Flash of Foam: Within a 2x6, 24-inch-wide wall cavity, spray a 1-inch-thick (minimum) layer of closed-cell expanding foam along the perimeter to seal the joints. The expansion of denser closed-cell foam is easier to control than open-cell and delivers a higher R-value per inch. Cost-saving tip: Flash only the perimeter of the inside of the cavity (shown above) and let the batts handle the middle to the inside edge.
Flat Batts: Carefully install unfaced fiberglass batt rolls flush to the studs and plates to fill the rest of the wall cavity. Be careful not to compress the batts, which can reduce their R-value. Cut and carefully work the insulation around intrusions, such as electrical wires or plumbing runs, to retain its thermal qualities.
Use a BIB: Blown-in-blanket (or BIB) systems blow loose-fill fiberglass through and behind a woven fabric membrane fastened to the narrow edge of the studs to completely fill the cavity and retain the full R-value of the insulation. The loose fill works its way completely around intrusions and the uneven surface of the “flash” layer in the wall cavity, eliminating voids that batts can leave.
Open-Cell Option: If you prefer less-expensive open-cell foam insulation, forget the batt or BIB and use foam to fill the entire cavity. Air-sealing quality will be similar to closed-cell with a slightly better R-value than a 4-inch-thick batt, while ensuring a complete cavity fill without voids. You’ll also air seal any penetrations running through the cavity, effectively eliminating air loss or thermal transfer between adjacent cavities.