, R-value measures resistance to heat flow. When calculating the effective R-value of insulation, it's common practice to use a certain percentage of the nominal thickness to account for variations in installation and material consistency.

The correct percentage is 75%. This figure reflects the practical reality that actual performance may be less than the theoretical maximum due to factors like compression, density variances, and aging of materials. Thus, using 75% of the nominal thickness provides a more realistic estimation of the duct insulation's thermal resistance in real-world conditions.

This methodology helps in ensuring that installations meet energy efficiency standards and performance expectations by providing a conservative estimate rather than an idealized one. Proper understanding of these percentages is crucial for HVAC professionals to design systems that operate efficiently and maintain desired indoor conditions effectively.