Is your question in reference to the de minimis safe harbor?
See https://www.irs.gov/businesses/small-businesses-self-employed/tangible-property-final-regulations
If so, note the following from the IRS notice:
The de minimis safe harbor provided under § 1.263(a)-1(f) was intended as an administrative convenience whereby a taxpayer is permitted to deduct small dollar expenditures for the acquisition or production of new property or for the improvement of existing property, which otherwise must be capitalized under § 263(a).
Sorry, for some reason the additional context field didn't post originally.
For personal taxes under home energy credits there is a section for qualifying products including insulation, but I cannot seem to find where "insulation" is defined. From an engineering perspective, I'd assume this translates to anything that mitigates unwanted heat transfer into/out of a home. Which is why I'd think HVAC ductwork + supply/return ducts would count.
Thanks!