When the wall is already open, batts are the right answer and the cheap one.
Batts perform to their label only when they fill the cavity completely and touch the sheathing without being compressed. Most of the poor batt work we open up has the same three faults: compressed behind wiring instead of split around it, gaps at the top and bottom of the bay, and nothing detailed where the batt meets a window frame or an electrical box.
We split batts around services, cut friction-fit to the bay rather than stuffing, and seal the air barrier at every penetration. That last part is what turns a nominal R-value into a real one.
Batts are the wrong answer in a closed cavity — that is an injection job — and in a basement wall against masonry, where you want continuous rigid or closed-cell against the concrete first or you will condense moisture behind the batt.
Batts plus a properly detailed air barrier get you most of the performance for a fraction of the cost. Spray foam earns its price where the geometry is awkward or the assembly needs the air seal and the insulation to be the same material.
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