TREAT does not allow improvements for only a portion of the distribution system to be entered. If not all the distribution in the unconditioned space is being insulated, then enter it on a visual inspection improvement instead.
If you are replacing the heating system, then the distribution improvements should be entered under “new distribution system” on the heating replacement. TREAT will not allow you to have a heating system improvement and a separate distribution improvement in the same package.
Duct sealing savings can be estimated using BPI’s “Distribution Efficiency Look-Up Table”.