Which trades are responsible? The framing and insulation crews.
What products are needed? Closed-cell sill-seal tape and construction glue.
Precise product selection is up to you, but I caution against having your trade partners decide. Instead, be specific about the products you want them to use or at least the level of performance you require (and will inspect).
Is there redundancy? Yes, at the connection between the floor and the sill plate.
Establish a Quality Standard and Apply It to Other Common Air Leakage Points
This was just an example of the process, but you can follow the same steps to address other common culprits of air leakage at windows and doors, drywall penetrations, interstitial cavity connections, penetrations through exterior walls, and at mechanical chases, among others.
A blower door test will indicate any air leaks or confirm problem spots. At a glance, this approach may appear to be overkill, but once you’ve applied it, you can document the process to establish a quality standard in your scopes of work.
Also, talk to your trades about those standards and why they matter, train them on best practices, and then verify that they are executing all details to your expectations.
Once you take the first few steps, it gets easier. And your code inspector and homeowners will appreciate the effort.
