icynene foam insulation

i'm planning onelt or anything as a barrier. I am replacing a portion
of the siding below the drip cap under the windows, so I plan to add
felt under this area. My question is, should I add builder's felt into all of the cavities, by cutting to size between the studs, or is this a waste of time, money and effort? I cannot afford to remove and replace all of the siding, and also not sure I'd want to even if I could afford it.

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