When you set your duct insulation for the existing home to a very low value, and you have very large levels of duct leakage, the attic becomes a heated space. Placing the insulation between the heated attic and the heated home will have minimal impact. However, when you then proceed to insulate and seal the ducts, the savings jumps up very high. Once the ducts are fixed, the attic is no longer heated and the insulation is now doing its job. Look at the improvements as a cumulative benefit and the numbers will make sense. If the program were to rank the duct improvements first, the benefit from the insulation would make more sense.
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