Isolation of spaces between studs or joists used as return ducts in air duct distribution systems, for heating systems in which the rated heat input does not exceed 120 kW, that serve only one dwelling unit, in buildings to which Part 9 applies [see Sentence 1.3.3.3.(1) of Division A for application of Part 9].

To limit the probability that the size of ducts will be larger than necessary or that return ducts will be connected to other spaces, which could lead to a drop in pressure, which could lead to a reduction in the flow of return air from rooms, which could lead to an inability to regulate the flow of warm air to rooms, which could lead to:

• an inability to maintain the minimum indoor air temperatures [see Sentence 9.33.3.1.(1)] assumed for the purpose of determining minimum required thermal insulation levels [see Sentence 9.25.2.1.(1)], or
• where required ventilation is provided by the forced-air heating system, inadequate ventilation.

This is to limit the probability of condensation, which could lead to the deterioration of building elements, which could lead to compromised structural performance, which could lead to harm to persons.